Science Update - A look at CfA discoveries from recent journals
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 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(s):
"Stochastic
Modeling of the Fermi/LAT γ -Ray Blazar Variability," M. A. Sobolewska,
A. Siemiginowska, B. C. Kelly, and K. Nalewajko, ApJ 786, 143, 2014
