Gamma-Ray Burst Caught in the Act

GRB 021004 appeared in a previously blank spot of sky.
Left: The visible-light counterpart of the October 4th gamma-ray burst, shining at 15th magnitude just nine minutes later. 'OT' stands for optical transient. The circle, about 4 arcminutes wide, shows a preliminary position uncertainty from the HETE spacecraft's Soft X-ray Camera. Right: The same field beforehand. Not even the burst's distant host galaxy is visible.
Left: Derek Fox / NEAT – Palomar Oschin Schmidt Telescope. Right: Palomar Digitized Sky Survey.
It came with no warning. At 8:06 a.m. EDT on Friday, October 4th, NASA's High Energy Transient Explorer satellite (HETE-2) detected a strong gamma-ray burst in Pisces. For years astronomers have been trying to pin down exactly what causes these stupendous explosions — but that requires acting fast. Within just 11 seconds the HETE satellite sent out a worldwide alert. Just 38 seconds later, while the burst was still in progress, HETE narrowed down its position to within 10 arcminutes and sent out a second notice containing this crucial information.

Astronomers in the global Gamma-ray burst Coordinates Network (GCN) sprang into action. In California, Derek W. Fox of Caltech awoke to a beeping page and, following a prearranged protocol, used a laptop in his home to seize remote control of the 48-inch Oschin Schmidt Telescope on Palomar Mountain, which was in the midst of an asteroid hunt. The telescope wheeled around to the burst's position and grabbed its first image just nine minutes after the burst began. This image, seen at top, captured the burst's fading visible-light afterglow at magnitude 15.5 — making it the second-brightest afterglow ever snared. It faded by one-third as the giant Schmidt telescope took more images in the next eight minutes.

Other optical and radio observatories followed suit. Japanese astronomers at Kyoto Observatory began tracking the fading afterglow 37 minutes after the burst. Within seven hours Australian astronomers had obtained a spectrum and a preliminary redshift of 1.60. Later spectra indicated that this was merely the redshift of a foreground gas cloud, and that the burster itself has a redshift of 2.3. This implies a distance of about 10.5 billion light-years, or 80 percent of the way across the observable universe.

Gamma-ray bursts (GRBs) are the most energetic and violent events in the cosmos. A typical one briefly outshines its entire galaxy by billions of times. Yet a GRB's short duration (the gamma radiation lasts from a fraction of a second to a few minutes) means the energy source must be incredibly small in size. Most astronomers believe that GRBs result from jets of matter that are expelled extremely close to the speed of light by the small, dense core of a massive star collapsing and spiralling into a newborn black hole.

Last Friday's event was named GRB 021004 for the date it occurred. Astronomers' efforts in the last week have made it the best-observed gamma-ray burst in the 30 years since the first ones were discovered. As of yesterday observations were reported from more than 100 telescopes worldwide, including CCD-equipped amateurs tuned in to the GRB Coordinates Network.

The afterglow is now fading below 21st magnitude. In coming weeks it should dim enough for the underlying host galaxy to show through and be studied. Astronomers hope GRB 021004 will become a turning point in the study of these awesome events. Said George Ricker (MIT), leader of the HETE team, "This is the big one that didn't get away."