1975 - Stellar black hole identified

Ken Pounds with an X-ray detector built for Ariel 5The science of X-ray astronomy began in the early 1960s, at the same time that a young Assistant Lecturer named Ken Pounds was helping to found the University of Leicester’s Space Research Group. The search for evidence of black holes was an early target of space research, and – remarkably – Leicester X-ray astronomers were able to play a leading role in confirming the existence of both now-established classes of black holes.

Collapsed stars so dense that not even light could escape their gravitational pull had been predicted by Einstein, but it was only after rockets started carrying instruments above the atmosphere that distant X-ray sources could be studied. In 1975, a powerful X-ray source was detected by the Leicester Sky Survey Instrument (SSI) aboard the Ariel 5 spacecraft – causing great excitement among an international group of astronomers who had coincidentally gathered in Leicester that same weekend for a conference.

Dubbed A0620-00, the X-ray source was identified as a highly unusual binary star system, one component of which was quickly established as a black hole: six times the mass of the Sun but about the same size as Leicester. Although not the first black hole candidate to be detected, A0620-00 was considered a particularly strong candidate because the relatively small companion star made studying the black hole easier than in other binary systems.

Ariel 5 satellite under constructionAriel 5 under construction (image: NASA)Further research by Leicester X-ray astronomers, again using Ariel 5, suggested that the extremely bright regions at the centre of many galaxies might also harbour black holes, but many millions of times more massive than A0620-00. That idea was subsequently confirmed when Leicester-led research using the European EXOSAT spacecraft revealed that these bright X-ray sources varied in strength in just a few hours, confirming a powerful energy source in a region so small to be only compatible with a super-massive black hole.

For years, all known or suspected black holes fell into these two groups: stellar or super-massive. Theoretically however there should have been a third group somewhere between these two sizes. In 2010, as the University celebrated 50 years of space research, a group of Leicester astronomers confirmed an X-ray source called HLX-1 as most probably an intermediate mass black hole, a few thousand times the mass of the Sun – thereby continuing the University’s remarkable reputation for advancing the study of these enigmatic objects.