Over The Horizon

Colliding Black Holes?

Alan Gilmore

Astronomers have been debating whether they have spotted the final death throes of a small black hole spiralling into a large one at the heart of a galaxy 400 million light years away. Recent satellite observations of the Seyfert galaxy Fairall 49 (a.k.a. IRAS 18325-5926) suggested that this may have been the case.

Most galactic nuclei are now believed to contain massive black holes, millions of times heavier than the sun. In most galaxies they are rather innocuous objects.

Only if a star or gas cloud ventures too close does the black hole light up. Then the remnants of the star or cloud form into a rapidly spinning disk. Internal friction in the disk -- the inner gas orbiting faster than the gas further out -- causes the disk to heat up. Material near the black hole's boundary gets so hot that it emits x-rays.

Seyfert galaxies are noted for having active nuclei, probably due to the central black hole currently swallowing something. The Fairall 49 Seyfert galaxy has been a focus of attention since observers suggested that its central black hole was about to swallow a smaller, but still massive, black hole. The prediction was based on rapidly decreasing period in X-ray pulsations of the galaxy.

When Fairall 49 was observed with the ASCA satellite in March last year, it showed 16-hour pulsations in its X-ray output. When observed again last Christmas with the RXTE satellite, the pulsation time had decreased to 11 hours.

Further measurements in February found the X-ray period had shrunk to 7.8 hours. The observations suggested that, if the pulsations did indeed originate from an object orbiting Fairall 49's galactic black hole, then the two would collide on April 21.

Well before that date, questions were raised about the observations and potential collision, even by the researchers involved.

The joint Japanese-American and British collaborators noted that the chance of stumbling across a galactic black hole merger in its final is highly improbable: about 1 in a thousand for a galaxy within 120 megaparsecs or 400 million light years of us, the distance of Fairall 49.

The researchers have considered whether possible quasi-periodic oscillations of the galactic black hole might produce the X-ray modulations, but find this mechanism unlikely. One obvious quasi-periodic variation could be caused by a luminous patch on the disk orbiting around the black hole.

The pulsations were best explained by having a small black hole (with a mass between 100 and 10,000 times the mass of the Sun) spiral into a large black hole of around 10 million solar masses.

The intense interest in the phenomenon, however, led to a more prosaic explanation -- an International Astronomical Union circular at the beginning of last month noted that the apparently diminishing X-ray period was merely an artifact produced by the standard background modelling software used with the RXTE images.

Alan Gilmore works at the Mt John Observatory in Tekapo