Multiwavelength observations of a TeV-Flare from W Com

📝 Abstract

We report results from an intensive campaign of multiwavelength observations of the intermediate-frequency-peaked BL Lacertae object W Com (z=0.102) during a strong outburst of very high energy (VHE; E > 180 GeV) gamma-ray emission in June 2008. The initial detection of this VHE flare by VERITAS, an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes, was followed by observations in high-energy gamma rays (AGILE, E > 100 MeV), X-rays (Swift), optical and radio wavelengths. The VHE gamma-ray signal was detected by VERITAS on June 7-8 with a flux about three times brighter than during the discovery of VHE gamma-ray emission from W Com by VERITAS in March 2008. A detailed study of the spectral energy distribution of W Com during this flare, including theoretical work, will be presented.

💡 Analysis

We report results from an intensive campaign of multiwavelength observations of the intermediate-frequency-peaked BL Lacertae object W Com (z=0.102) during a strong outburst of very high energy (VHE; E > 180 GeV) gamma-ray emission in June 2008. The initial detection of this VHE flare by VERITAS, an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes, was followed by observations in high-energy gamma rays (AGILE, E > 100 MeV), X-rays (Swift), optical and radio wavelengths. The VHE gamma-ray signal was detected by VERITAS on June 7-8 with a flux about three times brighter than during the discovery of VHE gamma-ray emission from W Com by VERITAS in March 2008. A detailed study of the spectral energy distribution of W Com during this flare, including theoretical work, will be presented.

📄 Content

arXiv:0907.4048v1 [astro-ph.HE] 23 Jul 2009 PROCEEDINGS OF THE 31st ICRC, Ł ´OD´Z 2009 1 Multiwavelength observations of a TeV-Flare from W Com Gernot Maier∗for the VERITAS collaboration† Elena Pian‡ for the AGILE collaboration§ ∗Department of Physics, McGill University, H3A 2T8 Montreal, QC, Canada (maierg@physics.mcgill.ca) † see R.A. Ong et al (these proceedings) or http://veritas.sao.arizona.edu/conferences/authors?icrc2009 ‡ Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34143 Trieste, Italy § see http://agile.rm.iasf.cnr.it/index.php for a full author list Abstract. We report results from an intensive campaign of multiwavelength observations of the intermediate-frequency-peaked BL Lacertae object W Com (z=0.102) during a strong outburst of very high energy (VHE; E > 180 GeV) gamma-ray emission in June 2008. The initial detection of this VHE flare by VERITAS, an array of four 12-m diameter imaging atmospheric-Cherenkov telescopes, was followed by observations in high-energy gamma rays (AGILE, E > 100 MeV), X-rays (Swift), optical and radio wavelengths. The VHE gamma-ray signal was detected by VERITAS on June 7-8 with a flux about three times brighter than during the discovery of VHE gamma-ray emission from W Com by VERITAS in March 2008. A detailed study of the spectral energy distribution of W Com during this flare, including theoretical work, will be presented. Keywords: gamma-ray observatory; blazars; W Com I. INTRODUCTION W Com is an intermediate-frequency-peaked BL Lac (IBL) at a redshift of z = 0.102. The object was discov- ered in high-energy gamma rays (from 100 MeV to 10 GeV) by EGRET [11] and in very high energy gamma rays in March 2008 during a strong outburst of about 4 days duration by VERITAS [1]. The broadband spectral energy distribution (SED) of W Com is characterized by two peaks, arising from synchrotron radiation at radio to X-ray frequencies and high energy emission due to inverse Compton scattering or hadronic interactions at hard X-ray to gamma-ray energies. The X-ray and gamma-ray emission from W Com has been found to be variable on a timescale of hours to days [16]. Here we present contemporaneous observations of W Com during a second strong flare in June, 2008 with VERITAS, AGILE, and Swift. II. OBSERVATIONS AND RESULTS A. VERITAS VERITAS is an array of four imaging atmospheric- Cherenkov telescopes located at the Fred Lawrence Whipple Observatory in southern Arizona. It combines a large effective area (up to 105 m2) over a wide energy range (100 GeV to 30 TeV) with good energy (15-20%) and angular (≈0.1o) resolution. The field ] σ significance [ -4 -2 0 2 4 6 8 10 12 [deg] J2000 declination 27.5 28 28.5 29 [hours] J2000 right ascension m 18 h 12 m 20 h 12 m 22 h 12 m 24 h 12 m 26 h 12 PSF Fig. 1. VERITAS significance map of the region around W Com for MJD 54624.16 to 54625.24. The background is estimated using the reflected region model. The position of W Com derived from radio data is indicated by a white cross. The circle at the bottom right indicates the angular resolution of the VERITAS observations. of view of the VERITAS telescopes is 3.5o. The high sensitivity of VERITAS enables the detection of sources with a flux of 1% of the Crab Nebula in less than 50 hours of observations. For more details on the VERITAS instrument, see e.g. [17]. VERITAS observed the sky around W Com for about four hours in June 2008. All observations passed run quality selection criteria, which remove data taken dur- ing bad weather or with hardware-related problems. Data were taken in wobble mode, wherein the source was positioned at a fixed offset from the camera center. Parts of the observations were undertaken in moonlight conditions, where the elevated background light levels lead to a lower flux sensitivity and higher energy thresh- old for the detection of γ-rays. The different elevations combined with the continuously changing background light conditions due to the moon results in a wide range of energy thresholds from 180 to 420 GeV1 for the present observations. Table I lists observation times, elevation range, and background light conditions. 1 The energy threshold is defined as the peak of the differential counting rate for a Crab Nebula-like spectrum. 2 G.MAIER et al. OBSERVATIONS OF A TEV-FLARE FROM W COM TABLE I DETAILS OF THE VERITAS OBSERVATIONS OF W COM IN 2008 JUNE. FLUXES AND UPPER FLUX LIMITS ARE GIVEN AT THE 99% CONFIDENCE LEVEL FOR AN ENERGY THRESHOLD OF 200 GEV. MJD elevation observation average pedestal significance flux or upper flux limit range time [min] variations [dc]2 [σ] [cm−2s−1] 54624.16 - 54624.23 53-73o 100.2 7.8-8.0 8.9 (5.0 ± 0.8) × 10−11 54625.17 - 54625.24 49-68o 100.2 8.1-9.7 7.9 (6.2 ± 1.2) × 10−11 54626.18 - 54626.20 59-60o 32.0 12.2-12.3 -1.0 < 3.15 × 10−11 The VERITAS data analysis steps consist of image calibration and cleaning, second-moment parameteriza- tion of these images [13], stereoscopic reconstruction of the event impact position and dire

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