Spectral Properties of Bright Fermi-detected Blazars in the Gamma-ray Band

TL;DR

This paper analyzes gamma-ray spectra of bright blazars detected by Fermi-LAT, revealing spectral differences among subclasses, correlations with flux, and common spectral features like breaks and curvature, enhancing understanding of blazar emission mechanisms.

Contribution

It provides a detailed spectral analysis of bright blazars, identifying subclass-specific spectral properties and common features like spectral breaks, which were not comprehensively characterized before.

Findings

Photon index correlates with blazar subclass.

Spectral index varies with flux, often hardening when brighter.

Broken power-law spectra are common in FSRQs and some LSP-BLLacs.

Abstract

The gamma-ray energy spectra of bright blazars of the LAT Bright AGN Sample (LBAS) are investigated using Fermi-LAT data. Spectral properties (hardness, curvature and variability) established using a data set accumulated over 6 months of operation are presented and discussed for different blazar classes and subclasses: Flat Spectrum Radio Quasars (FSRQs), Low-synchrotron peaked BLLacs (LSP-BLLacs), Intermediate-synchrotron peaked BLLacs (ISP-BLLacs) and High-synchrotron peaked BLLacs (HSP-BLLacs). The distribution of photon index (obtained from a power-law fit above 100 MeV) is found to correlate strongly with blazar subclass. The change in spectral index from that averaged over the six month observing period is < 0.2-0.3 when the flux varies by about an order of magnitude, with a tendency toward harder spectra when the flux is brighter for FSRQs and LSP-BLLacs. A strong departure from a single power-law spectrum appears to be a common feature for FSRQs. This feature is also present for some high-luminosity LSP-BLLacs, and a small number of ISP-BLLacs. It is absent in all LBAS HSP-BLLacs. For 3C 454.3 and AO 0235+164, the two brightest FSRQ source and LSP-BLLac source respectively, a broken power law gives the most acceptable of power law, broken power law, and curved forms. The consequences of these findings are discussed.