Article - GCNEWS, Vol. 4, February 1997


A Newsletter for Galactic Center Research
This Volume was edited by Angela Cotera & Heino Falcke

Volume 4, February 1997 - ARTICLE

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The High-Energy Gamma-Ray Source at the Galactic Center

John R. Mattox
(Astronomy Department, Boston University)

In the course of a full-sky survey, the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory detected a high-energy (30 MeV - 30 GeV) gamma-ray excess, 2EG J1746-2852, which is consistent in position with the Galactic center (within 0.2 degrees of l= 0.05, b=-0.05, with 95% confidence). The proximity indicates that it is probably associated with the Galactic center. The excess is observed to be distributed as the EGRET point spread function. The flux is 1.1+/-0.1*10-6 photons cm-2 s-1, corresponding to a high-energy gamma-ray luminosity of ~5*1036 ergs/s if it is at the Galactic center. It is possible that the gamma-ray emission is due to the ~106 Mo black hole candidate Sgr A*, but there is not significant evidence of variability to confirm this. A diffuse origin within a ~100 pc distance of the Galactic center is also possible.

1.0 Introduction

The Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Compton Gamma Ray Observatory is sensitive in the energy range 30 MeV to 30 GeV (Thompson et al. 1993). A catalog of EGRET sources based on the first 30 months of exposure is given by Thompson et al. (1995). It includes 129 sources. Five are pulsars, and ~50 are identified with radio sources which appear to be of the blazar class of AGN. A total of 71 are unidentified. Among the unidentified sources is 2EG J1746-2852.

Figure 1: The distribution of gamma-rays detected by EGRET during phase 1 and 2 with E>1GeV. The PSF-shaped Galactic center excess is immediately apparent. Pulsar PSR B1706-44 is also apparent at l=343 deg, b=-3 deg.

2.0 The EGRET Detection

The EGRET data clearly show an excess spatially coincident with the center of the Galaxy (Figure 1) which is consistent with the EGRET point spread function. A likelihood position estimate (Mattox et al. 1996) has been done for the event energy selection, E>1 GeV. Assuming a point source, the formal likelihood result is that it is located with 95% confidence within 0.13 deg of l=0.05 deg, b=-0.05 deg. Allowing for a systematic error equal to the statistical error, the radius of uncertainty is 0.2 deg. Thus, the low-energy gamma-ray source 1E1740.7-2942 is excluded with high confidence. The proximity indicates that this excess is probably associated with the Galactic center. With 9 EGRET sources in the region -20 deg<l<20 deg and -5 deg<b< 5 deg (Thompson et al. 1995), the probability that any non-Galactic-center source would be found within 0.1 deg of the Galactic center may be estimated as 9[pi 0.12/(40*10)]=0.001. The position analysis implies that if the emission is extended, it also must be centered within ~0.2 deg of l=0.0, b=0.0. This is narrower than the distribution of the wide-line molecular clouds at Galactic radius ~400 pc.

While it is not known what the occurrence rate of such compact diffuse emission regions might be, it must be less than or equal to 9 in the region -20<l<20 deg and -5<b<5. Therefore, with 99.9% confidence, the point source (or compact diffuse gamma-ray excess) is associated with the Galactic center.

The spectrum is shown in Figure 2. This has been derived by making a likelihood estimate of flux for each of the narrow standard EGRET energy intervals using the diffuse model of Hunter et al. (1997). Because the PSF blossoms into an uncertain diffuse model at low energy, the photons with E<150 MeV have not been used in the analysis. The best power law fit has a photon index of -1.7. The fit is not good (reduced chi2=3.6). The low energy departure from the power-law could be due to an error in the diffuse model. If the high energy point is not below the power-law extrapolation due to statistical fluctuation (5% chance), a spectral break occurs at ~3 GeV.

Figure 2: The spectrum of the Galactic center excess for the phase 1 and 2 data.
(Click on image to see full-size version).

3.0 Summary

EGRET detects an unresolved source of GeV emission which is coincident with the direction of the Galactic center. It is likely that this source is in fact at the Galactic center, but it is not clear whether this is a compact source, or whether it is extended over a ~100 pc region. Pohl (1997) suggests that the emission is due the Galactic center arc. A more complete description of the EGRET result is in preparation by Mayer-Hasselwander et al. (1997).

The GeV observations of the proposed GLAST satellite (Michelson et al. 1997) will be very useful in understanding this gamma-ray source. With ~10 times the effective area of EGRET and a point spread function which covers ~1/10 of the solid angle of EGRET's PSF as a function of energy, GLAST will determine the position of the source to better than an arc minute, and will allow for resolution of extended emission as small as a few pc. The expected stability of the response of GLAST will also provide for detecting small variation in flux. The GLAST mission is well defined (Michelson et al. 1997). It could fly within ~5 years of getting the required ~200M$ funding.


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