Light Shed on Dark Gamma Ray Bursts

Dark gamma ray bursts (GRBs) may not be all that dark

' alt='Artists illustration of a gamma ray burst' >after all, new evidence has shown.

The remnants of an energetic explosion heralding the death of a star, GRBs comprise an intense flash of short-wavelength energy followed by an "afterglow" of X-rays and, in about half of cases, visible light too.

But this has left astronomers wondering why 50% of the GRBs are "dark" in the optical wavelengths.

Many theories had been put forward to explain these mysterious dark objects: perhaps they were a different class of GRB, or the light from bursts in very distant galaxies.

Now, astronomer Patricia Schady, from the Max-Planck Institute for Extraterrestrial Physics in Germany, has found that these dark afterglows are actually the same as the visible ones, except that the optical light has been stripped from them as they travelled across the Universe.

"We found that actually the majority of these gamma ray bursts, that don't have an optical afterglow, need not have occurred in the very early Universe. They could have happened when the Universe was 50 per cent its current age," said Schady.

It would seem that the dust and gas found in the host galaxy, where the burst originated, and also to some extent the dust found in our own Milky Way, absorbed and extinguished the optical wavelengths as the light travelled towards us, making the afterglow appear artifically dark.

Using the specially designed Gamma-Ray burst Optical and Near-infrared Detector (GROND) was the key to this study. According to Schady, "it has seven simultaneous filters so can look in seven different colours, from the optical band up to the mid-infrared band, which is unusual for an instrument."

This, combined with the data collected from the Swift telescope which was deployed in 2004, gave a complete picture of a large number of GRB afterglows studied by the team. And by being able to observe the afterglows very soon after the gamma ray event, the team were able to determine that any loss of optical light was due to dust rather than because the dark GRBs were originating from very distant galaxies or an exotic new class of gamma ray events.

This, among other things, has meant that the current understanding of galaxy and star formation has been confirmed, and from a cosmological standpoint, one of the unresolved mysteries of astronomical observations has finally been put to bed.

Louise Ogden spoke to Patricia Schady from the Max Planck Institute in Munich about the mysterious dark afterglows left over from gamma ray bursts.