Gamma-ray burst leaves ephemeral afterglow

A ground-based telescope on automatic pilot has recorded the visible-light afterglow of a gamma-ray burst less than 2 minutes after the eruption. One of the most energetic flashes of radiation known in the universe, gamma-ray bursts seem to be generated when a massive star collapses on itself and becomes a black hole or when a black hole merges with a superdense neutron star.

GLOWING IMAGE. Visible-light afterglow just 108 seconds after a gamma-ray burst. Red circle indicates afterglow. This image is actually a double exposure including a supernova (pink circle) in a different patch of sky that the robotic telescope had been observing. Filippenko, S. Jhan, et al./UC-Berkeley

The telescope started taking pictures just 108 seconds after the burst was detected by the High Energy Transient Explorer (HETE)-2 spacecraft. The ground-based device, the Katzman Automatic Imaging Telescope in Santa Cruz, Calif., traced the afterglow for more than 2.5 hours, until dawn halted observations.

“For the first time, we have really good data showing the early time afterglow from a gamma-ray burst and the transition to late-time decline,” says Alex Filippenko of the University of California, Berkeley. His team reported the findings Jan. 9 at a meeting of the American Astronomical Society in Seattle.

HETE-2 detected the 2.5-second gamma-ray burst on Dec. 11, 2002. Just 34 seconds later, the craft put out an e-mail alert. Although another ground-based telescope, dubbed RAPTOR (Rapid Telescopes for Optical Response), recorded the afterglow 43 seconds earlier than the Katzman telescope did, RAPTOR acquired only a single, short exposure.

Because this afterglow was so transient, it may shed light on so-called dark gamma-ray bursts, which don’t seem to have visible-light components, Filippenko says.

Perhaps visible emanations fade away before most telescopes have a chance to record them, he suggests.


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