David Ballantyne

Research Summary

David Ballantyne's research concentrates on topics in high-energy astrophysics with an emphasis on intrepretation of X-ray data. His main interest is in the physics of accretion disks and AGN, often making use of data from X-ray telescopes such as XMM-Newton. David Ballantyne is also interested in gamma-ray bursts and the physics of compact objects.

Research Projects:
(September 2002 - August 2003)

X-ray observation of a Seyfert 1 galaxy

David Ballantyne, S. Vaughan and A.C. Fabian (both at Cambridge) analyzed a 300 ks XMM-Newton observation of the Seyfert 1 galaxy MCG-6-30-15. The X-ray spectrum exhibits a very strong and broad iron K line indicative of emission from a relativistic accretion disk a few Schwarzschild radii away from the black hole. They proposed a novel origin for the iron line by showing that it could be split in two: with the broad red wing arising from the inner-most ionized region of the accretion disk, and the central line core originating from the outer regions of a warped disk.

X-ray spectra of accreting black holes

David Ballantyne and A.C. Fabian (Cambridge) examined the possibility that at least part of the iron K line emission commonly observed in the X-ray spectra of accreting black hole may be caused by particle excitation rather than photoionization. They considered beams of thermal and non-thermal electrons, as well as thermal protons, incident on a layer of iron atoms, and calculated the amount of Fe K line flux produced by the impact of the incoming particle. After comparing this flux to that computed from photoionization (assuming the same total energy flux), they found that particle impact was a very inefficient means of producing Fe K line photons.

Absorbing dust in Seyfert 1 galaxies

David Ballantyne, Joe Weingartner and Norm Murray pointed out that part of the dusty warm absorber in the Seyfert 1 galaxy MCG-6-30-15 may originate in the dust lane that is observed to cross the galaxy just below the nucleus. Simple photoionization modeling showed that dusty interstellar gas can produce soft X-ray absorption features even if its 100-200 pc away from the active nucleus. They also discussed various mechanisms to deplete the silicate grains in such an environment since observations have shown that neutral oxygen edge may be very weak in this source.

Density inhomogeneities in accretion disks

David Ballantyne, Neal Turner and Omer Blaes (both at the University of California, Santa Barbara) began an investigation into the effects of density inhomogeneities in accretion disks on the X-ray reflection spectrum. Recent numerical simulations of radiation pressure dominated disks have shown that significant density contrasts can occur in the fluid, which may persist down to small scales near the surface. They found that if density changes occurred within two Thomson depths of the surface then, depending on the illumination strength, significant changes reflection features, such as the iron K line, may occur.

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