Monday, February 25, 2013

Diagnosing the X-Ray Variability of a Galaxy's Nucleus

An optical image of the active galaxy NGC 4507. New observations of the X-ray variability of this galaxy imply that the the simple, universal unification model for active galaxies is deficient.  Credit: Carnegie Institution of Washington . Low Resolution Image (jpg)

An active galaxy is one whose nucleus contains a massive black hole that is vigorously accreting material. In the process, the nucleus typically ejects jets of particles and radiates brightly at many wavelengths, in particular at X-ray wavelengths. Nearly half of all active galactic nuclei are seen emitting X-rays that are of relatively high energy, with less energetic X-rays absent. Since typical physical processes will generate both kinds, the usual explanation is that thick gas clouds swarm near the nuclei, and they absorb the lower energy X-ray emission, leaving the higher energy radiation relatively unaffected. X-ray emitting active galaxies are important to astronomers not only because they provide insights into black holes and their surroundings, but because the X-rays are often time variable, possibly reflecting the motions of these clouds around the nucleus. These kinematics in turn provide information on how the nucleus and its galaxy formed and evolved.

Active galaxies display a range of dramatically different properties. For example, some eject long bipolar jets from their nucleus and have spectral lines with small velocities; others show no jets and have lines with large velocities. The standard "unification model" for these objects posits that all these galaxies are intrinsically similar, only for some our viewing angle is nearly edge-on to the galaxy, and for others it is close to face-on, or some angle in between. Determining the reliability and applicability of the unification model is critical to understanding these luminous objects and what powers their activity. 

Five CfA astronomers, Andrea Marinucci, Guido Risaliti, Junfeng Wang, Martin Elvis, and Emanuele Nardini, together with three colleagues, used the Chandra and XMM-Newton satellites to study the X-ray variability of the active galaxy NGC 4507. They were trying to test whether the variation of X-rays indicated motions in absorbing clouds of gas around the black hole, and if so, whether the result was consistent with the idea that a single universal structure, viewed at different angles, could explain the basic differences between active galaxy types. 

The galaxy NGC 4507 has been known to vary in X-ray emission, but only over the rather long time scales of years. The astronomers report that when they monitored the source over shorter intervals, they found significant variations took place over timescales of months, which correspond to motions of gas clouds at distances of roughly a hundred light-years from the nucleus. This distance is much larger than what the unification model requires. The astronomers therefore conclude that a single, universal structure in active galaxies is not able to explain all the observed behaviors.