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.
CONTACTS
Sheila McBreen, Space Science Group, Department of Experimental Physics,
University College Dublin, Dublin 4
E-mail: smcbreen@bermuda.ucd.ie
Phone: (+353) (1) 716 2202
Brian McBreen, University College, Dublin
E-mail: brian.mcbreen@bermuda.ucd.ie
Phone: (+353) (1) 716 2202
Lorraine Hanlon, University College, Dublin
E-mail: lhanlon@bermuda.ucd.ie
Phone: (+353) (1) 716 2202
NOTE
Background information on gamma-ray bursts and more about the Swift space
mission are available from
UK contacts for the Swift mission are:
Alan Wells (University of Leicester) aw@star.le.ac.uk
Keith Mason (Mullard Space Science Laboratory, University College London)
kom@mssl.ucl.ac.uk
