Thursday, December 07, 2006

Black hole feeds

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

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.
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.

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.

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 and so can be found on short observations. However to find equivalent timescales is much harder as we must observe for months or years at a time.

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 active galactic nuclei AGN really are just scaled-up galactic black holes."

Read more @ PPARC press release 07 Dec 2006
Supermassive Blackhole consumes A Star @ Universe Today
Where do stars go when they die? Podcast @ Universe Today
Do Galaxies follow Darwinian Evolution? from ESO Outreach.

Labels: , ,