Tracking Spectral Changes in Blazars with the Energetic Gamma Ray Experiment Telescope (EGRET)
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I analyze the entire blazar data from the Energetic Gamma Ray Experiment Telescope (EGRET) on board the Compton Gamma Ray Observatory (CGRO), using the skymaps that were regenerated to include the changes in performance during the mission. The sample of 98 sources consists of 66 flat spectrum radio quasars (FSRQs), 17 low-frequency peaked BL Lac objects (LBLs), 4 high-frequency peaked BL Lac objects (HBLs), 10 flat spectrum radio sources and 1 radio galaxy. I 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. I examine the EGRET spectral energy distribution (SED) for all the sources to identify these trends. I also observe a previously unreported spectral hysteresis in the spectral index Vs. flux space at weekly timescales, in all the three FSRQs for which data from flares lasting 3-4 weeks were available. All three sources show a counterclockwise rotation despite the widely different flux profiles. The time-averaged spectra of the HBLs are inconsistent with the predictions of the current theoretical models that have had success in describing simultaneous X-ray/TeV observations, and suggest additional components in the GeV band, as well as complex time variability. Current theoretical pictures explain the GeV emission as comptonization of the synchrotron photons in the jet, and predict hard spectra that should join smoothly with the TeV emission. The current analysis shows that the situation is more complex. The spectrum ranges from hard to soft during individual epochs, and the Mrk 421 SED shows a convex break in the aggregated data. The mission averaged EGRET spectrum for PKS~2155-304 also shows a similar (but not as pronounced) convex curvature. Simultaneous GLAST and X-ray observations of high X-ray states will address the issue of the convex curvature in the future. Such data will also explore the possibility of the steep EGRET emission originating from photons produced by electrons accelerated close to the limit of diffusive shock acceleration.