No matter their size black holes "feed" in the same way

Research by UK astronomers, published today in Nature (7th 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.

An accreting black hole and a binary star
Credit: R Hynes

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 Dr Elmar Koerding, Dr Christian Knigge, 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."

An Artist's impression of an intermediate sized black hole
Credit: NASA Goddard

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

Contacts

• Professor Ian McHardy
  School of Physics and Astronomy
  University of Southampton (UK)
  Tel: +44 (0) 238 059 2101
• 
  
• Sarah Watts
  Media Relations
  University of Southampton
  Tel: +44 (0) 238 059 3807
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• Gill Ormrod
  PPARC Press Office
  Tel: +44 (0) 179 344 2012
  Mobile: +44 (0) 781 801 3509

Image and caption

• Image - An X-ray binary system consisting of an accreting black hole and a binary star.
  Credit: R Hynes
• Image - An Artist's impression of an intermediate sized black hole, which exist in the heart of spiral galaxies throughout the Universe.
  Credit: NASA Goddard

The paper 'Active galactic nuclei as scaled-up Galactic black holes' by Professor Ian McHardy, Dr Elmar Koerding, Dr Christian Knigge, Professor Rob Fender of the University of Southampton (UK) and Dr Phil Uttley of the University of Amsterdam, is published in Nature today (7 December) pp 730-732.

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.

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

Professor Ian McHardy

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 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.

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