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03 Aug 2012
Astronomers have detected tell-tale luminosity fluctuations in the X-ray signal from a star that was torn apart and devoured by the supermassive black hole at the centre of a distant galaxy. The fluctuations, which have a period of 200 seconds, originate from the innermost stable orbit around the black hole and represent the last signal sent by the debris of the disrupted star before disappearing beyond the black hole's event horizon. The discovery, based on data from ESA's XMM-Newton and the Japan/US Suzaku space observatories, has allowed astronomers to probe the details of matter accretion onto a supermassive black hole in the distant Universe for the first time.
Black holes exist on a variety of scales, from the stellar-mass ones that derive from the collapse of massive stars to the supermassive black holes that reside at the centre of most galaxies and have masses that are millions or even billions of times larger than the Sun's. Regardless of their mass, the phenomena that arise in the proximity of these extremely dense and compact objects due to their intense gravitational fields are quite similar. An effect of the accretion of the surrounding matter onto a black hole is the emission of radiation across the electromagnetic spectrum, which has been detected and studied extensively around stellar-mass and supermassive black holes alike. These observations allow astronomers to probe the behaviour of gravity in its strongest regime and to test general relativity in a wide range of environments, both in our Galaxy (the Milky Way) and in more distant galaxies.
Artist's impression of the debris of a disrupted star being accreted by a supermassive black hole. Credit: NASA's Goddard Space Flight Center.
A small fraction of the supermassive black holes hosted at the centre of galaxies are undergoing 'active' accretion and feeding on a supply of gas at tremendous rates – these are the so-called active galactic nuclei (AGN). However, the majority of supermassive black holes, including the one at the centre of the Milky Way, are in a dormant state and only accrete matter on rare occasions, when a star happens to pass too close to it. In this case, matter on the side of the star facing the black hole experiences a stronger pull with respect to the other side, and this eventually tears the star apart. This phenomenon, referred to as tidal disruption, temporarily switches on the black hole's activity: debris from the shattered star starts orbiting around the black hole in a disc and part of it is rapidly accreted, causing a sudden boost in the luminosity of the galaxy's centre, especially at the highest energies.
Tidal disruption has been spotted in several galaxies in recent years, but the first case of such an event to be observed since its very onset was discovered only in 2011. Originally detected as a gamma-ray burst (GRB) by the NASA satellite Swift, the source, named Sw J1644+57, remained exceptionally bright for a few weeks after its discovery, unlike any other known GRB. After further observations, astronomers were able to link the flaring source to a star that was being disrupted and subsequently devoured by the supermassive black hole at the centre of a distant galaxy.
"The first observations of this intriguing object revealed that, besides being unusually bright, it also displayed rather curious variability," comments Rubens Reis from the University of Michigan, USA. Reis led a study of the luminosity fluctuations of Sw J1644+57, whose results are published online on 2 August 2012 in the journal Science. "We thus exploited target of opportunity observations on the ESA XMM-Newton and the Japan/US Suzaku X-ray observatories to study this source and investigate the origin of its variability," he adds. In particular, XMM-Newton was used to monitor the source for several months.
According to the data collected by Reis and his collaborators, Sw J1644+57 exhibits what astronomers call quasi-periodic oscillations: luminosity fluctuations that occur in a regular fashion but are only seen for a certain period of time before disappearing. In this particular case, the oscillations have a very short period – about 200 seconds – and were only detected in the first observations from both XMM-Newton and Suzaku, which were performed a few days after the discovery of the flare.
XMM-Newton Space Telescope
"When we see a cosmic source undergoing regular variations, it usually means that something exciting is taking place in its vicinity," explains Reis. In particular, quasi-periodic oscillations are known to arise in a very special site around a black hole: the so-called innermost stable circular orbit, which depends on the black hole's mass and spin and defines its range of action. At distances larger than this limiting orbit, matter can revolve around the black hole on stable trajectories, but anything located within this orbit will inexorably precipitate towards the black hole and be quickly accreted onto it. "The quasi-periodic oscillations we detected in Sw J1644+57 tell us that the flare was produced by matter on the edge of an accretion disc around the black hole. This was the last signal that we received from the debris of the disrupted star before being engulfed and disappearing beyond the black hole's event horizon," notes Reis.
Astronomers have identified quasi-periodic oscillations in several stellar-mass black holes across the Galaxy, but so far they had only spotted them once in the vicinity of a supermassive black hole. The discovery of quasi-periodic oscillations in the nearby, AGN-hosting galaxy RE J1034+396, in 2008, was also made with XMM-Newton. "With its large collecting area, XMM-Newton provides us with high signal-to-noise data, thus making it possible to unambiguously identify fluctuations and to detect quasi-periodic oscillations even in the faint light from other galaxies," comments Norbert Schartel, XMM-Newton Project Scientist at ESA.
Sw J1644+57 is the second supermassive black hole in the proximity of which quasi-periodic oscillations have been seen, and the first one hosted in a galaxy that is at a cosmologically significant distance from us – its light having travelled for over 3 billion years before reaching us. "The discovery confirms once more the scale-invariant nature of accretion onto black holes: no matter what their mass is, these compact objects give rise to the same physical processes," says Reis. Besides, having observed the same phenomenon both in a nearby and a distant galaxy indicates that the mechanisms underlying black hole accretion have not changed across the past few billion years of cosmic history. "With this finding, we are finally able to probe the predictions from general relativity in great detail beyond the local Universe," Reis concludes.
Notes for editors:
The findings presented here are based on a series of 12 observations of the source Sw J1644+57 performed with ESA's XMM-Newton X-ray observatory, as well as on one observation performed with the Japan/US Suzaku X-ray observatory .
Sw J1644+57 was discovered on 28 March 2011 by the Burst Alert Telescope (BAT) on board NASA's Swift space observatories and was originally identified as a gamma-ray burst (GRB). However, unlike any other known GRB, the source remained exceptionally bright and highly variable for several weeks. Further observations across the electromagnetic spectrum associated the event with the centre of a small, star-forming galaxy at redshift z~0.35. The flare appears to be due to a tidal disruption event, during which the dormant supermassive black hole at the centre of the galaxy has torn apart and accreted a star.
The XMM-Newton monitoring began about 19 days after the detection of the source and consisted of 12 bi-weekly observations at energies between 0.2 and 10 keV. The team of astronomers also used one Suzaku observation, performed in the same energy range about 9 days after the source was detected.
Related publications:
R. Reis, et al., "A 200-s quasi-periodicity following the tidal disruption of a star by a dormant black hole", 2012, Science, published online on 2 August 2012; DOI: 10.1126/science.1223940.
Related links:
XMM-Newton Science Operations Centre: http://xmm.esac.esa.int/ and http://sci.esa.int/science-e/www/area/index.cfm?fareaid=23
Images, Text, Credit: ESA / Norbert Schartel / University of Michigan / Rubens C. Reis / NASA's Goddard Space Flight Center.
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