Supermassive black holes are billions of times heavier than the Sun and astronomers believe that they lie at the heart of every galaxy. The gravitational pull of these and other black holes is so strong that nothing - not even light - can escape. Black holes appear to be fed by a steady stream of gas and other material that spirals around the body in an "accretion disk", before plunging in. This material gets very hot as it accelerates towards the black hole and emits copious X-rays, which reveal the position of the otherwise invisible black hole.

Although extremely bright, the X-ray emitting region of these disks is very small and astronomers had not been able to measure the size of the emitters directly. It had been predicted that the NGC 1365 X-ray source was about ten times the size of the black hole’s event horizon – the point at which not even light can escape from the black hole. This was confirmed by the sheer luck of a cloud eclipsing the black hole while it was being observed by Chandra. The transit took about two days, which makes the X-ray emitter about one billion kilometres in diameter.

Martin Elvis of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts and a member of the Chandra team told Physics Web that he expects more eclipses of NGC 1365 to occur. Indeed, the team already have plans to use the ESA’s XMM-Newton X-ray telescope satellite to observe these eclipses with the aim of reconstructing the shape of the X-ray source.