VERITAS 2008 - 2009 monitoring of the variable gamma-ray source M87
Creators: name: The VERITAS Collaboration
name: Acciari, V. A.
name: Aliu, E.
name: Arlen, T.
name: Aune, T.
name: Beilicke, M.
name: Benbow, W.
name: Boltuch, D.
name: Bradbury, S. M.
name: Buckley, J. H.
name: Bugaev, V.
name: Byrum, K.
name: Cannon, A.
name: Cesarini, A.
name: Chow, Y. C.
name: Ciupik, L.
name: Cogan, P.
name: Cui, W.
name: Dickherber, R.
name: Duke, C.
name: Finley, J. P.
name: Finnegan, G.
name: Fortin, P.
name: Fortson, L.
name: Furniss, A.
name: Galante, N.
name: Gall, D.
name: Gillanders, G. H.
name: Godambe, S.
name: Grube, J.
name: Guenette, R.
name: Gyuk, G.
name: Hanna, D.
name: Holder, J.
name: Hui, C. M.
name: Humensky, T. B.
name: Imran, A.
name: Kaaret, P.
name: Karlsson, N.
name: Kertzman, M.
name: Kieda, D.
name: Konopelko, A.
name: Krawczynski, H.
name: Krennrich, F.
name: Lang, M. J.
name: LeBohec, S.
name: Maier, G.
name: McArthur, S.
name: McCann, A.
name: McCutcheon, M.
name: Millis, J.
name: Moriarty, P.
name: Ong, R. A.
name: Otte, A. N.
name: Pandel, D.
name: Perkins, J. S.
name: Quinn, A. M. J.
name: Ragan, K.
name: Reyes, L. C.
name: Reynolds, P. T.
name: Roache, E.
name: Rose, H. J.
name: Rovero, A. C.
name: Schroedter, M.
name: Sembroski, G. H.
name: Senturk, G. Demet
name: Smith, A. W.
name: Steele, D.
name: Swordy, S. P.
name: Theiling, M.
name: Thibadeau, S.
name: Varlotta, A.
name: Vincent, S.
name: Wagner, R. G.
name: Wakely, S. P.
name: Ward, J. E.
name: Weekes, T. C.
name: Weinstein, A.
name: Weisgarber, T.
name: Williams, D. A.
name: Wissel, S.
name: Wood, M.
name: Zitzer, B.
name: Monitoring, Chandra M87
name: :
name: Harris, D. E.
name: Massaro, F.
Publisher: name: arXiv
Year: 2010
Subjects: subject: Cosmology and Nongalactic Astrophysics (astro-ph.CO)
subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: M87 is a nearby radio galaxy that is detected at energies ranging from radio to VHE gamma-rays. Its proximity and its jet, misaligned from our line-of-sight, enable detailed morphological studies and extensive modeling at radio, optical, and X-ray energies. Flaring activity was observed at all energies, and multi-wavelength correlations would help clarify the origin of the VHE emission. In this paper, we describe a detailed temporal and spectral analysis of the VERITAS VHE gamma-ray observations of M87 in 2008 and 2009. In the 2008 observing season, VERITAS detected an excess with a statistical significance of 7.2 sigma from M87 during a joint multi-wavelength monitoring campaign conducted by three major VHE experiments along with the Chandra X-ray Observatory. In February 2008, VERITAS observed a VHE flare from M87 occurring over a 4-day timespan. The peak nightly flux above 250GeV was 7.7% of the Crab Nebula flux. M87 was marginally detected before this 4-day flare period, and was not detected afterwards. Spectral analysis of the VERITAS observations showed no significant change in the photon index between the flare and pre-flare states. Shortly after the VHE flare seen by VERITAS, the Chandra X-ray Observatory detected the flux from the core of M87 at a historical maximum, while the flux from the nearby knot HST-1 remained quiescent. Acciari et al. (2009) presented the 2008 contemporaneous VHE gamma-ray, Chandra X-ray, and VLBA radio observations which suggest the core as the most likely source of VHE emission, in contrast to the 2005 VHE flare that was simultaneous with an X-ray flare in the HST-1 knot. In 2009, VERITAS continued its monitoring of M87 and marginally detected a 4.2 sigma excess corresponding to a flux of ~1% of the Crab Nebula. No VHE flaring activity was observed in 2009.
description: 17 pages, 5 figures, accepted for publication in ApJ