杏吧原创

GAMMA-RAY bursters are among the most powerful explosions in the Universe. In
less than a second they emit as much energy as a supernova releases鈥攁nd
all of it is in the gamma-ray part of the spectrum. Theorists have come up with
scores of proposals, including some quite outlandish ones, to explain how energy
is released at such a phenomenal rate, but now a team of astronomers is
speculating that it could be the result of a relatively simple act of creation.
鈥淲e may actually be witnessing the birth cry of a black hole,鈥� says Roy Maartens
of the University of Portsmouth.

Astronomers agree that the fading of gamma-ray bursts fits the
鈥渦ltra-relativistic fireball鈥� model put forward by Martin Rees of the University
of Cambridge and Peter M茅sz谩ros of Pennsylvania State University.
In this model, matter heated to billions of degrees explodes at close to the
speed of light. 鈥淪hocks鈥� in the fireball generate gamma rays. But till now
nobody has come up with a convincing power source for the fireball.

Among the suggestions have been mergers of neutron stars, and a black hole
swallowing a neutron star. But the problem of accounting for how energy could
get out fast enough has led theorists to focus on stars collapsing
catastrophically in on themselves.

The end result of such a collapse is a black hole surrounded by a 鈥渉orizon鈥�
from which nothing can escape, not even light. Inside, the gravitational field,
density and temperature reach extraordinary levels, but astronomers have long
discounted this as the source of gamma-ray bursts because the horizon would
prevent any of the super-hot matter from escaping. However, this may not always
be the case, say Maartens and his colleagues Pankaj Joshi of the Tata Institute
of Fundamental Research in Bombay and Naresh Dadhich of the Inter-University
Centre for Astronomy and Astrophysics in Poona.

Joshi and others have shown that Einstein鈥檚 theory of gravity allows the
possibility that the horizon may not form immediately after the star collapses.
There could be a brief window during which high-energy particles can escape.
鈥淭he window may be open for only a few milliseconds, but it could be enough to
energise a relativistic fireball,鈥� says Maartens. 鈥淭he gamma-ray burst would be
the brief and only `cry鈥� of the black hole during its birth.鈥�

Some other theorists are not yet convinced. According to Bohdan Paczynski of
Princeton University, there is plenty of observational evidence that gamma-ray
bursts are a special type of supernova caused when massive stars die with a more
spectacular bang than most. 鈥淭he real mystery is what concentrates the energy
that leads to the ultra-relativistic fireball and the gamma-ray emission.鈥�

The proposal is supported, however, by very recent calculations by Tomohiro
Harada of Waseda University, Hideo Iguchi of Osaka University and Ken-ichi Nakao
of Osaka City University. They also favour nascent black holes as the source of
bursters, and calculate they may indeed be able to produce sufficient energy.
But proving that the model is right is going to be difficult. 鈥淲e have not yet
probed it in detail to see if it has any distinctive observational signature,鈥�
says Maartens.