Searching for Black Holes in Space

As part of the ‘Legacy of Leicester‘ project, Professor Ken Pounds reflects on Leicester’s role in confirming the existence of both now-established classes of black holes.

John Michell, geologist and rector of a church near Leeds, was the first to contemplate, in a lecture to the Royal Society in 1783, the existence of stars so massive that light could not escape their gravitational pull. The physical basis of `light bending’ came over a century later with Einstein’s theory of General Relativity, and Karl Schwarzschild calculating the scale of the `event horizon’ where that bending would be total.

Evidence that such stellar `black holes’ actually existed was delayed a further half century, when astronomers were first able to observe the sky unimpeded by the Earth’s atmosphere. The key turned out to be the new space-age science of X-ray Astronomy, a field in which Leicester researchers have been involved from the start. After initial exploration using Skylark rocket launches from Woomera in South Australia, we were fortunate to have access to a dedicated satellite, Ariel 5, launched in 1974 as part of an early UK collaboration with NASA.

The Leicester Sky Survey Instrument (SSI) viewed from the side of the slowly spinning satellite with the spin axis being controlled on command from the ground. In the first two years of operation we planned several extended scans along the Milky Way, each lasting several weeks, with the aim of studying how X-ray emission from stars varied. That was to prove an excellent plan, not least in the discovery of several `transient’ sources. The most remarkable was A0620-00, first detected on the 3rd of August 1975 – as astronomers were gathering at Stamford Hall in Oadby for an international conference. Over the following week the X-ray signal increased to become – for a while – the brightest X-ray source in the sky, the Sun apart, a record that was to remain for over 30 years.

Optical and radio telescopes around the world were quickly trained on the direction of A0620-00, soon finding a normal stellar companion. Detailed study of the optical star when the X-ray flare had faded showed that A0620-00 was in a binary star system, the X-ray emission arising from a black hole where some 6 solar masses were confined within a region of space no larger than Leicester. We had been fortunate to be observing that region of the sky with our ‘x-ray eyes’  when a disc of gas drawn from the companion star and surrounding the black hole had become unstable, dumping matter onto the black hole, with gravitational energy being released as heat (and X-radiation) prior to disappearing beneath the hole’s event horizon.

Historically, though A0620-00 was considered by many astronomers to be more secure, the prior claim to have found evidence for a stellar black hole belongs to a group at Harvard led by Riccardo Giacconi, who was awarded the 2002 Physics Nobel prize for his pioneering research in X-ray Astronomy. The Leicester group were to take the next step, however, again with Ariel 5, in establishing powerful X-ray emission to be a key property of `active’ galaxies (AGN). The unusually bright nuclei of many galaxies had intrigued astronomers for many years, with some non-stellar energy source suspected. Finding strong X-ray emission from AGN strengthened conjectures that a supermassive object might lurk in the galactic nucleus, offering a gravitational energy source via a large scale analogue of A0620-00.

A further key step was needed, however, to verify that idea, in particular to show that the X-radiation came from a sufficiently compact region. Again Leicester researchers were able to take that crucial step, taking advantage of the deep space orbit of the European EXOSAT spacecraft to make continuous observations of a number of active galaxies over several days. The remarkable outcome was to find the X-ray flux varied with large amplitude over a few hours, demonstrating the emission region was no more than light hours across. The analogy with A0620-00 was clear, essentially confirming that active galaxies are driven by a supermassive black hole (SMBH) in their nucleus.

It now seems likely that most galaxies contain a SMBH, although – as with A0620-00 – the presence of the black hole is only apparent when being fed with matter, perhaps a passing star or massive gas cloud such as is anticipated to ‘light up’ the quiescent SMBH at the centre of our own Milky Way Galaxy over the next few months.

There are now over 20 confirmed stellar mass black holes in the Milky Way Galaxy, with a great many more lying unseen. Meanwhile, the study of SMBH is a highly topical field and continues to take up most of my research time, with the current focus on understanding how SMBH appear to control the growth of the surrounding galaxy. It now seems that they do so – perhaps counter-intuitively – by emitting powerful winds which blow away the gas in star-forming regions, thereby terminating further growth. At least until the merger with another galaxy triggers a further cycle of growth!

Professor Kenneth Pounds is Emeritus Professor of physics at the University of Leicester and was one of the pioneers of using rockets and satellites for research in the UK. His research is in the area of active galaxies, and one of his many discoveries is that black holes are common in the universe. He was a member of the Science and Engineering Research Council; President of the Royal Astronomical Society; and was seconded as the first Chief Executive of the newly formed Particle Physics and Astronomy Research Council.

Professor Pounds was elected a Fellow of the Royal Society in 1981, and appointed a CBE in 1984.

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