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Published January 10, 2013 | Published
Journal Article Open

Rapid TeV Gamma-Ray Flaring of BL Lacertae


We report on the detection of a very rapid TeV gamma-ray flare from BL Lacertae on 2011 June 28 with the Very Energetic Radiation Imaging Telescope Array System (VERITAS). The flaring activity was observed during a 34.6 minute exposure, when the integral flux above 200 GeV reached (3.4 ± 0.6) × 10^(–6) photons m^(–2) s^(–1), roughly 125% of the Crab Nebula flux measured by VERITAS. The light curve indicates that the observations missed the rising phase of the flare but covered a significant portion of the decaying phase. The exponential decay time was determined to be 13 ± 4 minutes, making it one of the most rapid gamma-ray flares seen from a TeV blazar. The gamma-ray spectrum of BL Lacertae during the flare was soft, with a photon index of 3.6 ± 0.4, which is in agreement with the measurement made previously by MAGIC in a lower flaring state. Contemporaneous radio observations of the source with the Very Long Baseline Array revealed the emergence of a new, superluminal component from the core around the time of the TeV gamma-ray flare, accompanied by changes in the optical polarization angle. Changes in flux also appear to have occurred at optical, UV, and GeV gamma-ray wavelengths at the time of the flare, although they are difficult to quantify precisely due to sparse coverage. A strong flare was seen at radio wavelengths roughly four months later, which might be related to the gamma-ray flaring activities. We discuss the implications of these multiwavelength results.

Additional Information

© 2013 American Astronomical Society. Received 2012 September 11; accepted 2012 November 13; published 2012 December 19. Q.F. and W.C. thank Dimitrios Giannios for useful comments on the manuscript. The work of the VERITAS Collaboration was supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland (SFI 10/RFP/AST2748) and by STFC in the U.K. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument. The Boston University effort was supported in part by NASA through Fermi grants NNX08AV65G, NNX08AV61G, NNX09AT99G, and NNX11AQ03G, and by NSF grant AST-0907893. The OVRO 40 m monitoring program is supported in part by NASA grants NNX08AW31G and NNX11A043G, and NSF grants AST-0808050 and AST-1109911. The MOJAVE project is supported under NASA-Fermi grant NNX08AV67G. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Y.Y.K. and K.V.S. were partly supported by the Russian Foundation for Basic Research (project 11-02-00368, 12-02-33101) and the basic research program "Active processes in galactic and extragalactic objects" of the Physical Sciences Division of the Russian Academy of Sciences. Y.Y.K. was supported by the Dynasty Foundation. The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. The Steward Observatory spectropolarimetric monitoring project is supported by Fermi Guest Investigator grants NNX08AW56G and NNX09AU10G. The Metsähovi team acknowledges the support from the Academy of Finland to observing projects (numbers 212656, 210338, 121148, and others). This research has made use of data from the MOJAVE database that is maintained by the MOJAVE team (Lister et al. 2009), the Swinburne University of Technology software correlator that is developed as part of the Australian Major National Research Facilities Programme and operated under license, and the LAT public archive that is maintained by the Fermi Science Support Center.

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