The discovery of a black hole enjoying a feeding frenzy in our nearest neighbor galaxy, Andromeda, has provided new insights into a mysterious class of extreme astrophysical objects called “ultraluminous X-ray sources”.
It isn’t unusual for material falling into a black hole to generate copious X-ray emission, but ultraluminous X-ray sources are so bright that they sometimes outshine their entire host galaxy in the X-ray band. Astronomers have spent years debating the nature of these enigmatic objects and two main scenarios have emerged. Either ultraluminous X-ray sources are unusually massive black holes feeding steadily on gas from an orbiting companion star, or alternatively they may be black holes around ten or twenty times as massive as our Sun that are somehow being force-fed by the in-falling gas.
By using a number of Earth-orbiting X-ray telescopes, including NASA’s Swift and Fermi satellites, as well as the European Space Agency’s XMM-Newton observatory, a large international team of astronomers were able to watch as the X-ray emission from a black hole in the Andromeda galaxy – over 2 million light years away – brightened dramatically and then faded again over the course of 6 months during early 2012.
This pattern of X-ray emission is very reminiscent of behaviour that is exhibited by relatively low mass black holes found in our own galaxy – the Milky Way – when they are suddenly flooded by fresh material from their companion star. As they swallow the deluge of in-falling gas, these black holes also launch powerful beams of magnetized plasma called jets, which stream outwards at speeds close to that of light, and shine brightly at radio wavelengths.
Discovering such radio jets from an ultraluminous X-ray source provides a clear indication that these are just normal, everyday black holes.
To search for these telltale relativistic jets, the team trained the US National Science Foundation’s Karl G. Jansky Very Large Array on the black hole, and detected extremely bright, rapidly varying radio emission that dropped by a factor of two within just half an hour. This rapid variability tells us that the region producing radio waves is extremely small in size; no further across than the distance between Jupiter and the Sun.
This surprising finding was confirmed by zooming in using a super-high-resolution radio telescope called the Very Long Baseline Array. Despite the large distance to Andromeda, the absence of dust and gas in that direction allows an unimpeded view of the extreme astrophysical processes occurring in black hole’s immediate surroundings. In fact, this is the first time that radio jets have been detected from such a low mass black hole outside our own galaxy.
The research was published in the scientific journal Nature. M. J. Middleton, et al., “Bright radio emission from an ultraluminous stellar-mass microquasar in M31”, Nature, 2012. DOI: 10.1038/nature11697