Posted: September 22, 2008
Artist impression of material falling into a supermassive black hole, forming an accretion disc and jets, together with the averaged shape of the periodic X-ray signal from REJ1034+396 as detected by XMM. Image: Aurore Simonnet, Sonoma State University (Adapted by M. Gierlinski).
When matter falls into a black hole it is heated up such that it emits X-rays at a certain frequency, which is detected as a periodic signal. The frequency of the signal is linked to the size of the black hole, with smaller objects emitting X-rays more frequently than larger ones. Most black holes observed in this way have masses on the order of ten solar masses. But scientists think that the same signals should also be emitted by supermassive black holes lodged at galactic centres, or active galactic nuclei (AGN), since the process of black hole accretion should be the same for all sizes. But these signals, until now, have not been observed in the largest known black holes.
“With more massive black holes where expected periods [of signal emission] are days or even months, the problem might be in the method the power spectrum is constructed, as it might decrease sensitivity to periodic signals,” says Dr Marek Gierlinski of the University of Durham, lead author of the paper in which the findings are reported. “So, it is easier to detect periodicity in a smaller supermassive black hole, and there are not too many of them.”
Using XMM, astronomers have found the rosetta stone of supermassive black holes, residing in a galaxy known only as RE J1034+396. The black hole itself is thought to be a million times as massive as our Sun, and emits a periodic signal once per hour. The finding confirms that even for an increase in black hole mass of a factor of one million, the same basic process for gas being sucked into a black hole remains the same. Now the quest is on to find out why some black holes show this behaviour and others don't, and to learn more about how matter behaves just before it falls into a black hole.
XMM has been in Earth orbit since 1999, and detects X-rays from objects that the Earth's atmosphere would otherwise block out. Image: ESA/ C. Carreau.
“Perhaps this particular object was unique, or we were very lucky,” says Gierlinski. “We don't know its mass well, but we suspect it is rather low for a supermassive black hole.”
The research group plan to continue their studies of REJ1034+396 for a much longer period, since the reported results are based on just 16 clear cycles of the pulsation.
“The long-term behaviour of the periodic signal, its phase and frequency variations would tell us a lot about these oscillations,” Gierlinski tells Astronomy Now. “This is something we cannot obtain from small black holes in binaries, as periods are very short. Secondly, REJ1034+396 has a rather special X-ray spectral shape, and we want to look at other similar objects.”
The results of the study are presented in the current edition of the journal Nature.