Astronomers from the Harvard-Smithsonian Center for Astrophysics used Fermi to monitor the gamma-ray variability of thirteen blazars, finding evidence that the emission arises in several different zones and/or from several mechanisms.
A blazar is a galaxy whose central, supermassive black hole shines intensely as it accretes material from the surrounding region. Although black hole accretion happens in many galaxies and situations, in blazars the infalling material erupts into a powerful, narrow beam of high velocity charged particles that are fortuitously pointed in our direction. These particles produce gamma rays, each photon over a hundred million times more energetic than the highest energy X-ray photons seen by the Chandra X-ray Observatory. Blazars are also generally characterized by having rapid, strong, and incessant variability, among a host of effects resulting from its beam of rapidly moving electrons.
Astronomers suspect that clues to the inner workings of black holes and accretion disks can be discerned from modeling the details of the variability, but this has been a difficult task. The complexity of the variability indicates that the emitting structures are also complex, and constraining the locations and sizes of the emitting sites has been hampered by a lack of long-term, sensitive observations capable of steady monitoring of the changing activity.
CfA astronomers Malgosia Sobolewska and Aneta Siemiginowska and two colleagues tackled the problem using the Large Area Telescope (LAT), a gamma ray imaging telescope onboard the Fermi spacecraft. LAT is well suited for studying the variability of blazars, and has been taking continuous observations of the gamma-ray sky since Fermi was launched in 2008. It therefore has an excellent set of light curves (plots of the intensity versus time) for blazars. Recent analyses showed that the blazar light seemed to be produced in random processes, at least for the high-energy gamma-rays. The problem is that many of the brightest blazar episodes are thought to be flares from a distinctly different kind of process than the regular emission, and if so they should be identified as not arising from a single random process. For example, there are hints in two blazars of activity that is preferentially occurring in six- or seven-day intervals, pointing to shocks or colliding ejecta of some kind.
The scientists undertook a systematic analysis of the first four years of the Fermi/LAT dataset for thirteen bright blazars, and they developed new methods that are insensitive to the known observational biases. They find that three blazars have emission consistent with arising from a combination of random processes; in two they constrain the characteristic times to seventeen and thirty-eight days respectively – longer times than ever before seen and suggestive that the gamma-ray and X-ray emissions arise in different zones of the blazar. In four other blazars they report evidence of characteristic timescales faster than one hour, a finding that is not easily understood and, together with their other conclusions, points to new progress and new puzzles in deciphering what makes blazars blaze.
Reference: “Stochastic Modeling of the Fermi/LAT γ-Ray Blazar Variability” by M. A. Sobolewska, A. Siemiginowska, B. C. Kelly and K. Nalewajko, 25 April 2014, The Astrophysical Journal.
DOI: 10.1088/0004-637X/786/2/143
arXiv: 1403.5276
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