current MPhil/PhD study topics

S. V. Fuerst
Radiation transfer in General Relativity
Supervisor — K. Wu

I am investigating flowing media around rotating black holes. There are several cases where this is astrophysically relevant. The first is where there is absorbing material obscuring an accretion disk. The disk will emit light, which will then propagate to an observer far away. If there is an absorber along that path, then the resulting spectra will be modified. If the absorber is moving relativistically, then additional affects will come into play. The first part of my thesis deals with creating a realistic physical model of the velocity flow of absorbing material near a black hole, and the radiation transfer of X-ray photons through it.

Another astrophysical situation involving material flowing around a black hole is the accretion disk itself. I am investigating how the accretion disk is ‘puffed up’ by radiation pressure as the accretion rate increases. This causes the gas in the disk to fill a two-dimensional potential well, which is more complicated than the one-dimensional problem of the classical infinitely thin accretion disk model. The expansion of the accretion disk into a torus requires a model of the torus surface so that the resulting observed spectra can be predicted and then compared with the 'normal' accretion disk results. This may provide extra information about the accretion rate in AGN, and reduce the discrepancy between the standard models and the observed spectra from telescopes such as XMM and Chandra.

C. E. Ashton
A Study of Warm Absorbers in Active Galactic Nuclei
Supervisor — G. Branduardi-Raymont

This thesis explores the ‘warm absorber’ phenomenon observed in Active Galactic Nuclei (AGN). Warm absorbers are clouds of ionised gas within AGN, that cause absorption at soft X-ray wavelengths. They are observed in half of all Type 1 AGN, hence they play an important part in the framework of our understanding of Active Galactic Nuclei. Observations with the satellite XMM-Newton have given us the highest signal-to-noise data yet.

XMM-Newton observations of the quasars PG 1114+445 and PG 1309+355 are studied. Both quasars exhibit evidence for absorption by warm material in the line-of-sight. We define a 'phase' of absorption to have a single ionisation parameter and column density. From fits to the data, the absorption in PG 1114+445 is found to be in two phases, a ‘hot’ phase with a log ionisation parameter ξ of 2.57 and a column of 10²² cm⁻², and a ‘cooler’ one with log ξ of 0.83 and a column of 10²¹ cm⁻². The absorption in PG 1309+355 consists of a single phase, with log ξ of 1.87 and a column of 10²¹ cm⁻². The absorbing gas lies at distances of 10¹⁹ — 10²² cm from the continuum radiation sources in these AGN, suggesting origins in a wind emanating from a molecular torus, according to the ‘Standard Model’ of AGN. The kinetic luminosities of the outflowing absorbers represent insignificant fractions ( < 10⁻³) of the energy budgets of the AGN.

Using data for the Seyfert 1 H 0557−385, the warm absorption is characterised by two phases, a phase with log ξ of 0.48 and a column of 10²¹ cm⁻², and a phase with log ξ of 1.63 and a column of 10²² cm⁻². Neutral absorption is also present in the source, and possible origins for this are discussed.

For a large sample, observations of warm absorbers are collated and compared with models.

T. Dwelly
XMM-Newton Multi-wavelength studies of AGN, and the hard X-ray luminosity function
Supervisor — M. J. Page

The extreme luminosity of active galactic nuclei (AGN) mean they can be used as beacons to chart the evolution of the universe back to early epochs. By accurately measuring the space density of AGN as a function of their luminosity and redshift (known as the X-ray luminosity function, XLF), we can map the development of structures in the universe as a whole. Previous optical and X-ray surveys have pricipally detected AGN by their strong broad optical lines and their soft X-ray continua. However, both of these methods are biassed against detecting ‘absorbed’ AGN, which generally exhibit neither of these characteristics. Surveys of the nearby universe have found that these absorbed AGN outnumber their unabsorbed brethren by 4:1. If this ratio holds at higher redshifts, it implies that previous surveys may have missed around 80% of AGN! In fact, the situation may be worse if we are to believe current synthesis models which require an even larger population of absorbed AGN to produce the observed hard X-ray background. Using the XMM-Newton space telecope we are able to detect X-rays at energies harder than 5keV, where the spectra of obscured AGN are less absorbed. In addition, XMM's relatively high throughput permits multi-band X-ray colour analysis to very faint fluxes. I have been working on a Monte Carlo simulation process to enable comparison of current synthesis models with deep (200ks) XMM observations. My simulations incorporate the inherent characteristics of the XMM telscope, including the energy response, effective area, point spread function, vignetting and background. The process permits direct comparison between our observational data and the XLF/NH models suggested in the literature. Initial results suggest that these models underproduce the observed source counts above 5keV.

My work also includes using optical spectroscopic follow-up observations to identify the X-ray sources. I have recently been reducing some multi-slit spectroscopic data from an observing run at the 10m Keck-I Telescope in Hawaii.

P. Schady
A GRB Luminosity Function based on an ACF Width-Time Dilation Factor Correlation
Supervisor — K. O. Mason

The determination of a gamma-ray burst's (GRB) redshift is crucial in understanding the underlying physics. It is needed in order to translate the measured fluences and the angular burst-host galaxy offsets into physical meaningful values, as well as providing a better idea of the space density distribution. In light of the approaching launch of SWIFT a quick method to determine the redshift of a gamma-ray burst would be instrumental in identifying the bursts of greater interest, with those at higher redshifts having precendence.

I am investigating several suggested correlations between various prompt gamma-ray emission parameters which, if credited, would provide redshift estimates from the bursts' gamma-ray light curves alone. In particular, I apply a correlation between the width of a GRB light curve's auto-correlation function (ACF) and its time-dilation factor, 1+z, (Borgonovo 2004) to acquire a redshift estimate for 223 BATSE GRBs. The above correlation suggests a bimodal distribution of the width of the ACF in GRBs, dividing GRBs into ‘broad’ and ‘narrow’ bursts. A preliminary redshift estimate is required from which to categorise a GRB's ‘type’, and for this I use two correlations that relate a GRB's luminosity to the time lag between different energy channels (Norris et al. 2000), and to a variability parameter of the light curve (Reichart et al. 2001).

Using the 223 GRB redshifts and luminosities derived from the above method, a local luminosity function can be constructed with which I am investigating the overall luminosity function assuming the GRB formation rate follows the star formation rate (SFR), of which there are several parameterisations. Although the level of agreement between my results and other suggested GRB luminosity functions should provide an indication of the validity of the redshift estimate method used, satisfactory credence cannot be assigned to the correlations mentioned until a greater number of GRBs with measured redshifts are detected from which the empirical relations observed can be strengthened.