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Compact Stellar X-ray Sources in Normal Galaxies


arXiv:astro-ph/0307077v1 3 Jul 2003

11
Compact Stellar X-ray Sources in Normal Galaxies
G. Fabbiano1 & N. E. White2
1 Harvard-Smithsonian Center for Astrophysics 60 Garden St., Cambridge MA 02138, USA 2 NASA-GSFC Code 660, Greenbelt MD 20771, USA

11.1

Introduction

In the 1995 X-ray Binaries book edited by Lewin, van Paradijs and van den Heuvel, the chapter on Normal galaxies and their X-ray binary populations (Fabbiano 1995) began with the claim that “X-ray binaries are an important component of the X-ray emission of galaxies. Therefore the knowledge gathered from the study of Galactic X-ray sources can be used to interpret X-ray observations of external galaxies. Conversely, observations of external galaxies can provide us with uniform samples of X-ray binaries, in a variety of di?erent environments. ” This statement was based mostly on the Einstein Observatory survey of normal galaxies (e.g., Fabbiano 1989; Fabbiano, Kim & Trinchieri 1992). Those results have been borne out by later work, yet at the time the claim took a certain leap of faith. Now, nearly a decade later, the sensitive sub-arcsecond spectrally-resolved images of galaxies from Chandra (Weisskopf et al. 2000), complemented by the XMM-Newton (Jansen et al. 2001) data for the nearest galaxies (angular resolution of XMM-Newton is ? 15”), have made strikingly true what was then largely just wishful anticipation. While a substantial body of ROSAT and ASCA observations exists, which was not included in the 1995 Chapter, the revolutionary quality of the Chandra (and to a more limited degree of XMM-Newton) data is such that the present review will be based on these most recent results. In this Chapter we ?rst discuss the emerging awareness of X-ray (0.1 ? 10 keV band, approximately) stellar populations in spiral galaxies: we focus on four well studied galaxies (M31, M81, M83 and M101), and we then discuss the e?ect of recent widespread star formation on the luminosity functions of the X-ray emitting populations (Section 11.2). We then review the body of observational evidence on the ultraluminous X-ray sources (LX > 1039 ergs s?1 ), that are associated with active/recent star formation (Section 11.3; see the Chapter by King, in this book, for a review of theoretical work on this subject; see also the chapter by McClintock & Remillard on black hole binaries). We follow with a review of the X-ray population properties of old stellar systems (E and S0 galaxies; Section 11.4). We then discuss the results of correlation analyses of the integrated galaxy emission (Section 11.5), and we conclude with a look at the X-ray evolution of galaxies going back into the deep universe (Section 11.6). 1

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Compact Stellar X-ray Sources in Normal Galaxies

11.2

X-ray binary (XRB) Populations in Spiral Galaxies

Because of their proximity, nearby spiral galaxies is where the early work on extra-galactic XRB populations begun (see Fabbiano 1995). For the same reason, these are the galaxies where the deepest samples of sources have been acquired with Chandra and XMM-Newton. Here we will discuss ?rst the recent work done on M31, which, not surprisingly, is the galaxy that has been studied in most detail. We will then review the results on M81, M83, and M101, to provide examples of the XRB populations in a wider variety of spirals. We conclude this section with a summary of the work on actively star-forming galaxies. We note that this ?eld is evolving rapidly, with an increasing number of galaxies being surveyed and with the sensitivity limit being pushed to fainter ?uxes, with ever deeper Chandra observations. 11.2.1 M31 Being at a distance of only ? 700 kpc, M31 (NGC 224, the Andromeda nebula) is the spiral (Sb) galaxy closest to us. M31 has been observed by virtually all the X-ray observatories since Uhuru, the ?rst X-ray satellite (for a history of the Xray observations of galaxies, see Fabbiano & Kessler 2001). Starting with the Einstein Observatory and following on with ROSAT, M31 has been the prime target for systematic studies of a population of extragalactic XRBs, and for comparisons with our own Galactic XRBs (e.g., Long & Van Speybroeck 1983; Trinchieri & Fabbiano 1991; Primini, Forman & Jones 1993; Supper et al. 1997, 2001). Chandra and XMMNewton observations, both by themselves and in combination, are providing new insight on the characteristics of the XRB population of M31. With its subarcsecond resolution, Chandra is unique in resolving dense source regions, such as the circumnuclear region of M31, and detecting faint sources (Garcia et al. 2000). Given the proximity of M31 and the relatively low density of luminous XRBs, XMM-Newton provides valuable data on the XRB population of this galaxy, if one excludes the centermost crowded core (Shirey et al. 2001). Source variability and counterparts - Multiple observations of the same ?elds with these two observatories (and comparison with previous observations) have con?rmed the general source variability characteristic of XRBs. XMM-Newton work, following the ?rst statement of source variability (Osborne et al. 2001), includes detailed studies of interesting luminous sources. Trudolyubov, Borozdin & Priedhorsky (2001) report the discovery of three transient sources, with maximum X-ray emission in the 1037 ergs s?1 range: a candidate low-mass black-hole binary, a source with a long (>1 year) outburst, and a supersoft transient. Trudolyubov et al. (2002b) report an 83% modulation with a 2.78 hr period in the X-ray source associated with the globular cluster (GC) Bo 158. Comparison with earlier XMM-Newton observations and with the ROSAT PSPC data, allows these authors to conclude that the modulation is anticorrelated with the source ?ux, suggesting perhaps a larger less obscured emission region in high state. This source resembles Galactic ‘dip’ XRBs, and could be an accreting neutron star. Its period suggest a highly compact system (separation ? 1011 cm). Widespread source variability is evident from Chandra observations, both from a 47 ks HRC study of the bulge (Kaaret 2002), from a set of eight Chandra ACIS observations of the central 17’×17’ taken between 1999 and 2001 (Kong et al. 2002),

11.2 X-ray binary (XRB) Populations in Spiral Galaxies

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and from a 2.5 years 17 epochs survey with the Chandra HRC (Williams et al. 2003), which also includes the data from Kaaret (2002). Kong et al. ?nd 204 sources, including nine supersoft sources, with a detection limit of ? 2 × 1035 ergs s?1 . This detection limit is 5 times fainter than that of the ROSAT HRI catalog (Primini, Forman & Jones 1993), which lists only 77 sources in the surveyed area. They report 22 globular cluster (GC) identi?cations, 2 supernova remnants, and 9 planetary nebulae associations. By comparing the di?erent individual data sets, they establish that 50% of the sources vary on timescales of months, and 13 are transients. The spectra of the most luminous sources can be ?tted with power-laws with Γ ? 1.8, and, of these, 12 show coordinated ?ux and spectral variability. Two sources exhibit harder spectra with increasing count rate, reminiscent of Galactic Z sources (e.g. Hasinger & van der Klis 1989). All these characteristics point to an XRB population similar to that of the Milky Way. The HRC survey (Williams et al. 2003) reports ?uxes and light curves for 173 sources, and ?nds variability in 25% of the sources; 17 of these sources are transients, and two of these are identi?ed with variable HST WFPC2 U band counterparts. One of these two sources is also a transient in the optical and has global properties suggesting a ? 10 M⊙ black hole X-ray nova with a period ≥ 9 days. Williams et al. (2003) determine that at any given time there are 1.9 ± 1.3 active X-ray transients in M31, and from here they infer that the ratio of neutron star to black hole LMXBs in M31 is ? 1, comparable to that in the Galaxy. Globular Cluster (GC) sources - The recent X-ray populations studies of M31 with Chandra and XMM-Newton demonstrate the importance of large area surveys of the entire galaxian system. A targeted study of GCs with three Chandra ?elds at large galactocentric radii (Di Stefano et al. 2002) revives the old suggestion (Long & Van Speybroeck 1983) that the M31 GC sources are more X-ray luminous than Galactic GC sources. This hypothesis had been dismissed with the ROSAT M31 survey (Supper et al. 1997), which however covered only the central 34’ of M31. Di Stefano et al. (2002) ?nd that in their ?elds the most luminous sources are associated with GCs. They detect 28 GCs sources, 15 of which are new detections: 1/3 of these sources have LX (0.5 ? 7 keV) > 1037 ergs s?1 ; 1/10 of the sources have LX (0.5 ? 7 keV) > 1038 ergs s?1 . The X-ray luminosity function (XLF) of the M31 GC sources di?ers from the Galactic GC XLF, by both having a larger number of sources, and by extending a decade higher in X-ray luminosity (the most luminous M31 GC is Bo 375 with LX > 2 × 1038 ergs s?1 ; compare with Milky Way GCs, that emit less than 1037 ergs s?1 ). Supersoft sources (SSS) - SSS are very soft X-ray sources, with most of the emission below 1 keV, and spectra that can be ?tted with black body temperatures of ≤ 100 eV (see Chapter by Kahabka in this volume). SSS were ?rst discovered in M31 with ROSAT (Supper et al. 1997). As noted above, Kong et al. (2002) reported nine SSS in their Chandra observations of M31. Recent work by Di Stefano et al. (2003) reports 33 SSSs in the same ?elds surveyed for GCs by Di Stefano et al. (2002), of which only two were known since the ROSAT times. Two SSSs are identi?ed with symbiotic stars and two with supernova remnants, but the bulk are likely to be supersoft XRBs. These sources are highly variable, and may be classi?ed

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Compact Stellar X-ray Sources in Normal Galaxies

in two spectral groups: sources with kT≤ 100 eV, and other sources with harder emission, up to kT? 300 eV. Sixteen of them (on average the most luminous) cluster in the bulge, others are found in both the disk and the halo of M31. Di Stefano et al. (2003) point out that some of these sources are detected with luminosities well below 1037 ergs s?1 , the luminosity of a 0.6 M⊙ white dwarf steadily burning hydrogen, and are therefore likely to be lower mass white dwarfs or luminous cataclysmic variables. The bulge - The XLFs of the global core population [Kaaret 2002 (Chandra HRC); Kong et al. 2002 (Chandra ACIS); Trudolyubov et al. 2002a (XMM-Newton)] all are in general agreement with each other and with the Einstein (Trinchieri & Fabbiano 1991) and ROSAT studies (Primini, Forman & Jones 1993). However, because of the resolution and sensitivity of Chandra, both Kong et al. (2002) and Kaaret (2002) can can look at the bulge source population in greater detail than ever before. Kong et al. divide the detected sources in three groups, based on their galactocentric position: inner bulge (2‘×2’), outer bulge (8’×8’, excluding the inner bulge sources), and disk (17’×17’, excluding the two bulge regions). When considering the entire bulge population, these authors ?nd a general low luminosity break of the XLF at ? 2 × 1037 ergs s?1 , in agreement with Trudolyubov et al. (2002a). However, they also ?nd that the break appears to shift to lower luminosities with decreasing galactocentric radius, going from 0.18 ± 0.08 × 1037 ergs s?1 in the inner bulge to 2.10 ± 0.39 × 1037 ergs s?1 in the outermost ‘disk’ region. They note that if the breaks mark episodes of star formation, the more recent of these events must have occurred at larger radii. The slopes of the XLFs also vary (0.67 ± 0.08 in the center, 1.86 ± 0.40 in the outermost region), but this trend is the opposite of that expected from progressively young populations, where more luminous, short lived sources, may be found (see e.g. Kilgard et al. 2002; Zezas & Fabbiano 2002; Section 11.2.4). Kong et al. suggest that the XRB populations of the central regions of M31 may instead all be old (see Trudolyubov et al. 2002a), with the shifts of the break resulting from the inclusion of new classes of fainter sources in the inner regions, rather than from a disappearance of the most luminous sources. Kaaret (2002) contributes to the debate on the nature of the inner bulge sources by investigating their spatial distribution. He shows that the the number of X-ray sources detected in the centermost regions of the bulge (< 100′′ ) is in excess of what would be expected on the basis of the radial distribution of the optical surface brightness, and suggests that this result may be consistent with a GC origin for the LMXBs. X-ray source populations in di?erent galaxian ?elds - With the increased rate of papers on M31, resulting from the XMM-Newton and Chandra surveys of this galaxy, we are now realizing that the X-ray source population of M31 is more varied than previously thought, and that there are correlations between the properties of the X-ray sources and those of the stellar ?eld to which they belong. In contrast with previous reports (e.g. Trinchieri & Fabbiano 1991; Kong et al. 2002), Trudolyubov et al. (2002a), by using a larger de?nition for the radius of the bulge (15’), with XMM-Newton observations conclude that, although the XLFs of bulge and disk sources have a similar cumulative slope (-1.3), disk sources are all fainter than LX < 2 × 1037 ergs s?1 , while bulge sources can have luminosities

11.2 X-ray binary (XRB) Populations in Spiral Galaxies

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Fig. 11.1. Regions of M31 observed with Chandra and XMM-Newton. Dots are detected Chandra sources; yellow crosses and blue diamonds identify supernova remnants and OB associations in the ?eld (not X-ray sources), respectively (from Kong et al. 2003).

as high as LX ? 1038 ergs s?1 . They suggest that the most luminous sources are associated with the older stellar population, as in the Milky Way (Grimm, Gilfanov & Sunyaev 2002). However, the ?elds studied by Trudolyubov et al. (2002a) do not include the areas surveyed by Di Stefano et al. (2002), where the most luminous GC sources are found (see Fig. 11.1). A Chandra ACIS study of XLFs from di?erent regions of M31 (Fig. 11.1; Kong et al. 2003), uses a follow-up of the Di Stefano et al. (2002) survey. The results (Fig. 11.2) show that the sources in the central 17’×17’ region are overall more luminous than those from the outer ?elds (as noticed by Trudolyubov et al. 2002a), but only if one removes the GC population, which appears to have a relatively more numerous high luminosity component than the central sources. The slopes of the XLFs of the external ?elds also vary, and there is an indication that these di?erences are related to variations in the stellar populations of the di?erent ?elds: Field 1, which has the steepest slope (cumulative -1.7+0.34 ) and also the lowest density of X?0.15 ray sources, does not appear to have a large young population of stars; Field 2, with the largest X-ray source population and the ?attest XLF slope (cumulative -0.9) is in the region with the youngest stellar population. This slope is the closest to that (0.63 ± 0.13) derived by Grimm, Gilfanov & Sunyaev (2002) for the high mass X-ray binaries (HMXBs) in the Galaxy; Field 3, with an intermediate XLF slope instead does not appear to cover a large stellar population. The overall integrated slope is instead similar to that found by Grimm et al. for the Galactic low-mass X-ray binary

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Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.2. Cumulative XLFs and best-?t power-laws from di?erent ?elds of M31 (Kong et al. 2003).

(LMXB) population, suggesting that these sources dominate the X-ray emission of M31. Williams et al. (2003), using the Chandra HRC survey of M31, distinguish between a roughly radially symmetric bulge population (within a 7’ radius) and a ?eld population, outside this inner region. They report di?erent XLFs for bulge and disk sources, with a ?atter broken power-law representing well the disk distribution. Their survey has a wider (although shallower) coverage of the entire M31 galaxy, than the Trudolyubov et al. (2202a) work, and covers also the southern half of the disk, where the X-ray sources are signi?cantly more luminous than in the northern disk, surveyed with XMM-Newton by Trudolyubov et al.. The Trudolyubov et al. (2202a), Kong et al. (2003), and Williams et al . (2003) papers are illuminating in demonstrating the variability of the XLF in di?erent regions, and in pointing out how a good spatial sampling and supporting multi-wavelength information, are needed to get a complete picture of the XRB population of M31.

11.2 X-ray binary (XRB) Populations in Spiral Galaxies

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11.2.2 M81 As discussed in Fabbiano (1995), M81 (NGC 3031) is a nearby (3.6 Mpc, Freedman et al. 1994) Sb galaxy optically similar to M31; however, in X-rays it displays a signi?cantly more luminous population of individual sources (even discounting the nuclear AGN). To get a feel of the progress in sensitivity of X-ray telescopes in the last ? 20 years, it is interesting to compare the Einstein observations of M81, where 9 extra-nuclear sources with LX ≥ 2 × 1038 ergs s?1 were detected (Fabbiano 1988; total ? 35 ks exposure time), with the ROSAT results that led to detection of 26 extra-nuclear sources with LX > 1037 ergs s?1 (Immler & Wang 2001; 177 ks - HRI, 101 ks - PSPC), and ?nally with the Chandra results: 124 sources detected within the optical D25 isophote to a limiting luminosity of ? 3 × 1036 ergs s?1 in ?50 ks (Swartz et al. 2003). The Chandra results show that 88% of the non-nuclear emission is resolved into individual sources. The brightest of these sources have luminosities exceeding the Eddington luminosity for a spherically accreting neutron star (see Fabbiano 1995), i.e. they are among the sources dubbed ‘Ultraluminous X-ray Sources’ (ULX; see Section 11.3). Of the 66 sources that lie within Hubble Space Telescope (HST) ?elds, 34 have potential counterparts (but 20±4 chance coincidences are expected). Five sources are coincident with supernova remnants in the spiral arms (including the well studied SN 1993J), but one of them (the ULX X-6) is identi?ed with a XRB, based on it X-ray spectrum. Only four potential GC identi?cations are found. For one of the M81 sources, Ghosh et al. (2001) report a 10-year ROSAT-Chandra X-ray transient light curve. Nine of the sources found in the Chandra observation of M81 are supersoft (SSS; Swartz et al. 2002), with LX (0.2 ? 2.0 keV) in the range of > 2 × 1036 ? 3 × 1038 ergs s?1 , and a blackbody emission temperature of 40-80 eV. The fraction of SSS is consistent with the expected values, based on the Galaxy and M31. Four sources are in the bulge and ?ve in the disk; of the latter, four are on the spiral arms. With the exception of the most luminous of these systems, which has a bolometric luminosity Lbol ? 1.5 × 1039 ergs s?1 , and will be discussed in Section 11.3, all these sources are consistent with the nuclear-burning accreting white dwarf picture of SSS (van den Heuvel et al. 1992; see the Chapter by Kahabka in this book). The SSS associated with the spiral arms tend to have higher emission temperatures, suggesting more massive white dwarf counterparts, which would result from relatively massive stars in a relatively younger stellar population. The ?rst report of XLF studies in M81 (Tennant et al. 2001; Fig.11.3) showed dramatic di?erences in the XLFs of bulge and disk sources. While the XLF of the bulge is reminiscent of the bulge of M31, with a relatively steep power-law ?attening at LX (0.2 ? 8.0 keV) < 4 × 1037 ergs s?1 , the XLF of the disk follows a uninterrupted shallow power law (cumulative slope -0.50). The subsequent more complete study of Swartz et al. (2003) con?rms the break in the bulge XLF and suggests that it may be due to an aging ? 400 Myr old population of LMXBs. The extrapolation of this XLF to lower luminosities can only explain 10% of the unresolved bulge emission, which, however, has the same spatial distribution as the detected bulge sources: besides some gaseous emission, this may suggest an undetected steepening of the XLF due to a yet fainter older population of sources

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Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.3. Bulge and Disk XLFs for M81. The straight line is the best-?t power-law to the disk XLF (Tennant et al. 2001).

in the central regions. The disk population has di?erent XLFs, depending on the source distance from the spiral arms (Fig. 11.4): in particular, the very luminous (> 1038 ergs s?1 ) sources responsible for the ?at power law are all concentrated on the arms; a break at high luminosities appears when spiral arm sources are excluded. Swartz et al. (2003) suggest that these most luminous sources are likely to be very young XRBs resulting from the star formation stimulated by the spiral density waves. 11.2.3 M83 and M101 M83 (NGC 5236) and M101 (NGC 5457) are both face-on Sc galaxies. M83 is likely to be a member of the Centaurus group, with a distance of ? 4 Mpc (de Vaucouleurs et al. 1991); M101 is more distant (? 7 Mpc; Stetson et al. 1998), but still in the nearby universe. M83 is a grand design, barred spiral, with a starburst nucleus. Is has been observed extensively in the pre-Chandra era, but here we discuss only the Chandra observations, that are the most relevant for the study of the X-ray source population. M83 was observed with Chandra ACIS-S3 for ? 50 ks (Fig. 11.5). Soria & Wu (2002) detect 81 sources in these data, of which 18 had been detected previously with ROSAT; 15 sources are resolved in the previously confused nuclear region, which has the highest source density. The XLF of the sources in the nuclear-bar region, where a young stellar population is likely to prevale, follows a fairly ?at unbroken power-law (cumulative slope -0.8). The XLF of the disk sources is instead steeper (slope -1.3),

11.2 X-ray binary (XRB) Populations in Spiral Galaxies

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Fig. 11.4. Steepening XLFs of disk sources of M81, at increasing distance from the spiral arms (solid line; Swartz et al. 2003).

with a break at ? 6 × 1037 ergs s?1 , becoming ?atter at the lower luminosities. This behaviour is reminiscent of the XLFs of the bulges of M31 and M81, and suggests an older XRB population. In M101, 110 sources (27 of which are expected to be backround AGN) were detected in a 98 ks Chandra ACIS-S3 observation, with a limiting luminosity of

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Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.5. M83 as seen with Chandra. Note the population of point-like sources and the softs di?use emission (possibly from hot ISM), associated with the spiral arms (from http://chandra.cfa.harvard.edu/photo/2003/1154/index.html).

1036 ergs s?1 (Pence, Snowden & Mukai 2001). The sources cluster along the spiral arms, and, interestingly, sources in the interarm regions tend to have X-ray colors compatible with AGNs. Twelve sources are spatially coincident with supernova remnants, but, based on their variability, two of them are identi?ed with XRBs. Eight other luminous sources exhibit variability in the Chandra data, and two more are found variable by comparison with previous ROSAT observations. Ten sources are supersoft, and a correlation between black-body temperature and total source lumi-

11.2 X-ray binary (XRB) Populations in Spiral Galaxies

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nosity is suggested by the data. The XLF of the M101 sources can be modelled with a power-law (cumulative slope -0.8) in the 1036 ? 1038 ergs s?1 range. In summary, with Chandra, X-ray source population studies are ?nally coming of age. The sub-arcsecond resolution of the Chandra mirrors (Van Speybroeck et al. 1997) allows both the separation of discrete sources from surrounding di?use emission and the detection of much fainter sources than previously possible. The XLFs of sources in a given system re?ect the formation, evolution, and physical properties of the X-ray source population. These di?erences are evident in di?erent regions of M31, M81 and M83. Comparison of the XLFs of nearby galaxies (and components thereof) with the XLFs of more distant systems provides a general coherent picture, pointing to steeper XLFs in older stellar populations (relative lack of very luminous sources). The XLFs of E and S0 galaxies have cumulative slopes in the range -1.0 to -2.0 (see Section 11.4.3), generally consistent with those of the bulges of M31 and M81. These slopes are signi?cantly steeper than those of sources associated with younger stellar ?elds in M31, M81, and M83. A recent study of 32 nearby galaxies extracted from the Chandra archive (Colbert et al. 2003) con?rms this basic di?erence between XLFs of old and younger stellar populations, ?nding cumulative slopes of ? 1.4 and ? 0.6 ? 0.8 for elliptical and spiral galaxies respectively. 11.2.4 XRBs in Actively Starforming Galaxies Observations show ?atter XLF slopes (i.e., an increased presence of very luminous sources) in galaxies with more intense star formation. The best example is given by the merger system NGC 4038/39 (The Antennae), where nine ultraluminous X-ray sources (ULXs; LX > 1039 ergs s?1 , for a distance of 19 Mpc) were discovered with Chandra (Fabbiano, Zezas & Murray 2001). Other examples of exceptionally luminous sources are found in M82 (Kaaret et al. 2001; Matsumoto et al. 2001), the Circinus galaxy (Smith & Wilson 2001; Bauer et al. 2001) and NGC 1365 X-1 (Komossa & Schultz 1998). Consequently, ?atter XLFs occur in galaxies with more intense star formation: the cumulative XLF slope is - 0.45 in The Antennae (Zezas & Fabbiano 2002; Kilgard et al. 2002; Fig. 11.6). Grimm, Gilfanov & Sunyaev (2003) suggest that the XLFs of star forming galaxies scale with the star formation rate (SFR), thus advocating that HMXBs may be used as a star formation indicator in galaxies. They ?nd that at high SFRs the total X-ray luminosity of a galaxy is linearly correlated to the SFR, and suggest a ‘universal’ XLF of starforming galaxies described by a power law with cumulative slope of ? ?0.6 and a cut-o? at LX ?few×1040 ergs s?1 . This result of course depends on how well is the SFR of a given galaxy known. This is a subject of considerable interest at this point, since various indicators are di?erently a?ected by extinction. The conclusion of a universal slope of the XLF of starforming galaxies may be at odd with the reported correlation between the XLF slope and the 60?m luminosity from a minisurvey of spiral and starburst galaxies observed with Chandra (Kilgard et al. 2002). Also, theoretical models (Kalogera et al. 2003) suggest that XLF slopes depend on the age of the starburst, so it is possible that the ‘universal’ XLF slope is not truly universal, but re?ects a selection bias, in that the sample used by Grimm, Gilfanov & Sunyaev (2003) may be dominated by starburtsts of similar ages. Comparison of the XLFs for di?erent galaxies, and modeling of the same, provide

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Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.6. Left: Chandra ACIS image of The Antennae (Fabbiano et al. 2001); Right: the XLF of The Antennae (points with error bars) compared with other galaxies, as labelled. Note the steep XLFs of the Galactic HMLXBs (bulge) and of the early-type galaxy NGC 4697 (Zezas & Fabbiano 2002).

powerful tools for understanding the nature of the X-ray sources and for relating them to the evolution of the parent galaxy and its stellar population. Early theoretical work has attempted to interpret the XLFs, using ad hoc power-law models, and accounting for aging and impulsive birth of XRB populations (Wu 2001, Kaaret 2002, Kilgard et al. 2002). Spurred by the recent observational developments, Kalogera and collaborators have developed the ?rst models of synthetic XLFs, based on XRB evolutionary calculations (Belczynski et al. 2003). Such models provide us with a potentially powerful tool for studying the origin and evolution of XRB populations in stellar systems and their connection to galactic environments. A preliminary examination of such models for starburst galaxies (Kalogera et al. 2003; see Fig. 11.7) successfully shows that predictions and consistency checks for the shapes and normalizations of XLFs are possible with theoretical XRB modeling. These new developments demonstrate that the predictions of 1995 are coming true (see Section 11.1).

11.3

Ultra Luminous X-ray Sources - ULXs

ULXs are also named super-Eddington sources (see Fabbiano 1989, 1995), super-luminous sources, and intermediate luminosity X-ray objects (IXOs) (Roberts & Warwick 2000; Colbert & Mushotzky 1999; Colbert & Ptak 2003). All these names aim to convey the fact that they are extremely luminous X-ray sources, emitting well in excess of the Eddington luminosity of a spherically accreting and emitting neutron star (? 2 × 1038 ergs s?1 ). Usually, sources emitting at ? 1039 ergs s?1 or above are included in this category. If these sources are emitting isotropically at the Eddington limit, masses in excess of those expected from stellar black holes are implied, up to in some cases, ≥ 100M⊙ (e.g. Fabbiano 1989, 1995; Makishima et al. 2000). Colbert & Mushotzky (1999) dubbed this type of black holes ‘intermediate mass black holes’ (IMBH), to distinguish them from the stellar mass black holes found in Galactic

11.3 Ultra Luminous X-ray Sources - ULXs
0.5

13

0

-0.5

-1

-1.5

-2 35 36 37 38 39 40

Fig. 11.7. Comparison of XRB population models (from Kalogera et al. 2003) with the observed XLF of NGC 1569 (bottom points, in black; data taken from Martin et al. 2002, ApJ, 574, 663). Models were constructed to match the star-formation history of NGC 1569 (recent star-burst duration and metallicity) and model XLFs are shown at di?erent times since the beginning of the starburst. Top to bottom: 10Myr (blue), 50Myr (yellow), 110Myr (red), 150Myr (cyan), 200Myr (green). Note that based on observations in other wavelengths, the age of the starburst is estimated to be 105-110Myr.

black hole binaries, and also from the supermassive 107 ? 109 M⊙ found at the nuclei of galaxies that are responsible for AGNs. 11.3.1 Spectra and spectral variability Although young supernova remnants may be responsible for ULX emission in some cases (e.g Fabian & Terlevich 1996), there is now su?cient evidence from spectral and variability data, to establish that the majority ULXs are indeed compact systems, most likely accreting binaries. ASCA X-ray spectra suggested accretion disk emission. These spectra, however, also require temperatures much larger than those expected from black holes of the mass implied by the luminosities of these sources, leading to the suggestion of rotating Kerr black holes (Makishima et al. 2000; Mizuno, Kubota & Makishima 2001). In The Antennae ULXs the Chandra spectra (Zezas et al. 2002a, b) tend to be hard, and their average co-added spectrum requires both a power law (Γ ? 1.2) and a disk-blackbody component consistent with the ASCA results, with kT? 1.1 keV. A XMM-Newton survey of 10 galaxies reports ULX spectra consistent with black hole binaries in either high or low state (Foschini et al. 2002), but the data quality is too poor for detailed modelling. Similar general

14

Compact Stellar X-ray Sources in Normal Galaxies

spectral results can be found in a Chandra survey of ULXs in di?erent galaxies (Humphrey et al. 2003). Instead, XMM-Newton high quality spectra of two ULXs in NGC 1313 (X-1 and X-2) led to highly signi?cant detections of soft accretion disk components, with temperatures of kT? 150 eV, consistent with accretion disks of IMBHs (Miller et al. 2003a; Fig. 11.8). The XRB hypothesis is reinforced by observations of correlated luminosity-spectral variability similar to the ‘high/soft-low/hard’ behavior of Cyg X-1 (e.g., in M81 X-9, La Parola et al. 2001, with a variety of X-ray telescopes, Fig. 11.9; and in two ULXs in IC 342, Kubota et al. 2001 with ASCA). However, more recently, Kubota, Done & Makishima (2002) argue that these power-law ULX spectra should not be identi?ed with the low/hard state, but rather may be due to a strongly Comptonized optically thick accretion disk, analogous to the Comptonization-dominated ‘very high/anomalous state’ in Galactic black-hole binaries. ASCA observations of one of the IC 342 sources in high state (disk-dominated) revealed a ‘high/hard-low/soft’ low-level variability, with a possible 30-40 hr periodicity, as could be produced by a massive main sequence star orbiting a black hole (Sugiho et al. 2001). With Chandra and XMM-Newton an increasing number of ULXs are being discovered and studied in galaxies. Variability in the Chandra observations of M82 established that the ULXs in this galaxy are likely to be accreting compact objects (Matsumoto et al. 2001). The Chandra observations of NGC 3628 (Strickland et al. 2001) show the re-appearance of the 1040 ergs s?1 variable ULX ?rst discovered with ROSAT (Dahlem, Heckman & Fabbiano 1995). A new transient ULX was discovered in M74 (NGC 628) with (Soria & Kong 2002). Chandra observations of MF 16 in NGC 6946, formerly identi?ed as an extremely luminous supernova remnant (Schlegel 1994), reveal instead a point-like source with the typical X-ray spectrum of a black-hole binary (Holt et al. 2003; Roberts & Colbert 2003). Similarly, M81 X-6, which is positionally coincident with a supernova remnant, is identi?ed as a XRB by its X-ray spectrum (Swartz et al. 2003). Chandra observations of the nucleus of M33 have revealed a two-component (power-law and disk) spectrum and have established luminosity-spectral variability patterns in this ULX, reminiscent of the black hole binary LMC X-3 (La Parola et al. 2003; see also Long, Charles & Dubus 2002); Dubus & Rutledge (2002) compare this source with the Galactic microquasar GRS 1915+105. High/hard-low/soft variability was found in M51 X-7, together with a possible 2.1 hr period (but the time coverage is scant) by Liu et al. (2002). Both Cyg X1 like high/soft-low/hard as well as high/hard-low/soft variability was detected in the population of nine ULXs discovered with Chandra in the Antennae galaxies (Fabbiano ; Fig. 11.10). The latter type of variability can also be found in a few Galactic XRBs (1E 1740.7-2942, GRS 1758-258, GX 339-4, Smith et al. 2002; see also the XMM-Newton results on GRS 1758-258, Miller et al. 2002). This spectral variability may be indicative of the competition between the relative dominance of the accretion disk versus the innermost hot accretion ?ow; several scenarios for spectral variability are discussed in Fabbiano et al. 2003a and references therein.

11.3 Ultra Luminous X-ray Sources - ULXs

15

Fig. 11.8. Bottom: the XMM-Newton image of NGC 1313, showing the position of the two ULXs. Top: X-ray spectrum of ULX-1, compared with best-?t model requiring a cool accretion disk component (Miller et al. 2003).

16

Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.9. Light-curve of M81 X-9, covering ? 20 yrs of observations (La Parola et al. 2001).

11.3.2 Intermediate Mass Black Holes or Beamed XRBs? Although there is clear evidence pointing towards an XRB nature for ULXs, the presence of IMBHs in these systems is by no means universally accepted, and it may be quite possible that ULXs are indeed a heterogeneous population. As discussed above, the ASCA spectra were interpreted by Makishima et al. (2000) as evidence for rotating Kerr IMBH, to reconcile the high accretion disk temperature suggested by the model ?tting of these spectra with the large black hole masses implied by the bolometric luminosity of the ULXs, which would require much cooler disks for a non-rotating IMBH. Colbert & Mushotzky (1999) suggested that these cooler accretion disk components may be present in their ASCA survey of ULXs, but the statistical signi?cance of these early claims is not very high. The Chandra detections of super-soft ULXs (e.g., Swartz et al. 2002, in M81; Di Stefano et al. 2003 in M104; see also later in this Section) could be interpreted as evidence for IMBHs. More important, low-temperature components vere discovered in the XMM-Newton spectra of ‘normal’ ULXs: in the NGC 1313 ULXs, which do not require a Kerr black hole, and are entirely consistent with emission from an IMBH accretion disk (Miller et al. 2003a; Fig. 11.8); and in at least one of the ULXs in the Antennae galaxies (kT?0.13 keV) (Miller et al. 2003b). Considerable attention has been devoted to an extremely luminous variable 1040 ergs s?1 ULX detected with Chandra near the dynamical center of M82. In the picture of spherical accretion onto an IMBH, the luminosity of this source would imply masses

11.3 Ultra Luminous X-ray Sources - ULXs

17

Fig. 11.10. Left: Chandra light curves of the ULXs of The Antennae. Right: color-color diagrams of the most luminous sources (Fabbiano et al. 2003a).

in excess of 100 M⊙ for the accretor. This ULX appears to be at the center of an expanding molecular superbubble with 200 pc diameter (Matsushita et al. 2000). Based on its accurate Chandra position, which is not at the nucleus, Kaaret et al. (2001) set an upper limit of 105 ? 106 M⊙ to its mass. Strohmayer & Mushotzky (2003) report quasi periodic oscillations (QPOs) in the XMM-Newton data of this source. They argue that their discovery suggests emission from an accretion disk and is incompatible with the radiation being beamed, and therefore implying a less extreme emitted luminosity, as in King et al. (2001; see below). On the assumption that the highest QPO frequency is associated with the Kepler frequency at the innermost circular orbit around a Schwarzschild black hole, these authors set an upper limit of 1.87×104M⊙ to the black hole mass: this source could therefore be an IMBH, with masses in the 100-10,100 M⊙ range. However, as noted by Strohmayer & Mushotzky (2003), the crowded M82 ?eld cannot be spatially resolved with XMMNewton, making the association of the QPO with the most luminous ULX in the ?eld not entirely proven. Moreover, the spectral ?t of these data suggests a temperature kT? 3 keV, much higher than the one expected from an IMBH accretion disk. As we will discuss below, some results are hard to explain in the IMBH scenario. Two other models have been advanced, which do not require IMBH masses. The

18

Compact Stellar X-ray Sources in Normal Galaxies

large number of ULXs found in The Antennae led to the suggestion that they may represent a normal stage of XRB evolution (King et al. 2001). In the King et al. (2001) model, the apparent (spherical) accretion luminosity is boosted because of geometrical collimation of the emitting area in thick accretion disks, resulting from the large thermal-timescale mass transfer characterizing the later stages of a massive XRB (see Chapter by King in this book). Exploiting the similarity with Galactic microquasars, the jet emission model of K¨rding et al. (2002) produces o enhanced luminosity via relativistic beaming. In at least one case, the variable luminous ULX 2E1400.2-4108 in NGC 5408, there is observational evidence pointing to this relativistic jet model: Kaaret et al. (2003) ?nd weak radio emission associated with the X-ray source, and argue that the both the multi-wavelength spectral energy distribution, and the X-ray spectrum are consistent with the K¨rding et al. (2002) o scenario. In some cases at least the IMBH hypothesis is supported by the association of the ULX with di?use Hα nebulae, suggesting isotropic illumination of the interstellar medium by the ULX, and therefore absence of beaming (e.g. Pakull & Mirioni 2002 in the case of the NGC 1313 sources, see Miller et al. 2003). M81 X-9 is also associated with an optical nebula, which also contains hot gas (La Parola et al. 2001; Wang 2002). Wang (2002) considers the possibility that this nebula may be powered by the ULX and also speculates that it may be the remnant of the formation of the ULX. Weaver et al. (2002) discuss a heavily absorbed ULX in the nuclear starburst of NGC 253; this source appears to photoionize the surrounding gas. Weaver et al. speculate that it may be an IMBH, perhaps connected with either the beginning or the end of AGN activity. However, in at least one case (IC 342 X-1, Roberts et al. 2003), there is a suggestion of anisotropic photoionization, that may indicate beamed emission from the ULX. In The Antennae, comparison with HST data shows that the ULXs are o?set from starforming stellar clusters. While coincidence with a stellar cluster may be due to happenstance because of the crowded ?elds, the absence of an optical counterpart is a solid result and suggests that the ULXs may have received kicks at their formation (Zezas & Fabbiano 2002), which would be highly unlikely in the case of a massive IMBH forming in a dense stallar cluster (e.g. Miller & Hamilton 2002). An alternate IMBH scenario, discussed by Zezas & Fabbiano, is that of primordial IMBHs drifting through stellar clusters after capturing a companion (Madau & Rees 2001). Other optical studies ?nd counterparts to ULXs, and set indirect constraints on the nature of the accretor. A blue optical continuum counterpart to the variable ULX NGC 5204 X-1 was found by Roberts et al. (2001), and subsequently resolved by Goad et al. (2002) with HST. These authors conclude that the stellar counterpart points to an early-type binary. Similarly, Liu, Bregman & Seizer (2002) ?nd an 08V star conterpart for M81 X-1, a ULX with average LX ? 2 × 1039 ergs s?1 . These counterparts may be consistent with the picture of King et al. (2001), of ULXs as XRBs experiencing thermal timescale mass transfer. Recent results on supersoft variable ULXs suggest that the emitting region may not be associated with the inner regions of IMBH accretion disks in these sources, but may be due to Eddington-driven out?ows from a stellar mass black hole. The spectral variability (at constant bolometric luminosity) of the soft ULX P098 in

11.4 XRBs in Elliptical and S0 Galaxies

19

M101 (detected with Chandra; Mukai et al. 2003) led to the suggestion of an optically thick out?ow from a 15-25 M⊙ black hole, regulated by the Eddington limit. Chandra time monitoring observations of The Antennae have led to the discovery of a variable super-soft source (kT = 90 - 100 eV for a blackbody spectrum), reaching ULX luminosities of 2.4×1040 ergs s?1 (Fabbiano et al. 2003b). The assumption of unbeamed emission would suggest a black hole of ≥ 100M⊙. However the radiating area would have to vary by a factor ? 1000 in this case, inconsistent with gravitational energy release from within a few Schwarzschild radii of a black hole. As discussed in (Fabbiano et al. 2003b), a surprising possible solution is a white dwarf with M ? 1M⊙ , at the Eddington limit, with a variable beaming factor (up to a beaming factor b ? 10?2 ). A second possible solution involves out?ows from a stellar–mass black hole, accreting near the Eddington limit (as in Mukai et al. 2002) but with mildly anisotropic radiation patterns (b ? 0.1, as in King et al. 2001). Similar sources are reported in M81 (Swartz et al. 2002), NGC 300 (Kong & Di Stefano 2003), and other nearby spiral galaxies (Di Stefano & Kong 2003; see also Di Stefano et al (2003) for SSSs in M31). Transient behavior has been shown to be an important observational diagnostic that could allow us to distinguish between beamed models and IMBH accretion for the origin of ULXs in young, star-forming regions (Henninger et al. 2003). Accretion onto IMBH black holes can lead to unstable disks and hence transient behavior whereas beamed binary systems have transfer rates that are high enough for the disks to be stable and X-ray emission to be persistent. Therefore long-term monitoring can prove a valuable and possibly unique tool in unraveling the nature of ULXs.

11.4

XRBs in Elliptical and S0 Galaxies

As discussed in the 1995 chapter (Fabbiano 1995), XRBs could not be directly detected in E and S0 galaxies with pre-Chandra telescopes, because of the distance of these galaxies and the limited angular resolution of the telescopes. The presence of XRBs in E and S0 galaxies was predicted by Trinchieri & Fabbiano (1985), based on an analogy with the bulge of M31, for which such a population could be detected (Van Speybroeck et al. 1979; see also Fabbiano, Trinchieri & Van Speybroeck 1987). This early claim was reinforced by di?erences in the average spectral properties of E and S0 galaxies with di?erent X-ray-to-optical luminosity ratios, that suggested a baseline X-ray faint XRB emission (Kim, Fabbiano & Trinchieri 1992; Fabbiano, Kim & Trinchieri 1994), and by the ASCA discovery of a hard spectral component in virtually all E and S0 galaxies (Matsushita et al. 1994), which, however, could also have been due, at least in part, to accreting massive nuclear black holes (Allen, Di Matteo & Fabian, 2000). The Chandra images (Fig. 11.11) leave no doubt about the presence of rich populations of point-like sources in E and S0 galaxies. Published results, of which the ?rst one is the paper on NGC 4697 by Sarazin, Irwin & Bregman (2000), include point-source detections in a number of galaxies. These source populations have been detected with varying low-luminosity detection thresholds (a function of galaxy distance and observing time). While most of the detected sources have luminosities in the 1037 ?1039 ergs s?1 range, some were detected at luminosities above 1039 ergs s?1 , in the Ultra-Luminous-X-ray (ULX) source range (see Section 11.3). A representa-

20

Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.11. Chandra ACIS image of the Virgo elliptical NGC 4365, using archival data. The white ellipse is the D25 isophote, from de Vaucouleurs et al. (1991).

tive summary (limited to papers published or in press as of May 2003) in given in Table 11.1. The X-ray colors or co-added spectra of these sources are consistent with those of LMXBs (see above references, and Irwin, Athey & Bregman 2003); however, a variety of spectral properties have been reported in some cases, similar to the spectral variety of Galactic and Local Group XRBs, including a few instances of very soft and supersoft (i. e., all photons below ? 1 keV) sources (e.g., NGC 4697, Sarazin, Irwin & Bregman 2000; M84, Finoguenov & Jones 2002; NGC 1399, Angelini, Loewenstein & Mushotzky 2001; NGC 1316, Kim & Fabbiano 2002). The overall spatial distribution of these sources follows that of the stellar light, but there are exceptions, such as in NGC 720, where the most luminous sources follow arcs (Jeltema et al. 2003), NGC 4261 and NGC 4697, where the X-ray source distributions are highly asymmetric (Zezas et al. 2003), and NGC 4472, where the X-ray source distribution

11.4 XRBs in Elliptical and S0 Galaxies

21

Table 11.1. E & S0 Galaxies: Representative Summary of Chandra Results
Name NGC 720 No. of sources 42 LX (ergs s?1 ) band (keV) 4 × 1038 ? 1 × 1040 0.3-7 Comment 9 ULX in ‘arc’ pattern 12 associations with GCs (Jeltema et al. 2003) 3 associations with GCs (Irwin et al.. 2002) kT?5 keV average spectrum 5 associations with GCs (Kim & Fabbiano 2003) 70% associated with GCs (Angelini et al.. 2001) X-ray colors consistent with NGC 4697 3 associations with GCs (Blanton et al.. 2001) spectra consistent with Galactic LMXB (Finoguenov & Jones 2002) 40% associated with CGs (Kundu et al.. 2002) average spectrum kT?8 keV 7 (20%) in GCs (Sarazin et al.. 2001) 9 identi?cations with GCs (Kraft et al. 2001) (Trinchieri & Goudfrooij 2002)

NGC 1291

?50

< 3 × 1038 0.3-10 2 × 1037 ? 2 × 1039 0.3-8

NGC 1316

81

NGC 1399

? 140

5 × 1037 ? 5 × 1039 0.3-10 1.6 × 1038 ? ? ×1040 0.3-10

NGC 1553

49

NGC 4374 (M84)

? 100

3 × 1037 ? ? 2 × 1039 0.4-10

NGC 4472

? 120

1 × 1037 ? ? 1.5 × 1039 0.5-8 5 × 1037 ? 2.5 × 1039 0.3-10

NGC 4697

?80

NGC 5128 (CenA) NGC 5846

246

2 × 1036 ? 1 × 1039 0.4-10 3 × 1038 ? 2 × 1039 0.3-10

? 40

may be more consistent with that of Globular Clusters (GCs) than of the general ?eld stellar light (Kundu, Maccarone & Zepf 2002; Maccarone, Kundu & Zepf 2003). No ?rm conclusion on the origin and evolution of these sources exists. Given the old stellar population of the parent galaxies, and the life-times of LMXBs, it has ben suggested that these sources may be outbursting transients (Piro & Bildsten 2002). Alternatively, more recent formation and evolutions in GCs may result in steady sources (Maccarone, Kundu & Zepf 2003). With the exception of NGC 5128, which is near enough to allow detection of sources in the 1036 ergs s?1 luminosity

22

Compact Stellar X-ray Sources in Normal Galaxies

range, and for which multiple observations demonstrate widespread source variability (Kraft et al. 2001), the Chandra observations performed so far typically only give a single snapshot of the most luminous part of the XRB population in a given galaxy. In NGC 5128, a comparison of the two Chandra observations reveals at least ?ve transients (sources that disappear with a dimming factor of at least 10), supporting the Piro & Bildsten scenario. Chandra observations of highly signi?cant asymmetries in the spatial distribution of X-ray sources in otherwise regular old elliptical galaxies (Zezas et al. 2003) may suggest rejuvenation of the stellar population of these galaxies. In NGC 4261, the most signi?cant example, all the detected sources are luminous, above the Eddington limit for a neutron star accretor. If the X-ray sources were standard LMXBs belonging to the dominant old stellar population, we would expect their spatial distribution to be consistent (within statistics) with that of the stellar light. However this is not so, as indicated by Kolmogorov-Smirnov tests and Bayesian block analysis. On the basis of simulations of galaxy interactions (Hernquist & Spergel 1992; Mihos & Hernquist 1996), this result suggests that the luminous XRBs may belong to a younger stellar component, related to the rejuvenating fall-back of material in tidal tails onto a relaxed merger remnants. 11.4.1 ULXs in Early-Type Galaxies As can be seen from Table 11.1, in early-type galaxies the occurrence of sources with LX = 1 ? 2 × 1039 ergs s?1 is common, although generally limited to a few sources per galaxy. These sources could easily be explained with normal black hole binaries or moderately beamed neutron star binaries (King 2002). In their mini-survey of 14 galaxies observed with Chandra (which include some of the ones listed in Table 11.1), Irwin, Athey & Bregman (2003) ?nd that of the four sources with X-ray luminosities in the 1 ? 2 × 1039 ergs s?1 range for which they can derive spectra, three have soft spectra, similar to those of black hole binaries in high state (see also Finoguenov & Jones 2002). Not much can be said about the variability of ULXs in early-type galaxies, because repeated Chandra observations of a given galaxy are not generally available. In the case of NGC 5128, comparison with previous ROSAT images (see Colbert and Ptak 2002) shows considerable ?ux variability in these very luminous sources: two ULXs were detected in ROSAT observations, both have considerable lower luminosities in the Chandra data (Kraft et al. 2001), and one of them may have disappeared. While, in general, sources with LX > 2 × 1039 ergs s?1 are relatively rare in earlytype galaxies as compared to actively star-forming galaxies (see Section 11.3), and may be preferentially associated with GCs (e.g. Angelini, Loewenstein & Mushotzky 2001; see Irwin, Athey & Bregman 2003), this is not always the case, as exempli?ed by NGC 720. This galaxy (Jeltema et al. 2003) is peculiar in possessing nine ULXs (this number is of course dependendent on the assumed distance, 35 Mpc), a population as rich as that of the actively starforming merger galaxies The Antennae (Fabbiano, Zezas & Murray 2001, Zezas & Fabbiano 2002). Only three of these ULXs can be associated with GCs. The sources in NGC 720 are also peculiar in their spatial distribution, which does not follow the distribution of the optical light, as it would be expected from LMXBs evolving from low-mass bulge binaries: these sources are

11.4 XRBs in Elliptical and S0 Galaxies

23

distributed in arcs. Their large number and their spatial distribution may suggest that they are younger systems, perhaps the remnants of a recent merger event. The associations of some ULXs in early-type galaxies with GCs may support the possibility that a subset of these sources may be associated with IMBH (> 10M⊙ ) (see Fabbiano 1989 and refs. therein; Irwin, Athey & Bregman 2003). However, most of the ULXs in early-type galaxies are likely to be lower mass binaries, given the stellar population of the parent galaxy. King (2002; see also Piro & Bildsten 2002) suggests that they may be a class of ULXs associated with outbursts of soft X-ray transients, resulting in moderately beamed emission from the inner regions of a thick accretion disk. In the case of NGC 720 they may be related to a ‘hidden’ younger stellar population (Jeltema et al. 2003).

11.4.2 X-ray sources and Globular Clusters The association of X-ray sources in early type-galaxies with GCs has been widely discussed. As can be seen from Table 11.1, associations with GCs range from ≤ 10% in most galaxies, ? 40% in some Virgo galaxies (NGC 4472, NGC 4649), to 70% in NGC 1399, the dominant galaxy in a group . The statistics are somewhat fraught with uncertainty, since lists of GCs from HST are not available for all the galaxies studied with Chandra, and the detection thresholds di?er in di?erent galaxies. However, this association is interesting and has led to the suggestion that perhaps all the LMXBs in early-type galaxies may form in GCs, from whence they may be expelled if they receive strong enough kicks at their formation, or may be left behind if the GC is tidally disrupted. This suggestion was ?rst advanced by Sarazin, Irwin & Bregman (2000), and was more recently elaborated by White, Sarazin & Kulkarni (2002), on the basis of a correlation of the speci?c GC frequency with the ratio of the integrated LMXB luminosity to the optical luminosity of eleven galaxies. Kundu, Maccarone & Zepf (2002) explore the LMXB-GC connection in NGC 4472, where they ?nd that 40% of the sources detected at LX > 1 × 1037 ergs s?1 are associated with GCs. In this galaxy, the fraction of GCs hosting an X-ray source is 4%, the same as in the Galaxy and M31. More luminous, more metal rich, and more centrally located GCs are more likely to host LMXBs, re?ecting both an increased probability of binary formation with the numbers of stars in a GC, and also an e?ect of metallicity in aiding binary formation (Kundu, Maccarone & Zepf 2002). While the possibility of LMXB formation in GCs is intriguing, this is still an open question, since evolution of bulge stars may also produce LMXBs (e.g., Kalogera & Webbink 1998; Kalogera 1998). The spatial distribution of the LMXBs, if it follows the optical stellar light (e.g. in NGC 1316, Kim & Fabbiano 2003; Fig. 11.12), would be consistent with this hypothesis. However, in NGC4472 at least, no di?erences are found in the distributions of X-ray luminosities of the GC sources and the other LMXBs (Maccarone, Kundu & Zepf 2003). Moreover, in the inner bulge of M31, at radii that even with Chandra cannot be explored in elliptical galaxies because of their distances, the distribution of LMXBs appears more peaked than that of the optical light (Kaaret 2002).

24

Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.12. Radial distributions of the emission components of NGC 1316. The gaseous component (hot ISM) is represented by the inner distribution of points. The cumulative XRB contribution is given by the outer set of points; the dashed line through these points is the extrapolation of the stellar (I) surface brightness (Kim & Fabbiano 2003).

11.4.3 X-ray Luminosity Functions The XLFs of the early-type galaxies observed with Chandra are generally steeper than those of star-forming galaxies (see Section 11.2.4), i.e. with a relative lack of luminous HMXBs. These XLFs are generally well ?tted with power-laws or broken power laws with (cumulative) slopes ranging from -1.0 to -1.8, and breaks have been reported both at 2-3×1038 ergs s?1 , the Eddington luminosity of an accreting neutron star (Sarazin, Irwin & Bregman 2000; Blanton, Sarazin & Irwin 2001; Finoguenov & Jones 2002; Kundu, Maccarone & Zepf 2002), and at higher luminosities (1039 ergs s?1 ) (Jeltema et al. 2003, in NGC 720). While the former break may be related to a transition between neutron star and black hole binaries (Sarazin, Irwin & Bregman 2000), the latter, high luminosity break, could be produced by a decaying (aging) starburst component from binaries formed in past merging and star bursting episodes (Wu 2001). This possibility was suggested in the case of NGC 720 (Jeltema et al. 2003). The XLFs of NGC 5128 (Kraft et al. 2001), obtained at di?erent times and re?ecting source variability, are well ?tted with single power-laws

11.5 Multi-wavelength Correlations

25

in the luminosity range of 1037 ? 1039 ergs s?1 . In NGC 1291 (Irwin, Sarazin & Bregman 2002), no super-Eddington sources are detected. The e?ects of detection incompleteness have been considered by Finoguenov & Jones (2002), and have been recently explored extensively by Kim & Fabbiano (2003) in their derivation of the XLF of NGC 1316. Low-luminosity sources may be missed because of higher background/di?use emission levels in the inner parts of galaxies, and also because of the widening of the Chandra beam at larger radii. Correcting for these e?ects with an extensive set of simulations, Kim & Fabbiano (2003) found that an apparent 2-3×1038 ergs s?1 break in the XLF of NGC 1316 disappeared when incompleteness was taken into account, and the XLF of this galaxy could be represented by an unbroken power-law down to luminosities of ? 3 × 1037 ergs s?1 (Fig. 11.13). This result shows that caution must be exercised in the derivation of XLFs, and that perhaps some of the previous reports should be reconsidered. If the XLFs extend unbroken to lower luminosities, the amount of X-ray emission from undetected LMXBs in early-type galaxies can be sizeable, as it is the case in NGC 1316. This result is important not only for our understanding of the XRB populations, but also for the derivations of the parameters of the hot interstellar medium in these system (see Kim & Fabbiano 2003). Ignoring the contribution to the emission of hidden XRBs results in biases and erroneous results and may give the wrong picture of the overall galaxy dynamics and evolution. Moreover, the dominance at large radii of XRB emission over the hot ISM (see Fig. 11.12) in some (X-ray faint) ellipticals, does also a?ect adversely mass measurements of these galaxies from low-resolution X-ray data (Kim & Fabbiano 2003).

11.5

Multi-wavelength Correlations

Although this chapter is focussed on the XRB populations that we can now resolve and study with Chandra in galaxies as distant as ? 20 Mpc, the study of the integrated emission properties of samples of galaxies (either more distant, or observed at lower resolution) can also give useful information on the average properties of their XRB components. We will summarize here some of these studies, that were pursued mostly by using the samples of galaxies observed with Einstein and ROSAT. Most of the early work in this area was done by Fabbiano and collaborators, using the ?rst sample of galaxies ever observed in X-rays, the Einstein sample (see reviews in Fabbiano 1989; 1995). Besides suggesting the baseline XRB emission in E and S0 galaxy, that is now con?rmed with Chandra (Section 11.4), these results suggested a general scaling of the integrated X-ray emission with the optical luminosity (and therefore stellar population) of the galaxies, and pointed to a strong association of the XRB populations of disk/arm-dominated spirals with the far-IR emission, i.e. the younger component of the stellar population (e.g., Fabbiano, Gioia & Trinchieri 1988; see also David, Jones and Forman 1992). More recent work on the Einstein sample (Shapley, Fabbiano & Eskridge 2001; Fabbiano & Shapley 2002?), on ROSATobserved galaxies (Read & Ponman 2001), and on Beppo-SAX and ASCA data (Ramalli, Comastri & Setti 2003) has examined some of these correlations afresh. Given the di?erent pass-bands of these observatories, these studies have a varied
?

probably the last paper to be published on the Einstein data

26

Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.13. Observed (empty squares) and corrected (?lled points) XLFs of NGC 1316 (Kim & Fabbiano (2003).

sensitivity to the e?ect of hard XRB emission and soft hot ISM emission in the galaxies. The Einstein sample is the largest, consisting of 234 S0/a-Irr galaxies observed in the 0.2-4. keV band. The X-ray luminosities are compared with B, H, 12 ?m, 60 ?m, 100 ?m, global FIR, and 6 cm luminosities (Shapley, Fabbiano & Eskridge 2001; Fabbiano & Shapley 2002). Both ?uxes and upper limits were used in this work, to avoid obvious selection biases. This work provides baseline distributions of LX and of LX /LB for the entire Hubble sequence (including E and S0 galaxies), and a critical compilation of distances for the sample. Multi-variable correlation analysis shows clear dependencies of the emission properties on the morphological type of the galaxies (and therefore indirectly on the stellar population and star formation activity). In Sc-Irr galaxies, all the emission properties (including the X-rays) are tightly correlated, suggesting a strong connection to the stellar population. This is not true for S0/a-Sab, where there is a general connection of the X-ray luminosity with the B and H-band emission (stellar population), but not with either radio or FIR. In Sc-Irr galaxies the strongest link of the X-ray emission is a linear correlation with the FIR,

11.6 The X-ray Evolution of Galaxies

27

suggesting a connection with the star-forming stellar component. This conclusion is reinforced by a correlation between LX /LB and L60 /L100 , which associates more intense X-ray emission with hotter IR colors. The X-ray emission / star-formation connection is also discussed as a result of the analysis of a small sample (17 nearby spirals) observed with ROSAT in a softer energy band (0.1-2.0 keV; Read & Ponman 2001), and more recently from the analysis of another small sample (also 17 galaxies) observed in the 2-10 keV band (Ranalli, Comastri & Setti 2003). The advantage of this harder band is that the emission is predominantly due to the XRB population (if the sample does not include AGNs). These authors suggest that the hard X-ray emission can be used as a clean indicator of star formation, because extinction is not a problem at these energies. These correlation analyses are now being extended to the XRB populations detected with Chandra. Colbert et al. (2003) report good correlations between the total point source X-ray luminosity in a sample of 32 galaxies of di?erent morphological type extracted from the Chandra archive and the stellar luminosity (both B and K bands). While correlations are still present in the spiral and merger/irregular galaxies with FIR and UV luminosities, the ellipticals do not follow this trend and show a clear lack of FIR and UV emission, consistent with their older stellar populations. This results is consistent with the conclusions of Fabbiano & Shapley (2002; see above), which were however based on the analysis of the integrated x-ray luminosity of bulge dominated and disk/arm dominated spiral and irregular galaxies. In summary, there is a correlation between X-ray emission and SFR in star-forming galaxies, that may lead to a new indicator of the SFR. However, one has to exercise caution, because this conclusion is only true for star-forming galaxies. In old stellar systems (bulges, gas-poor E and S0s), the X-ray emission is connected with the older stellar population of these systems. This conclusion is also in agreement with the recent studies of XLFs (Section 11.2; Section 11.4.3)

11.6

The X-ray Evolution of Galaxies

X-ray images of the extragalactic sky routinely taken with Chandra and XMM-Newton do not typically detect normal galaxies as serendipitous sources in the ?eld. Instead the images reveal a relativity sparse population of point sources, the majority of which are Active Galatic Nuclei (AGN) with a space density of order a thousand per square degree. Normal galaxies are not detected because the X-ray luminosity of normal galaxies is relatively low and the predicted ?uxes very faint. However, in the deepest few million second or more exposures made with Chandra (the Chandra Deep Fields – CDFs; Giacconi et al. 2002, Alexander et al. 2003) faint X-ray emission has been detected from optically bright galaxies at redshifts of 0.1 to 0.5 (Hornschemeier et al. 2001). These are amongst the faintest Xray sources in the CDF, with ?uxes of ? 10?16 erg cm?2 s?1 , corresponding to a luminosity of 1039 to 1041 erg s?1 – the range seen from nearby galaxies (e.g. see Shapley, Fabbiano & Eskridge 2001). Some of these might be galaxies containing a low luminosity AGN, but most are likely to be part of an emerging population of normal galaxies at faint X-ray ?uxes. The detection sensitivity of Chandra can be increased by ‘stacking’ analysis, i. e. by ‘stacking’ sub-images centered on the positions of galaxies in comparable

28

Compact Stellar X-ray Sources in Normal Galaxies

redshift ranges. This can push the threshold of Chandra to ? 10?18 erg cm?2 s?1 – equivalent to an e?ective exposure time of several months or more. Brandt et al. (2001) used this technique for 24 Lyman Break galaxies at z ? 3 in the Hubble Deep Field North (Steidel et al. 1996) and detected a signal with an average luminosity of 3 × 1041 erg s?1 – similar to that of nearby starburst galaxies. Nandra et al. (2002) con?rmed this result by increasing the number of Lyman Break galaxies to 144 and then extended it to also include 95 Balmer Break galaxies at z ? 1. The Balmer Break galaxies were detected with a lower average luminosity of 7 × 1040 erg s?1 , but a similar X-ray to optical luminosity ratio as the Lyman Break galaxies. Hornschemeier et al. (2002) report ‘stacking’ detections of optically luminous spiral galaxies at 0.4 < z < 1.5. These Chandra X-ray Observatory detections of normal galaxies at high redshifts have initiated the study of the X-ray evolution of normal galaxies over cosmologically interesting distances. Evolution of the X-ray properties of galaxies is to be expected because the star formation rate (SFR) of the universe was at least a factor of 10 higher at redshifts of 1–3 (Madau et al. 1996). Since the X-ray luminosity of galaxies scales with the infra-red and optical luminosity (see Section 11.5; Fabbiano, Gioia & Trinchieri 1988; David, Jones and Forman 1992; Shapley, Fabbiano & Eskridge 2001; Fabbiano & Shapley 2002) the increased star formation will have a corresponding impact on the X-ray properties of galaxies at high redshift (White and Ghosh 1998). For spiral galaxies without an AGN, the overall X-ray luminosity in the 1–10 keV band will typically be dominated by the galaxy’s X-ray binary population. There is expected to be a corresponding increase in the number of high mass X-ray binaries associated with the increased star formation rate. The ‘detection’ of the Balmer and Lyman Break galaxies by Nanda et al. (2001) and the factor of 5 increase in the X-ray luminosity from redshift 1 to 3 is consistent with an increasing star formation rate. Nandra et al. (2001) point out that the X-ray luminosity of galaxies provides a new ‘dust free’ method to estimate the star formation rate, as also pointed in the Beppo-Sax study of Ranalli, Comastri & Setti (2003), and by Grimm, Gilfanov & Sunyaev (2003). The low mass X-ray binary (LMXB) population created by the burst in star formation at z > 1 may not emerge as bright X-ray sources until several billion years later (White and Ghosh 1998; Ghosh and White 2001). This is due to the fact that the evolutionary timescales of LMXBs, their progenitors, and their descendants are thought be signi?cant fractions of the time-interval between the SFR peak and the present epoch. In addition to an enhancement near the peak (z ≈ 1.5) of the SFR due to the prompt turn-on of the relatively short-lived massive X-ray binaries, there may be a second enhancement, by up to a factor ? 10, at a redshift between ? 0.5 and ? 1 due to the delayed turn-on of the LMXB population (Ghosh and White 2001). This second enhancement will not be associated with an overall increase in the optical or infrared luminosity of the galaxy, resulting in an increase in the Xray to optical luminosity ratio. Hornschemeier et al. (2001) using the ‘stacking’ technique detected X-ray emission from L? redshift 0.4 to 1.5 spiral galaxies in the HDF-N. The X-ray to optical luminosity ratios are consistent with those of galaxies in the local universe (e.g., Shapley, Fabbiano & Eskridge 2001), although the data indicate a possible increase in this ratio by a factor of 2–3.

11.7 Conclusions

29

Ptak et al. (2001) discuss the observable consequences of the increased SFR at high redshifts for the X-ray detection of galaxies at redshift > 1 in the HDF-N. To do this Ptak et al. (2001) used the Ghosh and White (2001) models for the evolution of the underlying X-ray binary populations for several di?erent possible SFR models (the SFR with redshift is not well known). Depending on the SFR model used, the average X-ray luminosity of galaxies in the HDF-N can be an order of magnitude higher than in the local universe. These model predictions can be translated into a prediction of the number counts verses ?ux. Fig. 11.14 taken from Hornschemeier et al. (2003) shows the number counts from the CDF-N (which are dominated by AGN), along with the predictions from Ptak et al. (2001) for two di?erent SFR models. The emerging population of optically bright, X-ray faint (OBXF) galaxies detected in the CDF-N is also shown, along with the extension of the source counts to fainter ?uxes using a ?uctuation analysis of the CDF-N (Miyaji & Gri?ths 2002) . The predictions are that emission from normal galaxies, largely at redshift of 1–3, will start to dominate the source counts somewhere between ?uxes of 10?17 and 10?18 erg cm?2 s?1 . The cross on Fig. 11.14 shows the constraint from the stacking analysis of Hornschemeier et al. (2002) for relatively nearby spiral galaxies (z<1.5), which is in agreement with the predictions from Ptak et al. (2001) for the lower SFR models. Much deeper Chandra exposures of several months or even a year long will be able to eventually reach ?uxes of 10?18 erg cm?2 s?1 and directly test the models for the X-ray evolution of galaxies – given the projected long lifetime of Chandra and good luck, these very deep exposures will hopefully will eventually happen as the mission matures. To obtain spectra of these galaxies, which are typically at a redshift of 1–3, and to see higher redshift objects at a similar faint ?ux level, will require 100-1000 times more collecting area with 1 arc sec angular resolution to avoid confusion (e.g., Fabbiano 1995, 2000; Elvis & Fabbiano 1997; Fabbiano & Kessler 2001). Even more challenging, to resolve an AGN or an o?set ULX from more extended emission from the galaxy will require an angular resolution of order 0.1 arc sec. Such mission parameters are technologically extremely challenging, but nonetheless are being pursued by NASA, ESA and ISAS as a long term goal for X-ray astronomy (Parmar et al. 2002, Zhang et al. 2002).

11.7

Conclusions

As we have shown in this review, X-ray studies of galaxies are now yielding copious information on the properties of their XRB populations. The classi?cation and study of these di?erent populations is providing a unique tool for understanding the origin and evolution of XRBs, and for relating these sources to the evolution of the stellar populations of the parent galaxies, both in the nearby and the far-away universe. This work would not have happened without the vision of Riccardo Giacconi, who pushed forward the high resolution X-ray telescope concept, and the work of Leon Van Speybroeck, who designed the Chandra optics. We thank the colleagues that have provided ?gures and comments (Martin Elvis, Albert Kong, Ann Hornschemeier, Vicky Kalogera, Jon Miller, Doug Swartz, Andreas Zezas, Harvey Tananbaum, Phil Kaaret, Je? McClintock, Andrew King). This work bene?tted by the

30

Compact Stellar X-ray Sources in Normal Galaxies

Fig. 11.14. CDF-N number counts, with predictions (ar the faint end) based on di?erent SRF at high redshift. The open boxes are the counts from the optically bright, X-ray faint sources - these are mainly normal and starburst galaxies, but some low-luminosity AGN may be present -. The cross is the result of the ‘stacking’ analysis using z ≤ 1.4 galaxies in the CDF-N ?eld. The solid and dashed lines at the lowest ?uxes are the the predictions of the galaxy number counts from Ptak et al 2001 (from Hornschemeier et al 2003). The leftward pointing arrow indicates the number density of ?eld galaxies at I = 24 mag.

Aspen Summer Workshop on Compact X-ray Sources (Summer 2002), and would not have been completed without the relentless prodding of Walter Lewin, to whom we are indebted. We aknowledge partial support from the Chandra X-ray Center under NASA contract NAS 8-39073.

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Van Speybroeck, L., Jerius D., Edgar, R. J., Gaetz, T. J., Zhao, P. & Reid, P. B.1997, Proc. SPIE 3113, 89 Shirey, R., et al. 2001, A&A, 365, L195 Supper, R., Hasinger, G., Pietsch, W., Truemper, J., Jain, A., Magnier, E. A., Lewin, W. H. G. and van Paradijs, J. 1997, A&A, 317, 328 Wang, Q. D. 2002, MNRAS, 332, 764 Weaver, K. A., Heckman, T. M., Strickland, D. K. & Dahlem, M. 2002, ApJ, 576, L19 Weisskopf, M., Tananbaum, H., Van Speybroeck, L. & O’Dell, S. 2000, Proc. SPIE 4012 (astro-ph 0004127) White, N.E. & Ghosh, P., 1998, ApJ, 504, L31 White, R. E., III, Sarazin, C. L. & Kulkarni, S. R. 2002, ApJ, 571, L23 Williams, B. F., Garcia, M. R., Kong, A. K. H., Primini, F. A., King, A. R., and Murray, S. S. 2003, ApJ, submitted (astro-ph/0306421) Wu, K. 2001, Pub. Astron. Soc. Australia, 18, 443 Zezas, A. & Fabbiano, G. 2002, ApJ, 577, 726 Zezas, A., Fabbiano, G., Rots, A. H. & Murray, S. S. 2002a, ApJ Suppl., 142, 239 Zezas, A., Fabbiano, G., Rots, A. H. & Murray, S. S. 2002b, ApJ, 577, 710 Zezas, A., Hernquist, L., Fabbiano, G. & Miller, J. 2003, ApJL, submitted Zhang, W.; Petre, R., & White, N.E. 2001, X-ray Astronomy 2000, ASP Conference Proceeding Vol. 234. Edited by Riccardo Giacconi, Salvatore Serio, and Luigi Stella. San Francisco: Astronomical Society of the Paci?c.



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