Lifting the Veil on Dark Gamma-Ray Bursts

Credit: Aurore Simonnet/Sonoma State University, NASA Education & Public Outreach

Gamma-ray bursts are some of the most energetic explosions in the universe. They come in two varieties: long and short. The short bursts are thought to be the result of the collision and merger of objects like neutron stars and black holes. The long burst (those that last longer than 2 seconds!) are the result of the collapse and explosion of a massive star.

While the burst of actual gamma-rays is short they are often accompanied by a flash of visible light that can last minutes to hours. Astronomers use a variety of telescopes to track down this "afterglow". Gamma-ray bursts (GRBs) are first identified by NASA's SWIFT or other orbiting satellite (gamma-rays do not penetrate our atmosphere). Rapid follow-up observations are often made from ground-based telescopes like the Palomar 60-inch telescope. Through March 2008, the 60-inch telescope conducted follow-up observations of 29 GRBs discovered by Swift. 14 of those bursts were classified as "dark" - meaning there was little or no visible light observed.

GRBs are usually bright enough to be observed from billions of light years, essentially from about as far away as can be observed. Why were these GRBs dark? Astronomers like Brad Cenko and others (formerly with Caltech and now with UC Berkeley) have been checking into the mystery by looking following up on the dark GRBs identified at Palomar with the twin 10-meter Keck telescopes in Hawai'i.

Massive stars live fast and die young. It makes sense that such stars would lie in or near the nebulae from which they were born (like the artwork above). The findings on dark GRBs, announced at the AAS meeting this week, suggest that these GRBs are dark because the massive stars that formed them were hidden in vast, dusty nebulae. These nebulae are dense enough that they absorb much of the light from the explosions that the GRBs.

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