Catching a burst's visible glow

When James Wren's beeper woke him just before 3 a.m. last Saturday, he knew the routine. From his home computer, he checked that a robotically operated telescope 8 kilometers away was recording images from the correct patch of sky. Then Wren, an astronomer at the Los Alamos (N.M.) National Laboratory, went back to sleep.

 Little did he know that the event that made the telescope swing into action was the most energetic cosmic eruption ever detected. Or that the telescope had for the first time captured the visible glow of a gamma-ray burst while it was still spewing high-energy radiation.

 Although optical telescopes have recorded the glowing embers of some 12 bursts, "this is the first time we've seen the counterpart of the fire," says Bradley E. Schaefer of Yale University. Carl W. Akerlof and Timothy A. McKay of the University of Michigan in Ann Arbor reported the find in a Jan. 23 circular of the Gamma-Ray Burst Coordinates Network.

 The optical observations, taken at Los Alamos with a telephoto-camera array known as ROTSE-I (Robotic Optical Transient Search Experiment I), owe their success to an early warning network set up by Scott D. Barthelmy of NASA's Goddard Space Flight Center in Greenbelt, Md. Whenever one of several satellites detects a burst, the network instantly alerts ground-based telescopes to search for a visible glow.

 On Jan. 23, just 22 seconds after NASA's Compton Gamma Ray Observatory found a burst in the constellation Bootes, ROTSE-I began scanning the same region. To the surprise of many astronomers, the glow spied by the telescope was so bright that it could have been seen with a pair of binoculars.

 Spectra reveal that the burst came from a galaxy some 9 billion light-years away. At that distance, the intensity of the gamma-rays unleashed during the 100-second-long burst indicates that it was the most energetic ever recorded, surpassing a burst dubbed the second Big Bang. If the burst emitted radiation equally in all directions, its total output equaled the explosive energy of 2,000 supernovas.

 The actual energy could be less. Stan E. Woosley of the University of California, Santa Cruz has proposed that bursts are produced by asymmetric explosions that beam their energy in one direction. One aimed at Earth could appear 100 times more energetic than it is.

 In addition, a cosmic mirage might have caused the Jan. 23 burst to appear brighter, says S. George Djorgovski of the California Institute of Technology in Pasadena. Either of two galaxies that apparently lie in front of the burst's home galaxy could have acted as a gravitational lens, bending and focusing the light. Such lenses can also create time-delayed images, raising the possibility that new pictures of the burst may only appear after days or months.