Kilonova solves the short GRB puzzle

27 September 2013

Follow-up observations of a recent short-duration gamma-ray burst (GRB) provide the strongest evidence yet that these elusive bursts result from the merger of two neutron stars. The evidence is in the detection with the Hubble Space Telescope (HST) of a new kind of stellar blast – a kilonova.

During the 1990s, the detection of thousands of GRBs by the Burst and Transient Source Experiment (BATSE) revealed two bumps in the distribution of their duration. GRBs were therefore classified as being of either short or long duration, with a dividing line at 2 s. The origin of these brief flashes of gamma rays remained mysterious until the “rosetta stone” burst, GRB 030329 (CERN Courier September 2003 p15). A supernova explosion was found to be associated with this bright, relatively nearby burst of 29 March 2003 and therefore proved that long-duration GRBs result from core-collapse in massive stars. The collapse of the core forms a black hole, which powers a pair of relativistic jets that drill their way through the remains of the dying star and produce an energetic flash of gamma rays (CERN Courier June 2013 p12).

So what is the origin of the short-duration GRBs? Are they really of a different nature? The favoured hypothesis is that they are produced by the merger of two neutron stars, or a neutron star and a black hole (CERN Courier December 2005 p20). Theorists expect such mergers to produce neutron-rich radioactive isotopes, whose decay within days would lead to a transient infrared source. Such a hypothetical transient is called a kilonova because its brightness is about a thousand times that of a typical stellar nova, but is still 10 to 100 times less bright than a supernova explosion.

A team of astronomers led by Nial Tanvir of the University of Leicester now claims to have detected the first kilonova associated with the short GRB 130603B. The burst was detected on 3 June by the Burst Alert Telescope on the Swift spacecraft. The subsequent detection of an optical afterglow allowed the team to pinpoint the location of this genuine short GRB, which lasted only about 0.2 s. The burst occurred in a known galaxy at a redshift of z = 0.356, an ideal target for the sharp vision of the Hubble Space Telescope (HST).

Two HST observations have been performed: one nine days after the burst and the second after 30 days. While no transient source is detected in visible light, the earlier near-infrared image has a point source at the position of the burst’s afterglow, which is no longer present in the later observation. Furthermore, the brightness of this source is found to be significantly in excess of the extrapolation of the afterglow decay to nine days after the burst. This discrepancy reveals the presence of an additional component that Tanvir and his team suggest is the expected kilonova. The time delay, infrared brightness and the absence of emission in the visible light are characteristics that are all consistent with recent calculations for the emission of a kilonova.

If the infrared transient observed by the HST is correctly interpreted, this would be a new milestone in the understanding of GRBs. It would confirm that short GRBs are indeed produced by the merger of two compact stellar objects ejecting neutron-rich radioactive elements decaying in a kilonova blast. This would also be good news for searches for gravitational-wave signals from the merger of compact objects. Detecting the kilonova transient associated with a gravitational-wave signal would allow the location and distance of the source to be obtained, even in the absence of a detectable short GRB when the gamma-ray emission is pointing away from the Earth.

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