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Title:
Does the Blazar Gamma-Ray Spectrum Harden with Increasing Flux? Analysis of 9 Years of EGRET Data
Authors:
Nandikotkur, Giridhar; Jahoda, Keith M.; Hartman, R. C.; Mukherjee, R.; Sreekumar, P.; Böttcher, M.; Sambruna, R. M.; Swank, Jean H.
Affiliation:
AA(Department of Physics, University of Maryland at College Park, College Park, MD; .; Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD.), AB(Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD.), AC(Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD.), AD(Department of Physics and Astronomy, Barnard College and Columbia University, New York, NY.), AE(Space Astronomy and Instrumentation Division, ISRO Satellite Centre, Bangalore, Karnataka, India.), AF(Astrophysical Institute, Department of Physics and Astronomy, Ohio University, Athens, OH.), AG(Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD.), AH(Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD.)
Publication:
The Astrophysical Journal, Volume 657, Issue 2, pp. 706-724. (ApJ Homepage)
Publication Date:
03/2007
Origin:
UCP
ApJ Keywords:
Galaxies: Active, Methods: Data Analysis, Methods: Statistical, Radiation Mechanisms: Nonthermal
Abstract Copyright:
(c) 2007: The American Astronomical Society
DOI:
10.1086/510795
Bibliographic Code:
2007ApJ...657..706N

Abstract

The Energetic Gamma-Ray Experiment Telescope (EGRET) on the Compton Gamma-Ray Observatory (CGRO) discovered gamma-ray emission from more than 67 blazars during its 9 yr lifetime. We conducted an exhaustive search of the EGRET archives and selected all the blazars that were observed multiple times and were bright enough to enable a spectral analysis using standard power-law models. The sample consists of 18 flat-spectrum radio quasars (FSRQs), 6 low-frequency peaked BL Lac objects (LBLs) and 2 high-frequency peaked BL Lac objects (HBLs). We do not detect any clear pattern in the variation of spectral index with flux. Some of the blazars do not show any statistical evidence for spectral variability. The spectrum hardens with increasing flux in a few cases. There is also evidence for a flux-hardness anticorrelation at low fluxes in five blazars. The well-observed blazars (3C 279, 3C 273, PKS 0528+134, PKS 1622-297, PKS 0208-512) do not show any overall trend in the long-term spectral dependence on flux, but the sample shows a mixture of hard and soft states. We observed a previously unreported spectral hysteresis at weekly timescales in all three FSRQs for which data from flares lasting for ~(3-4) weeks were available. All three sources show a counterclockwise rotation, despite the widely different flux profiles. We analyze the observed spectral behavior in the context of various inverse Compton mechanisms believed to be responsible for emission in the EGRET energy range. Our analysis uses the EGRET skymaps that were regenerated to include the changes in performance during the mission.
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