Does the Blazar Gamma-ray Spectrum Harden with Increasing Flux?-What We Learned From EGRET

Journal Article · Thu Jul 12 04:00:00 EDT 2007 · AIP Conference Proceedings

DOI:https://doi.org/10.1063/1.2757348· OSTI ID:21067248

Nandikotkur, Giridhar ^[1]; Jahoda, Keith M; Hartman, R C; Sambruna, R M; Swank, J H ^[2]; Mukherjee, R ^[3]; Sreekumar, P ^[4]; Boettcher, M ^[5]

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1. University of Maryland, College Park, MD 20742 (United States)
2. Exploration of the Universe Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771 (United States)
3. Department of Physics and Astronomy, Barnard College and Columbia University, New York, NY 10027 (United States)
4. Space Astronomy and Instrumentation Division, ISRO Satellite Centre, Bangalore, Karnataka, 560017 (India)
5. Astrophysical Institute, Department of Physics and Astronomy, Ohio University, Athens, OH 45701 (United States)

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 nine-year lifetime. We conducted an exhaustive search of the EGRET archives and selected all the blazars that have been 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 Lacs (LBLs) and 2 high-frequency-peaked BL Lacs (HBL). 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 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 spectral dependence on flux but the sample shows a mixture of hard and soft states. Our analysis uses the EGRET skymaps that were regenerated to include changes in performance during the mission.

• OSTI ID: 21067248
• Journal Information: AIP Conference Proceedings, Journal Name: AIP Conference Proceedings Journal Issue: 1 Vol. 921; ISSN APCPCS; ISSN 0094-243X
• Country of Publication: United States
• Language: English

Related Subjects

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY
BLACK HOLES
COSMIC GAMMA SOURCES
GAMMA DETECTION
GAMMA RADIATION
GAMMA SPECTRA
INDEXES
LIFETIME
PERFORMANCE
QUASARS
RADIATION FLUX
TELESCOPE COUNTERS
VARIATIONS