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                              G C N E W S
                             * Newsflash *

            - The Newsletter for Galactic Center Research -

gcnews@mpifr-bonn.mpg.de             http://www.mpifr-bonn.mpg.de/gcnews
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Vol. 8, No. 9                                              May 19, 1998


Recently submitted paper:
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Email    : hrm@mpe.mpg.de
Title    : High-Energy Gamma-Ray Emission from the Galactic Center
Author(s): H.A. Mayer-Hasselwander^1, D.L. Bertsch^2, B.L. Dingus^3, A. 
           Eckart^1,  J.A. Esposito^2,10,  R. Genzel^1, 
           R.C. Hartman^2, S.D. Hunter^2, G. Kanbach61, D.A. Kniffen^4, 
           Y.C.   Lin^5,   P.F. Michelson^5, A. Muecke^1, C. 
           von Montigny^6, R. Mukherjee^7, P.L.Nolan^5, M.
           Pohl^8,   O. Reimer^1, E.J. Schneid^9, P. Sreekumar^2,10, 
           D.J. Thompson^2
Institute: ^1 Max-Planck-Institut fuer extraterrestrische 
           Physik, D-85748 Garching,    Germany   ^2 NASA Goddard 
           Space Flight Center, Greenbelt, MD 20771, USA   ^3 
           University of Utah, Salt Lake City, Utah, USA   ^4 Dept. of 
           Physics and Astronomy, Hampden-Sydney College, 
           Hampden-Sydney,    VA 23943, USA   ^5 W.W. Hansen 
           Experimental Physics Laboratory and Department of=20    
           Physics, Stanford University, Stanford CA 94305, USA=20   ^6 
           Landessternwarte K=F6nigstuhl, D-69117 Heidelberg, Germany   
           ^7 Barnard College & Columbia University, Columbia Astroph. 
           Lab, New York,    NY 10027, USA   ^8 Danish Space 
           Research Institute, 2100 Copenhagen O, Denmark   ^9 Northrop 
           Grumman Corporation, Bethpage, NY 11714, USA   ^10 USRA 
           Research Associate
Paper    : A&A, in press

Abstract:
The EGRET instrument on the Compton Gamma-Ray Observatory has observed
the Galactic Center (GC) region with good coverage at a number of
epochs.  A strong excess of emission is observed, peaking at energies
>500 MeV in an error circle of 0.2 degree radius including the
position l=3Degr and b=3Degr.  The close coincidence of this excess with
the GC direction and the fact that it is the strongest emission
maximum within 15 degrees from the GC is taken as compelling evidence
for the sources location in the GC region.  The history of the
emission intensity, observed over 5 years, leaves room for possible
time variation; however, it does not provide evidence.  The angular
extent of the excess appears only marginally compatible with the
signature expected for a single compact object.  The emission
therefore may stem from one or more compact objects or may originate
from diffuse interactions within 85 pc from the center of the Galaxy
at 8.5 kpc distance.  The spatial distribution of the emission does
not correlate with the details of the CO-line surveys.  Thus, in spite
of the existence of a strong emission peak, earlier conclusions based
on an apparent gamma-ray deficit, postulating the masses of the wide-
line clouds in the GC area to be an order of magnitude lower than
indicated by naive CO interpretation, are supported.  However, the
total gas mass in the Nuclear Bulge (NB) derived from the gamma-ray
emission is found to be in agreement with the mass which in recent
studies has been derived from molecular-line and FIR surveys.  The
g-ray emission spectrum is peculiar and different from the spectrum of
the large-scale galactic diffuse emission.  A diffuse emission
scenario requires an enhanced and peculiar Cosmic Ray (CR) spectrum as
suggested for the electrons in the Radio Arc.  A compact sources model
hints at an origin in pulsars.  While the spectrum suggests
middle-aged pulsars like Vela, too many are required to produce the
observed flux.  The only detected very young pulsar, the Crab pulsar,
has an incompatible spectrum.  However, it is not proven that the Crab
spectrum is characteristic for all young pulsars: thus, a single or a
few very young pulsars (at the GC not detectable in radio emission),
provided their gamma-ray emission is larger than that of the Crab
pulsar by a factor of 13, are likely candidates.  Alternatively, more
exotic scenarios, related to the postulated central black hole or dark
matter (neutralino) annihilation, may be invoked.
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			       Edited by
Angela Cotera                                Heino Falcke
(cotera@ipac.caltech.edu)                    (hfalcke@mpifr-bonn.mpg.de)
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