Searching for Black Holes in Space

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!

A brief history of A0620-00 at Leicester

On Friday 1 August 1975 Richard Griffiths, a young researcher in the X-ray Astronomy Group at Leicester University, was on duty examining the data tapes from the Sky Survey Instrument on board the small (135kg) Anglo-US satellite Ariel 5. Mounted on the side of the spinning spacecraft, with spin axis pointing to the Galactic Pole, the SSI was programmed to scan the Milky Way for the next 10 days. In addition to several dozen well-known x-ray sources, he noticed a previously unseen blip as the scan passed through the constellation Monoceros. However, having arranged a weekend away, daily duties were passed to fellow student Martin Elvis. He had won the lottery.

Over the following 2 weeks the ‘blip’ grew rapidly, by 15 August becoming the brightest cosmic x-ray source ever seen, a record to be held for the next 30 years.

Observatories around the world were alerted and within days the new x-ray source (A0620-00, denoting its sky coordinates) had been linked with an optical nova. Further observations had to wait several months while the optical – and x-ray – fluxes fell. When the first optical spectra were obtained, Doppler shifted lines showed a solar-type star racing around a massive but unseen companion at 500km/sec.

That unseen object, with a mass at least 3 times that of the Sun, was to become the first widely accepted stellar mass black hole, and an appropriate object to receive a voice message from Stephen Hawking.

While the closest recorded black hole, at 3000 light years, Hawking’s words will take some time to arrive. Meanwhile, historical records of the 20th magnitude solar-type companion show that it brightens every 58 years, with an expanding atmosphere apparently dumping matter onto the black hole being the origin of the discovery of A0620, and a prediction for budding x-ray astronomers to look out for in 2033, shortly after the planned launch of ESA’s Athena Observatory!

Footnote:

In 1976 Dr Griffiths took a research position at the Harvard Center for Astrophysics, before joining the Hubble Telescope Institute in Baltimore. He later moved to John Hopkins University and then as a full Professor to Carnegie Mellon in Pittsburgh, from where he was seconded in 2006 to the space science office at NASA HQ in Washington. He left NASA in 2013 and now lives on Big Island in Hawaii, with an associate position at the University in Hilo.

Dr Elvis also moved to CfA after completing his PhD and has since enjoyed a long and successful career in X-ray astronomy at Harvard, where he continues to play a leading role as Professor of Astrophysics.

Ken Pounds

Department of Physics and Astronomy

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