In the Media

Here is a collection of online press releases, magazine and newspaper articles that feature my work:

Using black holes to measure the expansion of the Universe

In this paper, we show that X-ray reverberation mapping can be used to measure the distance to nearby active galactic nuclei (AGN). I say nearby, but they are still millions of light years away. On cosmological scales that it a tiny distance though! Anyway, it is easy to measure the redshift to AGN, but difficult to measure the distance. If you can measure both, that gives you the Hubble constant, which is the current expansion rate of the Universe. We show that we can in theory measure the Hubble constant quite accurately if we conduct reverberation mapping analyses on about 25 or more AGN. This is important because the two main ways of measuring the Hubble constant currently disagree with one another, and so new methods are needed to break the impasse. The paper can be found at this link.

Astronomy Magazine article on black hole accretion discs

This is a really nice article all about what you can do with the relativistically smeared iron line that we see in the X-ray signal from black hole accretion discs. I feature in a bit about measuring black hole mass with X-ray reverberation mapping. In particular, there is a tale about a paper led by my former PhD student Guglielmo "Gullo" Mastroserio. We used reverberation mapping to measure the mass of the black hole in the famous X-ray binary system Cygnus X-1. This was a really exciting result because it was the first time anyone had ever measured the mass of a stellar black hole with this technique. But we measured the mass to be about 25 solar masses, whereas a much more mature method had previously returned a mass of about 15 solar masses. We thought there must have been something wrong with our assumptions, but then the other mass estimate was revised to about 22 solar masses. It turns out Cygnus X-1 is further away than we used to think it was, and having the wrong distance affected the established method, but not ours. So the new mass value agrees remarkably well with ours! Gullo's paper can be found here.

Numerical Simulations of black hole accretion flows

This paper was led by Matthew Liska, a University of Amsterdam PhD student who I co-supervised. We ran very detailed numerical simulations of a spinning black hole accreting material from a direction tilted with respect to the black hole spin plane. We saw that the accretion disk was made to precess due to the relativistic frame dragging effect. What was new was that we also saw that a jet of material was launched away from the black hole, and this jet also precessed! The paper can be found at this link.

Lense-Thirring precession in H 1743-322

In this paper, I found very strong evidence that the inner accretion flow in the black hole X-ray binary H 1743-322 is precessing due to the frame dragging effect. I measured Doppler shifts in the X-ray radiation from close to the black hole that would result from the inner accretion flow wobbling back and forth. This is currently the best observational evidence that so-called Lense-Thirring precession happens around astrophysical black holes. The paper can be found at this link.