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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) x 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.
The quasar PKS 1510-089 (z = 0.361) was observed with the H.E.S.S. array of imaging atmospheric Cherenkov telescopes during high states in the optical and GeV bands, to search for very high energy (VHE, defined as E >= 0.1 TeV) emission. VHE gamma-rays were detected with a statistical significance of 9.2 standard deviations in 15.8 h of H. E. S. S. data taken during March and April 2009. A VHE integral flux of I(0.15 TeV < E < 1.0TeV) = (1.0 +/- 0.2(stat) +/- 0.2(sys)) x 10(-11) cm(-2) s(-1) is measured. The best-fit power law to the VHE data has a photon index of G = 5.4 +/- 0.7(stat) +/- 0.3(sys). The GeV and optical light curves show pronounced variability during the period of H.E.S.S. observations. However, there is insufficient evidence to claim statistically significant variability in the VHE data. Because of its relatively high redshift, the VHE flux from PKS 1510-089 should suffer considerable attenuation in the intergalactic space due to the extragalactic background light (EBL). Hence, the measured gamma-ray spectrum is used to derive upper limits on the opacity due to EBL, which are found to be comparable with the previously derived limits from relatively-nearby BL Lac objects. Unlike typical VHE-detected blazars where the broadband spectrum is dominated by nonthermal radiation at all wavelengths, the quasar PKS 1510-089 has a bright thermal component in the optical to UV frequency band. Among all VHE detected blazars, PKS 1510-089 has the most luminous broad line region. The detection of VHE emission from this quasar indicates a low level of gamma - gamma absorption on the internal optical to UV photon field.
We report on the VERITAS observations of the high-frequency peaked BL Lac object 1ES 1959+650 in the period 2007-2011. This source is detected at TeV energies by VERITAS at 16.4 standard deviation (sigma) significance in 7.6 hr of observation in a low flux state. A multiwavelength spectral energy distribution (SED) is constructed from contemporaneous data from VERITAS, Fermi-LAT, RXTE PCA, and Swift UVOT. Swift XRT data is not included in the SED due to a lack of simultaneous observations with VERITAS. In contrast to the orphan gamma-ray flare exhibited by this source in 2002, the X-ray flux of the source is found to vary by an order of magnitude, while other energy regimes exhibit less variable emission. A quasi-equilibrium synchrotron self-Compton model with an additional external radiation field is used to describe three SEDs corresponding to the lowest, highest, and average X-ray states. The variation in the X-ray spectrum is modeled by changing the electron injection spectral index, with minor adjustments of the kinetic luminosity in electrons. This scenario produces small-scale flux variability of the order of less than or similar to 2 in the high energy (E > 1MeV) and very high energy (E > 100 GeV) gamma-ray regimes, which is corroborated by the Fermi-LAT, VERITAS, and Whipple 10 m telescope light curves.
We report the detection of a new TeV gamma-ray source, VER J0521+211, based on observations made with the VERITAS imaging atmospheric Cherenkov Telescope Array. These observations were motivated by the discovery of a cluster of >30 GeV photons in the first year of Fermi Large Area Telescope observations. VER J0521+211 is relatively bright at TeV energies, with a mean photon flux of (1.93 +/- 0.13(stat) +/- 0.78(sys)) x 10(-11) cm(-2) s(-1) above 0.2 TeV during the period of the VERITAS observations. The source is strongly variable on a daily timescale across all wavebands, from optical to TeV, with a peak flux corresponding to similar to 0.3 times the steady Crab Nebula flux at TeV energies. Follow-up observations in the optical and X-ray bands classify the newly discovered TeV source as a BL Lac-type blazar with uncertain redshift, although recent measurements suggest z = 0.108. VER J0521+211 exhibits all the defining properties of blazars in radio, optical, X-ray, and gamma-ray wavelengths.