University astrophysicists recently have discovered two of the brightest optical flashes from gamma-ray bursts (GRBs).
GRBs are the brightest explosions in the universe, releasing as much energy in tens of seconds as the sun will release in its entire lifetime. They are luminous enough to be visible from over 10 billion light-years away with modestly sized telescopes.
The bursts are believed to be formed when a massive rotating star collapses in a “hypernova” explosion, releasing energy in gamma rays as well as a short, bright optical flash. On average, two GRBs per day are visible on Earth; they are detected in the gamma rays with orbiting space telescopes such as NASA’s Swift Satellite.
The Robotic Optical Transient Search Experiment (ROTSE) telescopes comprise a global array of small, automated telescopes designed for rapid (five-second) responses to GRB triggers from satellites such as Swift. The two bright record-breaking flashes detected recently, GRB060927 and GRB061007, both were observed by the ROTSE-IIIa instrument at Siding Spring Observatory, Australia.
GRB060927 (see picture) was bright enough to be visible on Earth, although it occurred 12 billion light-years away, when the universe was just 8 percent of its current age. The fact that time is stretched out at such a large distance means that scientists can probe the very beginning of the optical event—less than three seconds after the start of gamma-ray emission. At that time, the optical counterpart was the brightest object in the Universe, although it faded very rapidly.
GRB061007 was much nearer to Earth—only 5 billion light-years away—but was bright enough to be seen with high-powered binoculars. This was a dramatic event that increased in brightness by a factor of 15 in just six seconds. The automated analysis software run by the ROTSE team was able to rapidly identify this new transient, and an alert to the community was released in just four minutes from the start of the event, allowing other observers to plan their follow-up observations when the object was brightest and easiest to detect.
“Each of these bursts was bright enough that if they had occurred in the neighboring Andromeda galaxy, they would have been visible to observers on the Earth as brighter than the full moon,” says Eli Rykoff, Physics Department visiting scholar. He further explains that fortunately for people on the Earth, GRBs are extremely infrequent events, particularly in the neighborhood of our local galaxies.
“These two events demonstrate the power of fast robotic systems such as ROTSE to explore even very distant reaches of the Universe,” says Carl Akerlof, physics professor and leader of the ROTSE collaboration.
