VERITAS Detection of a Bright VHE Gamma-Ray Flare from BL Lacertae

ATel #3459; Rene A. Ong (UCLA) for the VERITAS Collaboration
on 29 Jun 2011; 21:01 UT
Credential Certification: Rene Ong (rene@astro.ucla.edu)

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On June 28, 2011 UTC (MJD 55740) a bright flare from BL Lacertae was detected with the VERITAS array of atmospheric Cherenkov telescopes. VERITAS is sensitive to very high energy (VHE) gamma rays between 100 GeV and 30 GeV. On this night, a total of 40 minutes of data (beginning at 10:22 UTC) were taken with VERITAS during low levels of moonlight (~10% illumination). Preliminary analysis of these data yields a detection of ~250 gamma rays corresponding to a pre-trials statistical significance of ~20 standard deviations. The observed VHE gamma-ray flux above 200 GeV varies within these data; it is ~50% of the Crab Nebula flux during the first 20-minute data segment and ~10% of the Crab Nebula flux during the second 20-minute data segment. The variations are among the fastest observed from any blazar at these energies. BL Lacertae was previously detected at in VHE gamma rays by the MAGIC collaboration during a flare in 2005 (Albert et al., ApJ 666, L17, 2007) with a reported flux of 3% of the Crab Nebula flux.

The VERITAS observations of this nearby (z = 0.0686) blazar were taken as part of a target-of-opportunity campaign initiated in response to reports of an increase in flux from BL Lacertae by several observatories operating at a variety of wavelengths: radio (ATel #3380), NIR (ATel #3375), optical (ATel #3371), UV and X-ray (Swift UVOT/XRT: ATel #3377), and MeV-GeV gamma rays (Fermi-LAT: ATel #3368; AGILE: ATel #3387). Following these reports, a total of 7.3 hours of VERITAS observations were taken on ten different nights with good weather conditions. The blazar was not detected on the first nine of these nights.

VERITAS plans to continue to observe BL Lacertae (weather-permitting) and multi-wavelength observations are encouraged.

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