Does the blazar gamma-ray spectrum harden with increasing flux?: Analysis of 9 years of EGRET data

被引:52
作者
Nandikotkur, Giridhar [1 ]
Jahoda, Keith M.
Hartman, R. C.
Mukherjee, R.
Sreekumar, P.
Bottcher, M.
Sambruna, R. M.
Swank, Jean H.
机构
[1] Univ Maryland, Dept Phys, College Pk, MD 20742 USA
[2] NASA, Goddard Space Flight Ctr, Explorat Universe Div, Greenbelt, MD 20771 USA
[3] CUNY Bernard M Baruch Coll, Dept Phys & Astron, New York, NY 10010 USA
[4] Columbia Univ, New York, NY USA
[5] ISRO, Space Astron & Instrumentat Div, Satellite Ctr, Bangalore, Karnataka, India
[6] Ohio Univ, Inst Astrophys, Dept Phys & Astron, Athens, OH 45701 USA
关键词
galaxies : active; methods : data analysis; methods : statistical; radiation mechanisms : nonthermal;
D O I
10.1086/510795
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
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 similar to(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.
引用
收藏
页码:706 / 724
页数:19
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