ESO 243-49 HLX-1
In the constellation Phoenix
Roughly 20,000 times the mass of the Sun
Diameter roughly equal to the diameter of the planet Saturn
ESO 243-49 HLX-1
One of the last remnants of a defunct galaxy -- a black hole perhaps 20,000 times the mass of the Sun -- may be slowly devouring a companion star, taking a bite when the star comes closest every year or so. As the black hole ingests the stolen gas, it flares brightly, producing an outburst of X-rays that slowly fades -- until the black hole takes yet another bite.
That scenario describes a possible intermediate-mass black hole known as ESO 243-49 HLX-1. (ESO 243-49 is the catalog designation of the black hole's host galaxy, a spiral galaxy similar to the Milky Way in the southern constellation Phoenix. HLX-1 stands for Hyper-Luminous X-ray source No. 1, which means it is the first X-ray source of its type discovered in the galaxy.)
HLX-1 was discovered by a team led by Sean Farrell of Centre d'Etude Spatiale des Rayonnements in Toulouse, France, in observations by XMM-Newton, a European X-ray space telescope. The intensity and wavelengths of the X-rays in these and follow-up observations suggested the object was an accretion disk -- a disk of superhot gas spiraling into a black hole. From the intensity of the X-rays, Farrell suggested the disk was orbiting a black hole at least a few hundred times the mass of the Sun.
After the discovery, Farrell and his collaborators obtained spectra of the system with the Very Large Telescope in Chile. The observations confirmed that HLX-1 is associated with ESO 243-49, although it is well outside the galaxy's flat disk, which is seen edge-on.
The astronomers also monitored the system frequently with NASA's Swift satellite, which sees X-ray and ultraviolet light, and with two of NASA's Great Observatories, Chandra X-Ray Observatory and Hubble Space Telescope.
The observations showed that the system's spectrum is similar to that of binary systems in our own galaxy in which a black hole a few times the mass of the Sun is pulling gas from the surface of a companion star. HLX-1 is hundreds of times brighter than such systems, however, indicating that it is much larger, and that the central black hole is much more massive.
HLX-1 brightens and fades over a period of roughly 367 days. Its X-ray brightness increases dramatically in about 10 days, falls slowly over the next 100-200 days, then stays low until the next flare-up.
A possible explanation is that the accretion disk is fed by a star in an elongated orbit. When the star comes closest, the black hole's gravity pulls gas from its surface. The gas funnels into the accretion disk, heating the disk and causing it to emit a burst of X-rays. Over the next few months, the black hole ingests some of the gas, so the disk fades.
Hubble observations show that in addition to the strong X-rays and ultraviolet light, which come primarily from the accretion disk, HLX-1 also emits blue and red light. Farrell and others interpret that as light from a cluster of stars around the black hole.
Their scenario says the cluster and its central black hole, which is at least 9,000 times the mass of the Sun and most likely about 20,000 solar masses, probably are the remnant core of a small galaxy that orbited ESO 243-49. The larger galaxy's gravity pulled away the stars, gas, and dust in the smaller galaxy's outer regions, leaving only the core.
Another team of astronomers, however, has found that the visible light from HLX-1 also appears to vary dramatically, which would not be the case if the system were a cluster of stars. Instead, the team, led by Australia's Roberto Soria, suggests that most of the visible light comes from the accretion disk.
Additional observations by space- and ground-based telescopes are scheduled to confirm the nature of HLX-1.
Interview with Sean Farrell, postdoctoral fellow at the University of Sydney, on the possible intermediate-mass black hole ESO 243-49 HLX-1.
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This document was last modified: April 30, 2012.