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NASA's Swift Locates a Short Gamma-Ray Burst

May 11, 2005

The NASA satellite Swift has located to high precision the position of a short-duration gamma-ray burst. This marks the first time that a location with sufficient accuracy to allow ground based observations has been found for a short burst. To this point, only the positions of long bursts had been located to high accuracy.

The gamma-ray burst is a phenomena discovered by nuclear test ban treaty satellites at the end of the 1960s. These events have been studied actively actively since that time, but have yielded their secrets slowly. They fall into two groups: long-duration bursts, which typically last from ten to several hundred seconds, and short-duration bursts, which are two seconds or less in duration. These two types of bursts differ in their gamma-ray spectra, with the short bursts containing a higher fraction of high-energy gamma-rays to low-energy gamma-rays than do the long bursts.

Core-collapse supernovae in the most distant galaxies were found to be the sources of the long-duration gamma-ray bursts. The association between these events was established in the late 1990s when accurate burst positions obtained from x-ray and gamma-ray instruments in the interplanetary network permitted deep observation with large ground-based telescopes. The interplanetary network consists of any interplanetary and Earth-orbiting satellite that carries an x-ray or a gamma-ray detector. By measuring the times of arrival of the gamma-ray burst at each instrument, an accurate direction to the burst can be derived from the time-travel delays between the instruments. When accurate boxes were surveyed in the hours following a burst by large telescopes, such as the Keck optical telescope and the VLA radio telescope array, they found type 2 supernovae. The galaxies containing these supernovae are at high cosmological redshift—the cosmological redshift parameter z ranges from around 1 to several, which implies that we are seeing the supernovae of the first stars to form in our visible universe.

Before this identification with a known object, meaningful theoretical research on gamma-ray bursts had fragmented into a shower of speculative papers attempting to associate gamma-ray bursts with any object, real or imagined, conventional or ludicrous, that provides a large reservoir of energy. The problem is that objects in astronomy are too complex to understand from fundamental physics alone. Not until a definite source was identified were theoreticians given enough guidance to develop reasonable theories for gamma-ray bursts.

Before this month, no short-duration gamma-ray burst had been accurately located; the locations on the sky of short bursts were in error by several degrees, which is much too large to permit counterpart searches with ground-based telescopes. They are known to be extragalactic, because they are distributed uniformly on the sky, but their source is only a speculation at this point. Many researchers have settled on the merger of two neutron stars or of two black holes as the burst engine, but this judgement has a large component of wishful thinking, of a hope that the short gamma-ray bursts produce gravitational waves that could be observed with one of the current or future gravitational wave experiments.

On May 9th, the Swift gamma-ray observatory was able to derive an accurate position for a short gamma-ray burst. Designated GRB 050509B, this burst of 50 milliseconds duration produced a weak x-ray afterglow that enabled Swift to determine a precise position. The burst is located at a right ascension of 12h36m18s and a declination of +28° 59' 28" (epoch J2000) with an error radius of 2.8' (95% containment of the x-rays by the error circle), which places it in the constellation Coma Berenices, not far from the galactic north pole. Observations from the ground show that the burst sits at the position of an elliptical galaxy in the galaxy cluster NSC J123610+285901. With a cosmological redshift of z = 0.225, the galaxy is roughly 1 gigaparsecs from Earth.

This is the first of many burst locations for short bursts. One observation is not sufficient to solve the mystery of their origin, but a large collection of observations should place constraints the possible progenitors.

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