Witnessing the Flash from a Black Hole's Cannibal Act (Forwarded)

ESO Education and Public Relations Dept.

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Contact:
Guido Chincarini
INAF-Osservatorio Astronomico di Brera
Merate, Italy
Phone: (+39) 039 999 1157

Under Embargo until 19:00 CET, 14 December 2005

ESO Press Release 32/05

Witnessing the Flash from a Black Hole's Cannibal Act

Short Gamma-Ray Burst Confirms Formation Scenario

An international team of astronomers reports the discovery
of a third short gamma-ray burst, associated with a nearby
elliptical galaxy. The low level of star formation in such
galaxies and the detection of a second long-lasting flare
indicate that this gamma-ray burst is most likely the final
scream of a neutron star as it is being devoured by a black
hole.

Gamma-ray bursts (GRBs), the most powerful type of explosion
known in the Universe, come in two different flavours, long
and short ones. Over the past few years, international
efforts have shown that long gamma-ray bursts are linked
with the ultimate explosion of massive stars (hypernovae;
see e.g. ESO PR 16/03).

Very recently, the observations by different teams --
including the GRACE and MISTICI collaborations that use
ESO's telescopes -- of the afterglows of two short gamma-ray
bursts provided the first conclusive evidence that this class
of objects originates most likely from the collision of
compact objects, neutron stars or black holes (see ESO PR
26/05).

ESO PR Photo 39/05
Gamma-Ray Burst GRB 050724 (FORS1/VLT)

Caption of ESO PR Photo 39/05: (Left) VLT optical image
taken on July 24, 12 hours after the burst, showing the
position of the gamma-ray burst GRB 050724 as measured by
the Swift X-Ray Telescope (XRT) and the Chandra X-ray
satellite. The blue cross is the position of the optical
afterglow. The burst positions are superimposed on a bright
red galaxy at redshift z=0.258. (Right) Difference between
two VLT images taken on July 24 and 29, clearly revealing
the presence of the GRB.

On July 24, 2005, the NASA/PPARC/ASI Swift satellite detected
another short gamma-ray burst, GRB 050724. Subsequent
observations, including some with the ESO Very Large Telescope,
allowed astronomers to precisely pinpoint the position of the
object, lying about 13,000 light-years away from the centre of
an elliptical galaxy that is located 3,000 million light-years
away (redshift 0.258).

"From its characteristics, we infer that this galaxy contains
only very old stars," says Guido Chincarini (INAF-Brera and
Milan University, Italy), co-author of the paper presenting
the results [1]. "This is similar to the host galaxy of the
previous short GRB which could be precisely localised, GRB
050509B, and very different from host galaxies of long bursts."

These observations thereby confirm that the parent populations
and consequently the mechanisms for short and long GRBs are
different in significant ways. The most likely scenario for
short GRBs is now the merger of two compact objects.

The observations also show this short burst has released
between 100 and 1000 less energy than typical long GRBs. "The
burst itself was followed after about 200-300 seconds by
another, less-energetic flare," says Sergio Campana (INAF-
Brera), co-author of the paper. "It is unlikely that this can
be produced by the merger of two neutron stars. We therefore
conclude that the most probable scenario for the origin of
this burst is the collision of a neutron star with a black
hole [2]."

Notes

[1]: The results are being published in the December 15 issue
of the journal Nature ("Unravelling the origin of short gamma-
ray bursts", by Scott Barthelmy, Guido Chincarini, Dave
Burrows, et al.).

[2]: In the case of a merger between a neutron star and a black
hole, the neutron star may be only partially disrupted in the
initial plunge. The remainder may orbit the black hole,
transferring mass at closest approach ("periastron"), until
the neutron star mass is reduced to less than 2 tenths of the
mass of the sun, where it expands to disruption. The activity
may thus extend over few tens of seconds, unlike the case of
a merging between two neutron stars.

National contacts for the media:

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