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Detection of TeV Gamma-Rays from extended sources with Milagro

22nd Texas Symposium on Relativistic Astrophysics, Stanford University, December 13-17, 2004


Detection of TeV Gamma-Rays from extended sources with Milagro
P. M. Saz Parkinson for the Milagro Collaboration?
Santa Cruz Institute for Particle Physics, University of California, 1156 High Street, Santa Cruz, CA 95064 The Milagro gamma-ray observatory employs a water Cherenkov detector to observe extensive air showers produced by high-energy particles impacting in the Earth’s atmosphere. A 4800 m2 pond instrumented with 723 8” PMTs detects Cherenkov light produced by secondary air-shower particles. An array of 175 4000 liter water tanks surrounding the central pond detector was recently added, extending the physical area of the Milagro observatory to 40,000 m2 and substantially increasing the sensitivity of the detector. Because of its wide ?eld of view and high duty cycle, Milagro is ideal for monitoring the northern sky almost continuously (>90% duty cycle) in the 100 GeV to 100 TeV energy range. Here we discuss the ?rst detection of TeV gamma-rays from the inner Galactic plane region. We also report the detection of an extended TeV source coincident with the EGRET source 3EG J0520+2556, as well as the observation of extended TeV emission from the Cygnus region of the Galactic plane.

arXiv:astro-ph/0503244v1 10 Mar 2005



Milagro is a TeV gamma-ray detector which uses the water Cherenkov technique to detect extensive airshowers produced by very high energy (VHE, > 100 GeV) gamma rays as they interact with the Earth’s atmosphere. Milagro is located in the Jemez Mountains of northern New Mexico, at an altitude of 2630 m, has a ?eld of view of ?2 sr and a duty cycle greater than 90%. The e?ective area of Milagro is a function of zenith angle and ranges from ? 10m2 at 100 GeV to ? 105 m2 at 10 TeV. A sparse array of 175 4000 liter water tanks, each containing an individual PMT, was recently added. These additional detectors, known as “outriggers” (Figure 1), extend the physical area of Milagro to 40,000 m2 , substantially increasing the sensitivity of the instrument and lowering the energy threshold. The angular resolution is approximately 0.75 degrees without the outriggers and 0.45 degrees with them.

FIG. 1: One of 175 outriggers (recently added to Milagro).



A full survey of the northern hemisphere (declination of 1.1–80?) for point sources was carried out [3]. Figure 2 shows the map of the northern hemisphere in TeV gamma rays for the Milagro data set between 15 December 2000 and 25 November 2003. Table I lists all point sources with an excess greater than 4σ.

The last column represents the 95% con?dence upper limit on the ?ux, in units of the Crab. The Crab and Mkn 421 are both clearly visible in the map, and are listed in the sixth and seventh row, respectively, of Table I. A square bin of 2.1? in declination (δ) and [2.1/cos(δ)]? in right ascension was used in this analysis. To detect other possible sources of VHE emission it is necessary to survey the sky at many di?erent timescales and at many di?erent bin sizes. In addition to searching for steady point sources of TeV gamma rays, Milagro has searched for short bursts of TeV gamma rays [2], for TeV emission from the direction of satellite detected GRBs [9], and for extended sources of TeV emission [11], as we summarize in this paper.

? R.

Atkins, W. Benbow, D. Berley, E. Blaufuss, D. G. Coyne, T. DeYoung, B. L. Dingus, D. E. Dorfan, R. W. Ellsworth, L. Fleysher, R. Fleysher, M. M. Gonzalez, J. A. Goodman, T. J. Haines, E. Hays, C. M. Ho?man, L. A. Kelley, C. P. Lansdell, J. T. Linnemann, J. E. McEnery, A. I. Mincer, M. F. Morales, P. Nemethy, D. Noyes, J. M. Ryan, F. W. Samuelson, P. M. Saz Parkinson, A. Shoup, G. Sinnis, A. J. Smith, G. W. Sullivan, D. A. Williams, M. E. Wilson, X. W. Xu and G. B. Yodh



Di?use emission from the Galactic plane is the dominant source in the gamma-ray sky [7]. Most of the di?use VHE emission from the Galactic plane is


22nd Texas Symposium on Relativistic Astrophysics, Stanford University, December 13-17, 2004

FIG. 2: Northern hemisphere as seen in TeV gamma rays. At each point, the excess is summed over a 2.1 by [2.1/ cos(δ)]? bin, and the signi?cance of the excess in standard deviations is shown by the color scale (Figure from [3]).

TABLE I: Locations of All Regions with an Excess Greater than 4σ R.A. 0.3 37.8 43.6 49.1 79.9 83.6 166.5 306.6 313.0 339.1 356.4 Decl. 34.3 6.7 4.8 22.5 26.8 22.0 38.6 38.9 32.2 72.5 29.5 ON 3.12308e+06 7.02166e+05 5.85952e+05 2.21431e+06 2.57841e+06 2.17188e+06 3.23552e+06 3.25329e+06 3.08380e+06 6.63534e+05 2.98656e+06 OFF Excess σ UL 3.11456e+06 8623 4.7 0.84 6.98667e+05 3498 4.0 1.8 5.82716e+05 3236 4.1 2.0 2.20813e+06 6175 4.0 0.87 2.57025e+06 8161 4.9 0.97 2.16222e+06 9665 6.3 NA 3.22467e+06 10850 5.8 NA 3.24531e+06 7983 4.2 0.78 3.07548e+06 8320 4.5 0.85 6.59727e+05 3807 4.2 3.02 2.97910e+06 7455 4.1 0.84

thought to be produced by the interaction of cosmicray hadrons with the interstellar matter. The ?ux measured by EGRET below 1 GeV ?ts models well, but that measured between 1 and 40 GeV is signi?cantly larger than what is predicted by most models. One possible explanation for this enhanced emission is the inverse-Compton scattering of cosmic-ray electrons [8]. If this turns out to be the dominant source of di?use gamma-ray emission from the Galactic plane, then the ?ux at TeV energies could be an order of magnitude higher than previously thought. Figure 3 shows the EGRET observations of the Galactic plane. The top panel shows the all-sky map, in Galactic coordinates, produced by EGRET, clearly showing the

Galactic plane. The bottom panel shows the EGRET di?use GeV ?ux (in black) along with the Milagro exposure (in red), indicating that the Milagro observation region is optimized for the detection of the diffuse emission predicted by the EGRET measurements. Using 36 months of data, from 19 July 2000 to 18 July 2003, we looked at the inner (40-100 degrees) and outer (140-220 degrees) regions of the Galaxy [6]. While the outer Galaxy shows no signi?cant excess, the inner Galaxy shows a 5 sigma excess [6]. Figure 4 shows the pro?le in latitude for the longitude band (40 to 100 degrees) of the inner Galactic region (left panel) and the pro?le in longitude for the latitude band (-5 to 5 degrees) of the inner Galactic region (right panel), where the enhancement can be seen just north of the equator. Figure 5 shows the signi?cance map of the Galaxy. The region of the inner Galaxy shows an enhancement along and just north of the Galactic equator. This is the same region where EGRET detected the strongest signal in the 100 MeV energy range. The 5 sigma excess is seen by summing the entire inner Galaxy with a +/- 5 degree latitude band, as was suggested by the EGRET results. We note that the Milagro observation of the Galactic plane remains signi?cant even when the region around the Cygnus Arm is excluded. This constitutes the ?rst detection of the Galactic plane at TeV energies.



A search for extended emission was carried out for the Milagro data collected between 17 August 2000

22nd Texas Symposium on Relativistic Astrophysics, Stanford University, December 13-17, 2004 though the results are highly correlated. Monte Carlo simulations were used to compute the post-trials probability for each source candidate.



3EG J0520+2556

The most signi?cant candidate found in our search of extended sources had a pre-trials signi?cance of 5.9 sigma, located at RA=79.8 degrees and Dec=26.0 degrees and was identi?ed using a 2.9 degree bin size. The probability of observing an excess this signi?cant at any point in the sky at any bin size is 0.8%. Figure 6 (left panel) shows the map of signi?cances around the source, which is located ?5.5 degrees from the Crab. This candidate was ?rst reported in 2002 [10]. The cumulative signi?cance using only data since it was ?rst reported is 3.7σ. The right panel of Figure 6 shows the accumulation of signi?cance with time, indicating that the excesss increases at a constant rate and shows no periods of signi?cant ?aring. This candidate is coincident with the EGRET unidenti?ed source 3EG J0520+2556 (See Figure 7).

B. FIG. 3: Top – EGRET all-sky map showing Galactic plane emission above 100 MeV. Bottom – EGRET diffuse GeV ?ux (in black) along with the Milagro exposure (in red).

The Cygnus Arm

and 5 May 2004 [11]. A set of standard cuts has been developed by the Milagro collaboration and validated by observations of the Crab [1] and Mkn 421 [3]. Events with 20 or more PMT hits used by the angle ?tter (NFIT ≥ 20) are kept (this rejects about 20% of events which have poor ?ts). In addition, we cut on a parameter known as “compactness” [3] (X2≥2.5) to retain 50% of the gammas while removing more than 90% of the background protons. The excess at each position is determined by counting the number of events in a particular bin and subtracting the estimated background. The background is computed from data collected at the same local detector coordinates, but at a di?erent time, ensuring the celestial angles of the background event sample do not overlap with the source position under consideration. The method of [5] is used to compute the signi?cance of each excess. While the optimal square bin for detection of point sources with Milagro is 2.1 degrees on each side [3], to look for di?use sources, the standard Milagro sky maps were searched using a range of bin sizes from 2.1 to 5.9 degrees in steps of 0.2. 20 separate searches were performed on the same maps,

The second extended source candidate is coincident with the region known as the Cygnus Arm, a spiral arm within our Galaxy that extends radially away from observers on Earth. This region is known to be a dense region of gas and dust and was observed by EGRET as the brightest source of GeV gamma rays in the northern sky, with a di?use GeV emission comparable to the Galactic bulge. Like the emission from the Galactic plane region, VHE emission from the Cygnus Arm is thought to originate mainly from interactions of cosmic rays with the interstellar gas and dust. A 5.5 sigma excess was detected using a 5.9 degree bin, at RA=308 degrees and Dec=42 degrees. The probability of observing an excess this signi?cant at any point in the sky at any bin size is 2.0%. Figure 8 shows the signi?cance of the region of the sky containing the Cygnus arm, showing also the Galactic plane from l=20 degrees to l=100 degrees and b=+/- 5 degrees superimposed on the plot. The excess observed by Milagro is inconsistent with a point source and the number of events coming from the entire 5.9? bin is approximately twice that of the Crab. Like in the case of 3EG J0520+2556, the accumulation of the excess is steady, and no evidence for ?aring is observed. While this is an extremely bright region, making it the hottest spot in the Galactic plane, it is not surprising that it has not been detected yet by any of the Atmospheric Cerenkov Telescopes, given the diffuse nature of the source, and the limited ?eld of view of such telescopes.


22nd Texas Symposium on Relativistic Astrophysics, Stanford University, December 13-17, 2004

FIG. 4: Left – Pro?le of the fractional excess as a function of Galactic latitude ( for a Galactic longitude between 40 and 100 degrees). Right – Pro?le of the fractional excess as a function of Galactic longitude (for a Galactic latitude between -5 and 5 degrees). The EGRET longitudinal source shape is superposed. (Figures from [4])

FIG. 5: Map in Galactic coordinates of Milagro signi?cances in 5? by 5? bins. The light area has no Milagro exposure (Figure from [4]).




Milagro has detected three previously unknown diffuse sources of TeV gamma rays. Milagro has reported the ?rst observation of di?use TeV emission from the Galactic plane. In addition to this, two other diffuse sources have been detected. The ?rst, coincident with 3EG J0520+2556, and the second coincident with the Cygnus Arm, at 5.9 and 5.5 sigma respectively. When all trials of the all-sky search are considered, the probabilities of these excesses being due to background ?uctuations are 0.8% and 2.0% respectively. The source coincident with 3EG J0520+2556 was previously reported by Milagro as a “hot spot”.

Many people helped bring Milagro to fruition. In particular, we acknowledge the e?orts of Scott DeLay, Neil Thompson and Michael Schneider. This work has been supported by the National Science Foundation (under grants PHY-0075326, -0096256, -0097315, -0206656, -0245143, -0245234, -0302000, and ATM0002744) the US Department of Energy (O?ce of High-Energy Physics and O?ce of Nuclear Physics), Los Alamos National Laboratory, the University of California, and the Institute of Geophysics and Planetary Physics.

22nd Texas Symposium on Relativistic Astrophysics, Stanford University, December 13-17, 2004


FIG. 6: Left – Milagro signi?cance map, showing the Crab in the center and the TeV source coincident with EGRET source 3EGJ0520+2556 to the left. Right – Accumulated signi?cance on TeV 0520+2556 as a function of time (Figure from [11]).

FIG. 7: EGRET Source Location and contours. We include the position reported by Milagro in 2002 [10], as well as the current location of maximum signi?cance (Figure from [11]).

[1] [2] [3] [4] [5] [6]

Atkins, R. et al. 2003, ApJ, 595, 803 Atkins, R., et al. 2004, ApJL, 604, L25 Atkins, R. et al. 2004, ApJ 608, 680 Atkins, R. et al. 2005, PRL, submitted Li, T. P., & Ma, Y. Q. 1983, ApJ, 272, 317 Fleysher, R., et al. 2004, in Proceedings of the International Symposium on High Energy Gamma-ray

Astronomy, Heidelberg, Germany, July 26-30 (2004) [7] Hunter, S. D., et al. 1997, ApJ, 481, 205 [8] Porter, T. A., & Protheroe, R. J. 1997, Journal of Physics G Nuclear Physics, 23, 1765 [9] Saz Parkinson, P. M. , et al. 2004, in Proceedings of the International Symposium on High Energy Gamma-ray Astronomy, Heidelberg, Germany, July 26-30 (2004)


22nd Texas Symposium on Relativistic Astrophysics, Stanford University, December 13-17, 2004

FIG. 8: Milagro signi?cance map, showing a clear excess in the Cygnus region. Also shown is the Galactic plane region from a latitude of 20 degrees to 100 degrees and longitude of -5 to 5 degrees (Figure from [11]).

[10] Sinnis, C. et al. 2002, in Proceedings of the American Physical Society and the High Energy Astrophysics AAS Meeting, Albuquerque, NM (2002) [11] Smith, A. J. , et al. 2004, in Proceedings of the In-

ternational Symposium on High Energy Gamma-ray Astronomy, Heidelberg, Germany, July 26-30 (2004)


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