Spinning black hole clue in gamma-ray burst signatures

Spinning black holes could be responsible for at least some of the immensely powerful gamma-ray bursts (GRBs) astronomers observe coming from distant galaxies. On Tuesday 8 April, Sheila McBreen of University College, Dublin, will tell the UK/Ireland National Astronomy Meeting in Dublin that her analysis of the way gamma-rays were emitted over the course of outbursts from a large sample of GRBs has revealed particular signatures, most likely to be those of a rotating black hole either being “spun up” or “spun down”.

The mechanism behind gamma-ray bursts is still not known for certain, though there are several plausible theories and much progress towards understanding these catastrophic cosmic events has been made over the last few years. “The detail in the structure of the gamma-ray signals holds key information about what is happening to the ’central engine’ during a burst event,” says Sheila McBreen, “but they are not easy to analyse”.

A typical burst consists of a series of pulses spanning a short period of time that can be as little as a fraction of a second or up to several minutes. The number of pulses, their shapes and the times between them, vary greatly, making the comparison of GRBs difficult. To tackle this problem, for each burst in her large sample McBreen and colleagues converted the record showing gamma-ray output changing with time into a cumulative form by adding together all the gamma-ray counts that had gone before. This had the effect of producing smoother profiles that were easier to compare and interpret.

Most of the cumulative profiles increased steadily with time, which means a remarkably stable output over the course of a burst. However, in the case of 19 bright GRBs, there were significant sections in the cumulative profile where it rose much more quickly (as the square of time) and in 11 others places where it rose more slowly (as one minus the square of time). “The most likely interpretation of these observations is that they are the signatures of black holes that are being spun up or down”, says Sheila McBreen.

Gamma-ray energy, she argues, could come from a disk of material swirling around a rotating black hole. Her findings are consistent with two possible ways of releasing energy from the disk. One of them involves the annihilation of neutrinos emitted from the disk, which is very hot. When the spin of the black hole increases during the process, it would give rise to the rapid rises seen in cumulative gamma-ray profile. Alternatively, the energy could come from the interaction between the black hole and its surrounding magnetic field. This mechanism could account for both the rapid and slow rises, according to whether the black hole’s spin is getting faster or slower. A third scenario, flares on the disk, appeared not to be consistent with the “spin up and spin down” observations.

The mystery of gamma ray bursts may be finally solved after the launch in December 2003 of Swift, a NASA-led space observatory with international participation, dedicated to observing GRBs. Several universities in the UK are involved with Swift.