Probing massive black hole with X-ray observations

Chris Reynolds (Astronomy Dept., Univ. of Maryland, College Park)

Recent years have seen dramatic developments in the observational study of accreting black holes. X-ray spectroscopy with ASCA has provided the first observational probe of the innermost regions of black hole accretion disks. X-ray irradiation of the surface layers of the inner accretion disk results in the emission of the prominent fluorescent Ka line of iron in the X-ray regime. The combined action of mildly-relativistic Doppler shifts and strong gravitational redshifts broaden and skew this line in a dramatic and very characteristic manner. Spectroscopy of bright Seyfert nuclei with ASCA confirms that such lines do indeed possess exactly this broadened/skewed profile (Tanaka et al. 1995; Nandra et al. 1997; Reynolds 1997). This is the first direct evidence for a relativistic accretion disk in an AGN. Furthermore, this emission line provides us with a well-understood ``clock'' in orbit about a black hole with which we can study strong gravity and the nature of the inner accretion disk.

Since moving to the University of Colorado, I have been interested in using the iron line to probe the physics of accreting massive black holes. This led to a series of projects involving both the collection of new high quality datasets on key objects, and careful application of physical models to existing data. In particular, I have critically addressed the robustness of claimed detections of spinning black holes in some AGN. It was found that black holes with very different spins can produce somewhat similar iron line profiles depending upon the astrophysical assumptions employed (Reynolds & Begelman 1997a). I have also studied the implications of the iron line strength for the elemental abundance of the accretion disk and the dynamics of the disk corona (Matt, Fabian & Reynolds 1997; Reynolds & Fabian 1997). Finally, I have examined how the iron line varies between different classes of AGN, such as broad-line radio galaxies (Reynolds et al. 1998) and low-luminosity AGN (Reynolds, Nowak & Maloney 2000), and the implications that this may have for the nature of the accretion disks in these objects.

A common theme in these investigations is that several different physical models can fit the same iron line data. In principle, many of these degeneracies can be lifted by studying iron line reverberation --- when the primary source flares, the X-ray echo propagates across the surface of the accretion disk at the (finite) speed of light and the iron line profile changes (reverberates) as different parts of the disk are illuminated. I initiated a program of theoretical study in order to assess the diagnostic power of iron line reverberation (the investigating team has since grown to include Prof. Mitch Begelman in Colorado, Prof. Andrew Fabian in Cambridge, and Dr. Andrew Young in Maryland). To date, we have calculated reverberation effects from isolated flares and shown that there are robust signatures of black hole mass and spin that are within reach of the NASA Constellation-X mission (Reynolds et al. 1999; Young & Reynolds 2000; see Fig.1).

With the launch of Chandra and XMM-Newton, this field has entered a new era. The spatial resolution of Chandra will allow us to bring iron line diagnostics to bear on classes of AGN which, until now, have been overwhelmed by the diffuse emission of the host galaxy or cluster of galaxies. More importantly, XMM--Newton will provide high-throughput data on those iron lines that have already been well studied. We will be able to examine iron line variability associated with structural changes in the disk and/or corona, and orbital motions of X-ray active regions on the accretion disk. XMM--Newton might also provide our first glimpse of X-ray line reverberation. On a related topic, we will be able to use XMM--Newton data to detail subtle lags between different energy bands within the X-ray continuum. Together with Comptonization models, these lags can be decoded to produce constraints on the physics, size, and geometry of the X-ray emitting corona.

I am involved with both Guaranteed-Time and Guest-Observer XMM--Newton observations (as Co-Investigator and Principal Investigator, respectively) of several bright Seyfert galaxies with the principal aim of studying continuum and iron line variability. The first of our datasets have been taken (the guaranteed-time observation of the Seyfert galaxy MCG--6-30-15) and is already producing huge surprises --- this AGN has soft X-ray spectral complexity that is well studied and was thought to arise from bound-free absorption by partially ionized material along the line of sight to the central engine (the so-called warm absorber). However, a careful analysis of the high resolution XMM--Newton grating data by the Columbia University component of our team call this interpretation into doubt. These data are, instead, consistent with there being powerful oxygen, nitrogen and carbon recombination line emission from the surface layers of the inner accretion disk. If vindicated by further work, this is a major change in our X-ray view of some AGN. We are continuing to analyse and model all aspects of these XMM--Newton data, with my personal emphasis being on continuum and iron line variability as seen by the CCD spectrometers. I am also part of the Science Definition Team for the Constellation-X mission, which will be launched c.2010 and has X-ray iron line reverberation as a primary science driver. The recent success of Constellation-X in the decadal report gives us confidence that these studies will be pursued well into the next decade.

While my emphasis is on the observational aspects of AGN, theoretical developments are vital if we are to interpret these new data meaningfully. Our reverberation models must be extended to include the effects of realistic X-ray source geometries and multiple flares. Most importantly, the surface of an AGN accretion disk will be (photo)ionized to some extent, and that ionization could have a dramatic effect on the reprocessed X-ray spectrum. For example, thermal instabilities might lead to the formation of a highly ionized boundary layer blanketing the colder parts of the disk and modifying the reprocessed X-ray spectrum. Also, intermediately ionized states of iron could resonantly trap the fluorescent line photons (which would eventually be destroyed via the Auger process or photoelectric absorption) thereby dramatically reducing the observed line strength. Working in collaboration with Dr.~Andrew Young (UMd) and Mateusz Ruszkowski (soon to be at the Univ. of Colorado), I will assess the effect that this physics will have on iron line reverberation signatures. Such a study must be performed so that we are in a position to interpret data from XMM--Newton and, eventually, Constellation-X.

References

• Matt G., Fabian A.C., Reynolds C.S., 1997, MNRAS, 289, 175
• Nandra K., George I.M., Mushotzky R.F., Turner T.J., Yaqoob  T., 1997, ApJ, 477, 602
• Reynolds C.S., 1997, MNRAS, 286, 513
• Reynolds C.S., Begelman M.C., 1997a, ApJ, 488, 109
• Reynolds C.S., Fabian A.C., 1997, MNRAS, 290, L1
• Reynolds C.S., Iwasawa K., Crawford C.S., Fabian A.C., 1998, MNRAS, 299, 410
• Reynolds C.S., Young A.J., Begelman M.C., Fabian A.C., 1999, ApJ, 514, 164
• Reynolds C.S., Nowak M.A., Maloney P.R., 2000, ApJ, 540, 143
• Tanaka Y. et al., 1995, Nat, 375, 659
• Young A.J., Reynolds C.S., 2000, ApJ, 529, 101

More information

For more information about the topics covered on this web site, please contact Chris Reynolds .