Studying the Andromeda galaxy with ESA's XMM-Newton X-ray space observatory, astronomers have discovered a new bright X-ray source that hosts a stellar-mass black hole accreting mass at a very high rate. The source's location in an external galaxy allowed the astronomers to probe the emission both from the black hole's accretion disc, at X-ray wavelengths, and from its jets, in radio waves.

These observations revealed, for the first time in an extragalactic stellar-mass black hole, the link between the source's X-ray brightening and the ejection of radio-bright material from the vicinity of the black hole into the jets, indicating an accretion rate close to the black hole's Eddington limit, or even above it.

Black holes are the densest objects in the Universe and, even though they do not emit light, astronomers can detect them indirectly because of their effect on the surrounding matter.

A black hole's intense gravity attracts nearby matter, which falls onto the dense compact object via an accretion disc. As it is accreted onto the black hole, the matter emits large amounts of X-rays.

Both in stellar-mass black holes - which derive from the collapse of very massive stars - and in the supermassive black holes that sit at the centre of massive galaxies, the accretion of matter is accompanied by the release of powerful jets.

These outflows of energetic particles can be observed across the electromagnetic spectrum, and are especially bright at radio wavelengths.

Accretion discs and jets are two aspects of the same phenomenon, and by studying their connection astronomers can investigate the physical mechanisms that take place close to a black hole.

"We have now found a black hole that allows us to see the coupling between accretion disc and jets 'in action'," comments Matthew Middleton from the University of Durham, UK, and the Astronomical Institute 'Anton Pannekoek' at the University of Amsterdam, in the Netherlands.

Middleton led a study of XMMU J004243.6+412519, an X-ray binary in the Andromeda galaxy which consists of a stellar-mass black hole that is accreting matter from a low-mass companion star. The source was discovered using ESA's XMM-Newton X-ray space observatory. The results of the study are published online on 12 December 2012 in the journal Nature.

"When we observe an accreting black hole in our Galaxy, the Milky Way, we look at it through the interstellar material in the Galactic Plane, which absorbs most of the X-rays from the black hole's accretion disc. When we observe other galaxies, instead, our line of sight goes through much smaller amounts of interstellar material in the Galactic Plane, so the X-ray emission from the disc is clearer," explains Middleton.

The source was discovered in January 2012 by an X-ray monitoring programme of the Andromeda galaxy led by Wolfgang Pietsch from the Max-Planck Institut fur Extraterrestrische Physik in Garching, Germany.

"Andromeda is the closest large galaxy to the Milky Way and hosts a rich variety of stellar populations, ranging from very young stars to the compact remnants of stars that have already concluded their life cycle," says Pietsch.

The programme has been observing Andromeda's population of novae - binary systems where a white dwarf is accreting mass from a companion star - with XMM-Newton and NASA's Chandra X-ray Observatory.

"If they are part of a binary system, stellar remnants such as white dwarfs, neutron stars and black holes may accrete matter from their companion and, in the process, shine brightly in X-rays.

Our programme was designed specifically to monitor novae, which emit mainly soft X-rays below 1 keV. However, an interesting by-product is the detection of X-ray binaries, which host neutron stars or black holes and emit well beyond 1 keV," Pietsch adds.

About ten days after its discovery, XMMU J004243.6+412519 underwent a dramatic brightness boost. With a luminosity in excess of 1039 erg/s, it was classified as an ultra-luminous X-ray source, or ULX. This is only the second ULX known in the Andromeda galaxy.

"After this exceptional brightening, the X-ray spectrum of the source changed too, revealing a stronger contribution from the accretion disc," says Middleton.

"These changes reflect the transition to a new regime of mass accretion: by then, the black hole was accreting matter at a rate that was very close to its Eddington limit, or even above it."

The Eddington limit identifies the maximum luminosity that can be radiated by matter as it falls onto a black hole, and depends on the black hole's mass. This key value is defined for a spherical accretion process, so can be exceeded in the context of disc accretion. However, the theoretical understanding of mass accretion breaks down at the Eddington limit.

Data from black holes that are accreting beyond this limit can help astronomers understand how the structure of the disc changes at such high accretion rates and, possibly, also what triggers the outflow of material from the vicinity of the black hole.

"We suspected that the transition to a higher mass accretion rate might be linked to the onset of radio jets, so we used radio telescopes to monitor the outflowing material," says Middleton.

"The radio data exhibit multiple flares on timescales of just a few days, suggesting that the increase in the mass accretion rate might have triggered a 'ballistic' jet - a series of consecutive ejections of material," he adds.

In the Milky Way, there are only four black holes known to accrete at a high enough rate to create these jets. But by extending the search to other galaxies, many more may be found - the astronomers already know about a hundred candidates.

Like the one discovered in Andromeda, these are valuable as they allow astronomers to study the connection between the accretion disc and jets without the interference of interstellar absorption.

"The discovery of a black hole in Andromeda accreting close to its Eddington limit extends the study of stellar-mass black hole accretion beyond the Milky Way," comments Norbert Schartel, XMM-Newton Project Scientist at ESA.

Stellar-mass black holes allow astronomers to study the dynamics of accretion in great detail as the process happens on much shorter temporal scales than in supermassive black holes.

"Further observations of similar sources in other galaxies will deepen our understanding of the nature and origin of the powerful jets that stem from stellar-mass black holes as well as from the supermassive black holes that are hosted in active galaxies," he concludes.

M. J. Middleton, et al., "Bright radio emission from an ultraluminous stellar-mass microquasar in M31", Nature, 2012. DOI: 10.1038/nature11697