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21 December 2006

Cosmic Gamma-Ray Bursts without Associated Supernovae?

Two young astrophysicists at the University of Heidelberg's Centre for Astronomy involved in the discovery of two cosmic gamma-ray bursts unexpectedly not associated with supernova explosions — Publication in today's issue of "Nature"

Two young astrophysicists at the University of Heidelberg's Centre for Astronomy (ZAH) were involved in the discovery of two cosmic gamma-ray bursts (GRBs) that were unexpectedly not accompanied by supernova explosions. This observation challenges the classical model for these high-energy phenomena, whose radiation comes to us from distant galaxies.

These brief gamma-ray bursts are of two kinds. The "shorter" variety lasts only for a few milliseconds and is explained by the fusion of two compact objects with a mass similar to the sun. The "longer" ones take anything from 2 seconds to a few minutes and are thought to evolve when a star several times larger than the sun explodes in a supernova at the end of its life cycle. The conclusion from the new observation (long gamma-ray bursts without supernovae) is that the proportion of GRBs without associated supernovae must be much larger than hitherto assumed.

Gamma-ray bursts (GRBs) are extremely high-energy phenomena associated either with the explosion of a massive star at the end of its life cycle or the fusion of two smaller stars. The data collected over the last few years suggested that short GRBs occur as a result of the collision and subsequent fusion of two celestial bodies of similar dimensions to the sun, while long GRBs indicate an extremely high-mass precursor star exploding in a supernova (this is also known as the "collapsar" model).

So far, this theory has been confirmed in all cases of long gamma-ray bursts by the identification of a corresponding supernova, whereas no indications of a supernova explosion have been detected in connection with short GRBs.

This simple model now appears to be challenged by two new observations. This Thursday, a team headed by Johan Fynbo and including two astrophysicists from the University of Heidelberg's Centre for Astronomy (ZAH), post-doc Arnaud Cassan and doctoral student Marta Zub, reports the discovery of two gamma-ray bursts of long duration for which no simultaneous supernova explosion has been detected, despite a very large number of high-exposure shots that should have identified even faint supernova signatures.

These two events, called GRB060505 and GRB060614 (after the dates on which they were measured), were discovered by the Swift satellite telescope. Their respective durations were 4 and 102 seconds. Measurements by optical telescopes in Chile and Hawaii almost immediately afterwards indicated that the objects were located in relatively close galaxies. In all such long gamma-ray bursts recorded so far, supernova explosions were registered in the days following the bursts. In the present two instances this has not been the case.

Two simple hypotheses might explain the absence of supernova explosions. First, these two GRBs might represent extreme cases of short gamma-ray bursts, though they are substantially longer than any short GRB observed so far and their position in the sky in star-forming regions of close galaxies argue very strongly in favour of massive precursor stars. A second possibility is that these two GRBs were much further away than originally thought and their positions in the sky just happen to coincide with the two foreground galaxies. It is however highly unlikely that this interpretation should square with the facts in both cases. At all events, these two new observations contradict the clear distinction between short and long gamma-ray bursts and their undisputed correlation with the respective mass of the precursor stars involved. This would also mean that the absence of a supernova does not necessarily preclude a massive precursor.

As these events are two out of the six nearest gamma-ray bursts, it is fair to suppose that the fraction of long gamma-ray bursts without associated supernova is much higher than originally assumed. Accordingly, this measurement represents a major challenge to our present understanding of the physical mechanisms of gamma-ray bursts. Obviously, very many more observations of such events are required to confirm or fine-tune possible models.

The article appears in the journal NATURE on 21 December 2006 under the title "No supernovae associated with two long-duration gamma-ray bursts".

Please address any inquiries to:
Prof. Dr. Joachim Wambsganss
Centre for Astronomy (ZAH)
University of Heidelberg
jkw@ari.uni-heidelberg.de

Journalists' inquiries should be addressed to
Dr. Michael Schwarz
Press Officer of the University of Heidelberg
phone: 06221/542310, fax: 54317
michael.schwarz@rektorat.uni-heidelberg.de
http://www.uni-heidelberg.de/presse/index.html


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