M 87 is a nearby radio galaxy that is detected at energies ranging from radio to very-high-energy ( 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 standard deviations ( \sigma ) from M 87 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 M 87 occurring over a 4-day timespan .
The peak nightly flux above 250 GeV was ( 1.14 \pm 0.26 ) \times 10 ^ { -11 } cm ^ { -2 } s ^ { -1 } , which corresponded to 7.7 % of the Crab Nebula flux .
M 87 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 M 87 at a historical maximum , while the flux from the nearby knot HST-1 remained quiescent . Acciari et al .
( 2 ) 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 M 87 and marginally detected a 4.2 \sigma excess corresponding to a flux of \sim 1 % of the Crab Nebula .
No VHE flaring activity was observed in 2009 .