Researchers at the Max-Planck-Institute for Astrophysics have developed new
relativistic models which allow predictions of so far unknown properties of
short gamma-ray bursts. Their simulations will come under scrutiny by the Swift
Gamma-Ray Burst Explorer, a NASA mission that is scheduled for launch in the
fall of 2004.

Gamma-ray bursts are among the most energetic and most luminous explosions in
the Universe. They occur roughly once a day, last from a few thousandths of a
second to a few hundred seconds, and come from all different directions of the
sky. Their gamma radiation is more energetic than visible light and can be
measured by satellites orbiting the Earth in space. The energy set free by the
bursts in just one second is comparable to the energy production of the Sun
during its whole life.

The more than 2700 observed bursts are grouped into two distinct classes, one of
which are the so-called long bursts that emit gamma radiation for more than two
seconds, and the other one are the short bursts with durations up to two seconds.

So far only long bursts could be observed in much detail. The detection of
associated afterglows in X-rays, visible light and at radio wavelengths allowed
the determination of their distances and confirmed their origin from host
galaxies at large redshifts, i.e., typically hundreds of millions to billions of
light years away. Until recently the source of these bursts was a mystery. But
evidence has accumulated that they are death throes that accompany the
catastrophic explosions which end the lives of very massive stars. A final
confirmation of this conjecture was provided by GRB030329, a gamma-ray burst
which was detected on March 29, 2003, by HETE, NASA’s High-Energy Transient
Explorer satellite. For the first time this burst could unambiguously be
identified as linked to a peculiar supernova named SN 2003dh at a distance of
about two billion light years.

But where does the gigantic energy come from which powers the gamma-ray burst?
Scientists have coined the theory that the "engine" is a rapidly spinning black
hole which forms when the central core of a dying star becomes unstable and
collapses under its own gravity. This newly formed black hole then swallows much
of the infalling stellar matter and thereby releases enormous amounts of energy
in two "jets". These expand "highly relativistically", i.e. with almost the
speed of light, along the rotation axis of the star. Before they break out from
the stellar surface, they have to drill their way through thick layers of
stellar material, thus getting collimated into very narrow beams with an opening
angle of only a few degrees (see Current Research — March 2000). Indeed,
observations not only confirm the origin of long gamma-ray bursts from exploding
massive stars, but also provide evidence that the gamma emission comes from
narrowly collimated, ultrarelativistic jets with velocities of more than 99.995
per cent of the speed of light.

Rotating, growing stellar mass black holes are also born in other cosmic events,
for example in the violent mergers encountered by binary neutron stars (Fig. 1)
or a neutron star and a black hole (Fig. 1) after hundreds of millions of years
of inspiral, driven by the emission of gravitational waves. The remnant of such
a catastrophy is a stellar-mass black hole sucking matter from a girding, thick
torus of gas (Fig. 2). Such events have long been considered as possible sources
of gamma-ray bursts, and they are still hot candidates for bursts of the short
type, which so far could not be studied by observations in the same way as
bursts from dying stars.

Researchers at the Max-Planck-Institute for Astrophysics have now developed
better computer models that take into account effects due to Einstein’s theory
of relativity. Their simulations can follow the highly relativistic ejection of
matter that is caused by energy release (e.g., due to particle reactions) in the
close vicinity of the black hole. The calculations confirm that short bursts
have properties that are distinctively different from those of long bursts.
Since the black hole — torus system is not buried inside of many solar masses
of stellar material as in case of dying stars, the polar jets do not have to
make their ways through dense stellar layers and quickly reach extremely high
velocities (Fig. 3). As a consequence, they are strongly collimated by the
presence of the accretion torus, but their opening angles are somewhat larger
than those measured for long bursts, typically around 5 to 10 degrees (Fig. 3).
The models predict that outside of these polar cones gamma emission should
become very weak (Fig. 4) so that a gamma-ray burst will be observable only from
one out of hundred mergers when the ultrarelativistic jet is sent towards Earth.
The models also suggest that short bursts can be nearly as bright as long
bursts, although their total energy release is 100 times lower.

Previous gamma-ray satellites were unable to make precise measurements for short
bursts, but there is hope that these predictions can be tested soon. The Swift
Gamma-Ray Burst Explorer, a NASA mission with international participation, is
scheduled for launch in the fall of 2004. One of its prime goals is to unravel
the mysteries of the short bursts.

Literature:

S. Setiawan. M. Ruffert und H.-Th. Janka, Monthly Not. R. Astron. Soc., 352,
753–758 (2004)

M.A. Aloy, H.-Th. Janka und E. M?ºller (2004), Astron. Astrophys., submitted
(astro-ph/0408291).

[NOTE: Images supporting this release are available at
http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2004-9/hl2004-9-en.html
]