Simultaneous Observations of Flaring Gamma-ray Blazar 3C 66A with Fermi-LAT and VERITAS

📝 Abstract

The intermediate-frequency-peaked BL Lac object 3C 66A was detected in a flaring state by the Fermi-LAT and VERITAS observatories in October 2008. These data and follow-up observations at other wavelengths create a rich sample of light curves and a constraining spectral energy distribution (SED). This is the first time that simultaneous observations at GeV and TeV energies were obtained for a flaring blazar. Results from these joint Fermi-LAT and VERITAS observations are presented in this conference proceeding.

💡 Analysis

The intermediate-frequency-peaked BL Lac object 3C 66A was detected in a flaring state by the Fermi-LAT and VERITAS observatories in October 2008. These data and follow-up observations at other wavelengths create a rich sample of light curves and a constraining spectral energy distribution (SED). This is the first time that simultaneous observations at GeV and TeV energies were obtained for a flaring blazar. Results from these joint Fermi-LAT and VERITAS observations are presented in this conference proceeding.

📄 Content

arXiv:0907.5175v1 [astro-ph.HE] 29 Jul 2009 PROCEEDINGS OF THE 31st ICRC, Ł ´OD´Z 2009 1 Simultaneous Observations of Flaring Gamma-ray Blazar 3C 66A with Fermi-LAT and VERITAS Luis C. Reyes∗on behalf of the Fermi-LAT and VERITAS† Collaborations ∗Kavli Institute for Cosmological Physics (KICP) at The University of Chicago, Chicago, IL 60605, USA †see R.A. Ong et al. (these proceedings) or http://veritas.sao.arizona.edu/conferences/authors?icrc2009 Abstract. The intermediate-frequency-peaked BL Lac object 3C 66A was detected in a flaring state by the Fermi-LAT and VERITAS observatories in October 2008. These data and follow-up observations at other wavelengths create a rich sample of light curves and a constraining spectral energy distribu- tion (SED). This is the first time that simultaneous observations at GeV and TeV energies were obtained for a flaring blazar. Results from these joint Fermi- LAT and VERITAS observations are presented in this paper. Keywords: Gamma-ray Astronomy, Active Galactic Nuclei, Fermi Gamma-ray Space Telescope, VERI- TAS I. INTRODUCTION Due to its significant optical and X-ray variability, 3C 66A was classified as a BL Lac object by Maccagni et al. [1], and given the location of its synchrotron peak (between 1015 and 1016 Hz), 3C 66A can be further sub-classified as an intermediate-frequency peaked BL Lac (IBL). BL Lacs are known for having very weak (if any) detectable emission lines, which makes the redshift determination quite difficult. The redshift of 3C 66A was reported as z = 0.44 by Miller et al. [2] and independently by Lanzetta et al. [3]. However, both measurements rely on the measurement of a single line, and as pointed out by Bramel et al. [4], the redshift of this object is still quite uncertain. Similarly to other blazars, the spectral energy distribu- tion (SED) of 3C 66A is known to have two pronounced peaks (see [5] for SED details), which suggests at least two different physical emission processes at play. The first peak (extending from radio to soft X-ray frequen- cies) is likely due to polarized synchrotron emission from high energy electrons, while different emission models (briefly introduced below) have been proposed to explain the second energy peak, which extends up to gamma-ray energies. It should be noted that the high- energy emission component dominates the bolometric luminosity of this source, a powerful demonstration of the extreme nature of this type of object. The models that have been proposed to explain gamma-ray emission in blazars can be roughly cat- egorized into leptonic and hadronic mechanisms, de- pending on whether the accelerated particles respon- sible for the gamma-ray emission are primarily elec- trons/positrons or protons. In leptonic models, high- energy electrons/positrons produce gamma rays via in- verse Compton scattering of low-energy photons. In synchrotron self-Compton (SSC) models, the same pop- ulation of electrons responsible for the observed gamma rays generates the low-energy photon field through syn- chrotron emission. In external Compton (EC) models the low-energy photons originate outside the jet. Possible sources of external photons include: accretion disk pho- tons radiated directly into the jet, accretion disk photons scattered by emission-line clouds or dust into the jet, or synchrotron radiation re-scattered back into the jet by broad-line emission clouds. In hadronic models gamma rays are produced by high-energy protons, either via proton synchrotron radiation, or via secondary (see [6] and references therein for a review of blazar gamma-ray emission processes). One of the main obstacles in the broadband study of gamma-ray blazars is the lack of simultaneity, or at least contemporaneity, of the data at the various wavelengths. At high energies the situation has been even more difficult due to the lack of objects that can be detected by MeV/GeV and TeV observatories in comparable time scales. Indeed, until recently most of our knowledge of blazars at gamma-ray energies was obtained from observations performed in two disjoint energy regimes: i) the high energy (HE) range (20 MeV< E < 10 GeV), studied in the 1990s by EGRET [7], and ii) the very-high-energy (VHE) regime (E > 100 GeV) observed by ground-based instruments [8]. Remarkably, most EGRET blazars have not been detected by TeV telescopes; even those that are nearby and bright [9]. Furthermore, blazars detected by EGRET at MeV/GeV energies are predominantly FSRQs, while blazars TeV blazars are BL Lacs. It is important to understand these observational differences since they are likely related to the physics of the AGN [10], or to the evolution of blazars over cosmic time [11]. Fortunately, the new generation of gamma-ray instru- ments (AGILE, Fermi, HESS, MAGIC and VERITAS) is closing the gap between both energy regimes due to their improved sensitivities, leading us towards a deeper and more complete characterization of blazars as a high-energy source and as a populat

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