The Shortest Flux Variations Detected from Active Galactic Nuclei
An Active Galactic Nucleus (AGN) is a compact region at the centre of a galaxy that can outshine some or all the electromagnetic radiation of the host galaxy by up to three orders of magnitude. The ultimate energy source of those objects is commonly believed to be the release of gravitational potential energy of matter from an accretion disk surrounding a super-massive black hole (10 million to 100 billion times the mass of the Sun). Although this general model has won broad support, discussion on the detailed emission processes underlying the broadband spectral energy distribution of AGNs is still ongoing. The observation of fast flux variations can potentially bring important information from the innermost regions of the AGN in a rather model-independent way. The most extreme flux variations have been observed in so-called blazars: AGNs containing jets of plasma moving at relativistic speed towards the observer. Although blazars comprise only several percent of the overall AGN population, they largely dominate the high-energy extragalactic sky. This is because most of the non-thermal power, which arises from relativistic jets that are narrowly beamed and boosted in the forward direction, is emitted in the gamma-ray band, whereas the presumably nearly-isotropic emission from the accretion disk is most luminous at optical, UV, and X-ray energies. Therefore, the study of the variable gamma-ray emission of blazars can potentially bring key data for the understanding of the working principle of the still rather unknown AGNs.
Recently, the MAGIC and the HESS instruments (Imaging Atmospheric Cherenkov Telescopes, IACT, for gamma-ray observation of the northern and southern hemisphere, respectively) recorded the fastest ever-detected flux variations from blazars. (Related papers about these blazars, Markarian 501 and PKS2155-304, are currently in publication and can be accessed via the links below.) Assuming the emission comes from a spherical blob in the jet, these observations imply that the source's radius is less than about 0.6 δ AU (an AU is an astronomical unit, or the mean distance between the Earth and the Sun, about 150 million kilometers), where δ is the Doppler factor of the source. Since the central engine of these objects is believed to be a super-massive black hole (with a mass about one billion times the mass of the sun), the most natural and smallest rule to measure distance in those systems is the Schwarzchild radius which defines the horizon of the black hole. For those black holes, the Schwarzchild radius is estimated to be about 3 billion kilometers. This implies that the radius of the gamma-ray source is just few percent the Schwarzchild radius times the Doppler factor, and thus that Doppler factors of about 100 are needed so that the emitting region is comparable to horizon of the black hole. The Doppler factor derived from the modeling of the blazars' spectral distribution is typically about 20; implying that the emitting region is smaller than the black hole horizon. This makes it unlikely that the observed flux variability is directly connected to the black hole.
The new generation of Cherenkov telescopes such as HESS and MAGIC (in operation for approximately 3 years now) are proving themselves to be very useful for the understanding of AGNs. Yet many questions still remain open: What is the composition of the jet, both in the initial and in the radiative phase? Where does the conversion between the kinetic power of the jet into radiation take place, and how? What role is played by relativistic hadrons? This year, another IACT named VERITAS came online, and next year GLAST (with a smaller collection area than IACTs, but a larger duty cycle) will start scanning the entire sky looking for, among other things, these outstanding flaring episodes. All these instruments will surely provide the community with very valuable data that will shed light into these mysterious objects.
David Paneque, SLAC Today, August 16, 2007
Above photo: The MAGIC (top) and HESS instruments. (Images courtesy of the MAGIC Telescope Project and W. Hofmann, respectively.)