XMM-Newton links black hole to microquasar
18 April 2001Astronomers using XMM-Newton have presented strong new evidence on the correlation between black hole binary systems and microquasars - celestial objects that expel subatomic particles at relativistic velocities, practically at the speed of light. An XMM-Newton observation of one such microquasar in the Milky Way implies that a black hole is almost certainly lurking within.
The source, named GRS 1758-258, is located approximately 26 000 light years from our Sun, at about 5 degrees from the centre of our Galaxy. It was discovered in 1990 by the SIGMA telescope on board the Franco-Russian gamma-ray satellite, Granat. At radio wavelengths this region of the sky also reveals a radio point source, with two nearly symmetrical jets, which is located very close to the X-ray and gamma-ray source position.
For ten years GRS 1758-258 had intrigued high-energy and radio astronomers alike and had come to be considered as a likely black hole candidate. Only three other galactic X-ray stellar systems, which are constantly emitting X-rays and gamma rays, are suspected of containing black holes and only one of them also shows ejections of material at such high speeds. Other candidates do exist but these are transient X-ray sources with outbursts which last only a few weeks or months.
XMM-Newton studied GRS 1758-258 on 19 September 2000 during a 10 000 second observation using the EPIC-MOS and EPIC-pn X-ray cameras. Results from a preliminary analysis of the data have just been presented at the Gamma 2001 Conference in Baltimore, Maryland, USA by Dr Andrea Goldwurm of the Service d'Astrophysique of Saclay, France, on behalf of a group of astronomers working at Saclay and at the University of Leicester, United Kingdom.
"GRS 1758-258 is part of a binary system in which an accretion disc of swirling matter torn from a companion star is being sucked into a black hole," explains Andrea Goldwurm. "The matter is moving so fast that it becomes sufficiently hot to produce X-rays, particularly in the soft X-ray band that can now be studied so well with XMM-Newton."
"GRS 1758-258 is one of two persistent sources which also display radio lobes. The heart of the system not only gives rise to an accretion disc, but the flow of matter is somehow able to expel particles in bipolar jets along the axis of the accretion disc at velocities close to the speed of light. Ripping through the magnetic fields, these fast-moving particles emit radio waves."
Such sources are referred to as 'microquasars' because, on a smaller scale, they resemble quasars which lie in distant galaxies and which also spew out matter in jets. The black holes which reside in quasars are millions of times more massive than our Sun; in nearby microquasars, the black holes are much more modest - usually three to twenty times the mass of the Sun.
Variable ejections are not seen in GRS 1758-258. However, in transient sources, ejections of relativistic particles are often linked to so-called 'state transitions' of the accretion process. These give rise to spectral features which can be observed in the X-ray spectrum of the source.
A 'state transition' describes a kind of 'pivoting' of the spectrum, when the low-energy component increases for a few weeks before returning to normal. The reasons for this change are still not understood. Perhaps a much greater amount of matter is being accreted. What is certain, however, is that when all these characteristics - disc emission, state transitions and relativistic jets - are present, then a black hole is probably lurking nearby.
The XMM-Newton observation of GRS 1758-258 obtained on 19 September 2000 produced spectra that display the typical signature of an accretion disc around a black hole and, for the first time, a 'state transition' was detected. XMM-Newton's improved positional accuracy reduces the error margin of previous observations and confirms that GRS 1758-258 and the microquasar source with its radio jets are one and the same object.
"I was part of a team studying GRS 1758-258 with ROSAT," says Andrea Goldwurm. "XMM-Newton, with its greater sensitivity, has confirmed what we had then presumed. With XMM-Newton we have been able to compare the temperatures of the accretion disc with the different models. It is very satisfying to observe such a source evolving over the years and what we have detected now is extremely important for improving our understanding of X-ray and gamma-ray black hole systems."
XMM-Newton's study of GRS 1758-258 last September had been a scheduled observation. Since then, the Rossi X-ray Timing Explorer satellite has detected that it has changed in an even more dramatic way. This latest transition has prompted a new 'Target of Opportunity' observation by XMM-Newton, which was carried out on 22 March this year. The additional data is currently being analysed by Andrea Goldwurm (with different collaborators) and promises even more interesting results.
GRS 1758-258 will also be a primary target for ESA's forthcoming INTEGRAL gamma-ray observatory.
Based on an oral presentation at the Gamma 2001 Conference:
"First XMM-Newton Results on the Galactic Black Hole Source GRS 1758-258" by A. Goldwurm, D. Israel, P. Goldoni, P. Ferrando, A. Decourchelle, F. Mirabel (Sap/CEA-Saclay) and R.S. Warwick (Leicester University).
Our thanks to Dr Andrea Goldwurm.