No matter their size black holes 'feed' in the same way
Research by Southampton astronomers, published in Nature (7 December 2006) reveals that the processes at work in black holes of all sizes are the same and that supermassive black holes are simply scaled up versions of small Galactic black holes.
For many years astronomers have been trying to understand the similarities between stellar-mass sized Galactic black hole systems and the supermassive black holes in active galactic nuclei (AGN). In particular, do they vary fundamentally in the same way, but perhaps with any characteristic timescales being scaled up in proportion to the mass of the black hole. If so, the researchers proposed, we could determine how AGN should behave on cosmological timescales by studying the brighter and much faster galactic systems.
Professor Ian McHardy, from the University of Southampton, heads up the research team whose findings are published today (along with colleagues Drs Elmar Koerding and Christian Knigge and Professor Rob Fender, and Dr Phil Uttley, currently working at the University of Amsterdam). Their observations were made using data from NASA's Rossi X-ray Timing Explorer and XMM Newton's X-ray Observatory.
Professor McHardy comments: "By studying the way in which the X-ray emission from black hole systems varies, we found that the accretion or "feeding" process - where the black hole is pulling in material from its surroundings - is the same in black holes of all sizes and that AGN are just scaled-up Galactic black holes. We also found that the way in which the X-ray emission varies is strongly correlated with the width of optical emission lines from black hole systems."
He adds: "These observations have important implications for our understanding of the different types of AGN, as classified by the width of their emission lines. Thus narrow line Seyfert galaxies, which are often discussed as being unusual, are no different to other AGN; they just have a smaller ratio of mass to accretion rate."
The research shows that the characteristic timescale changes linearly with black hole mass, but inversely with the accretion rate (when measured relative to the maximum possible accretion rate). This result means that the accretion process is the same in black holes of all sizes. By measuring the characteristic timescale and the accretion rate, the team argues this simple relationship can help determine black hole masses where other methods are very difficult, for example in obscured AGN or in the much sought after intermediate mass black holes.
Professor McHardy continues: "Accretion of matter into a black hole produces strong X-ray emission from very close to the black hole itself. So, studying the way in which the X-ray emission varies with time, known as the X-ray lightcurves, provides one of the best ways of understanding the behaviour of black holes.
"It has been known for over two decades that characteristic timescales can be seen in the X-ray lightcurves of Galactic black hole systems. The timescales are short (<second) and so can be found in short observations. However to find the equivalent timescales in AGN is much harder as we must observe for months or years."
Notes for editors
The paper 'Active galactic nuclei as scaled-up Galactic black holes' by Professor Ian McHardy and Drs Elmar Koerding and Christian Knigge and Professor Rob Fender of the University of Southampton (UK), and Dr Phil Uttley of the University of Amsterdam, is published in Nature (7 December) pp 730-732.
For further information:
Preliminary work by Professor McHardy in 1988 indicated that a similar characteristic timescale was present in the AGN NGC5506, but high-quality, long timescale monitoring observations, needed to measure the timescale accurately, were not possible until the launch of the NASA Rossi X-ray Timing Explorer in 1995. Since that time, a number of groups around the world, including one at the University of Southampton, have been making suitable observations. Combined with shorter timescale observations, for example with the ESA XMM-Newton X-ray Observatory, these observations have now enabled characteristic timescales to be found in over a dozen AGN.
A rough linear scaling of characteristic timescale with black hole mass was soon confirmed but it was clear that, for a given black hole mass, there was a large spread in characteristic timescale. The present paper shows that the spread is entirely accounted for by a spread in accretion rate rather than by any other parameter such as black hole spin. The origin of the characteristic timescale is not known, but it is suspected that it might be associated with the location of the inner edge of the accretion disc, close to the black hole. With higher accretion rates, this edge may be pushed closer towards the black hole, resulting in shorter characteristic timescales.
It has also been known for many years that the optical emission lines in AGN are narrower in AGN which are 'more variable'. However this observation has never previously been properly quantified or explained.
In their paper, the team show that the width of the lines is correlated very strongly with the characteristic X-ray timescales. "Using some basic physical assumptions about the gas which emits the emission lines, and some very simple mathematics, we showed that the observed relationship between line width and characteristic timescale is exactly what is expected, as long as the characteristic timescale is proportional to the ratio of the black hole mass and accretion rate," says Professor McHardy. "Our optical observations provide very strong confirmation that the characteristic timescale which links large and small black holes is just proportional to the ratio of the black hole mass to accretion rate. So AGN really are just scaled-up galactic black holes."
The work was supported by the UK Particle Physics and Astronomy Research Council (PPARC).
Dr Uttley, who obtained his PhD from the University of Southampton under the supervision of Professor McHardy, will be returning to the University of Southampton as a lecturer in February 2007.
The University of Southampton is a leading UK teaching and research institution with a global reputation for leading-edge research and scholarship. It is one of the UK's top 10 research universities, offering first-rate opportunities and facilities for study and research across a wide range of subjects in humanities, health, science and engineering. The University has around 20,000 students and over 5000 staff. Its annual turnover is in the region of £310 million.
The Particle Physics and Astronomy Research Council (PPARC) is the UK's strategic science investment agency. It funds research, education and public understanding in four broad areas of science - particle physics, astronomy, cosmology and space science. PPARC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Organisation for Nuclear Research, CERN, the European Space Agency and the European Southern Observatory. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, Australia and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility.