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
The High-Energy Gamma-Ray Source at the Galactic Center
John R. Mattox
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.
(Astronomy Department, Boston University)
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
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.
- Hunter et al., 1997, ApJ, in press
- Mattox, J.R. et al., 1996, Ap J, 461, 396
- Mayer-Hasselwander et al., 1997, in preparation
- Michelson, P.F., et al., 1997, in preparation; see also
- Pohl, M., 1997, A&A, 317, 441
- Thompson, D.J., etal., 1995, ApJ S, 101, 259
- Thompson, D.J., etal., 1993, ApJ S, 86, 629
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