8 Jun 2011

 

 

Black holes: feeding habits in X-rays

 

Although black holes are not observable directly, as they emit no light, the environment that surrounds them, and in particular an accretion disk that forms around them when material falls towards the hole, can provide a great deal of information about the central black hole.
A recent study has compared the rate at which matter falls into the super-massive black holes at the centre of active galaxies, with the level of ionization of the material in the accretion disk. In this study, led by David Ballantyne (photo at left), assistant professor at Georgia Tech's School of Physics, and with the collaboration of Jon McDuffie and John Rusin (centre and right of the other photo), archival X-ray data, from the Chandra and XMM-Newton satellites, for 8 Active Galactic Nuclei (AGN) were re-analysed.
They concentrated on the radiation emitted from the hottest, inner-most, regions of the accretion disks around black holes with precisely known masses. These studies are based on the same principle that allows anatomical X-rays to photograph the human skeleton. Although X-rays are very penetrating, they are absorbed to varying degrees as they pass through matter, such as accretion disks.
This differential absorption leaves very characteristic imprints especially on the X-ray spectra of AGN, and these can be used to infer the state of the material through which the X-rays have passed. In the case of black holes, the X-rays are influenced by the gravitational and magnetic fields, and the level of ionization of the material through which they pass. These effects are well demonstrated in laboratory experiments, and allow the observed X-ray spectra to be interpreted.
The main result of the work of Ballantyne's group (already available online in The Astrophysical Journal and in print on 20th June) is that the faster a black hole devours material falling from the accretion disk, the more ionized is the disk, something predicted by models, but not exactly like the relation now observed.
Because the evolution of galaxies is known to be closely connected with the evolution of their central black holes, the discrepancies between current models and the results of Ballantyne and colleagues is sure to stimulate new work also in the field of galaxy evolution.

 

by Michele Ferrara & Marcel Clemens

credit: Georgia Institute of Technology, Gary Meek