Abdo, A. A. ; Ackermann, M. ; Ajello, M. ; Cavazzuti, E. ; Colafrancesco, S. ; Giommi, P. ;
Dec - 2009
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 1
type: Article Journal

We report the Fermi Large Area Telescope (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 10 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) × 10–8 photons 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 × 10–8 photons cm–2 s–1, 2\sigma), thus there is no evidence for a significant MeV/GeV flare on decade timescales. Contemporaneous Chandra and Very Long Baseline Array 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 cannot be excluded.

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