Fermi Large Area Telescope Gamma-Ray Detection of the Radio Galaxy M87
  We report the Fermi-LAT discovery of high-energy (MeV/GeV) gamma-ray emission
positionally consistent with the center of the radio galaxy M87, at a source
significance of over 10 sigma in ten-months of all-sky survey data. Following
the detections of Cen A and Per A, this makes M87 the third radio galaxy seen
with the LAT. The faint point-like gamma-ray source has a >100 MeV flux of 2.45
(+/- 0.63) x 10^-8 ph cm^-2 s^-1 (photon index = 2.26 +/- 0.13) with no
significant variability detected within the LAT observation. This flux is
comparable with the previous EGRET upper limit (< 2.18 x 10^-8 ph cm^-2 s^-1, 2
sigma), thus there is no evidence for a significant MeV/GeV flare on decade
timescales. Contemporaneous Chandra and VLBA data indicate low activity in the
unresolved X-ray and radio core relative to previous observations, suggesting
M87 is in a quiescent overall level over the first year of Fermi-LAT
observations. The LAT gamma-ray spectrum is modeled as synchrotron self-Compton
(SSC) emission from the electron population producing the radio-to-X-ray
emission in the core. The resultant SSC spectrum extrapolates smoothly from the
LAT band to the historical-minimum TeV emission. Alternative models for the
core and possible contributions from the kiloparsec-scale jet in M87 are
considered, and can not be excluded.
