Hypernova Blast

Global Chase Ensues

Two billion years ago, in a far-away galaxy, a giant star exploded, releasing almost unbelievable amounts of energy as it collapsed to a black hole. The light from that explosion finally reached Earth at 6:37 a.m. EST on March 29, igniting a frenzy of activity among astronomers worldwide. This phenomenon has been called a hypernova, playing on the name of the supernova events that mark the violent end of massive stars.

With two telescopes separated by about 110 degrees longitude, the Robotic Optical Transient Search Experiment (ROTSE) will have one of the most continuous records of this explosion.

The changing intensity of a gamma-ray burst. On the left is an image of the gamma ray sky showing the burst becoming the brightest object. On the right is a plot of the changing brightness with time. The first gamma-ray burst was seen in the year 1967 (although it was not reported to the world until 1973) by satellite-borne detectors intended to look for violations of the Nuclear Test Ban Treaty. Credit: BATSE

"The optical brightness of this gamma ray burst is about 100 times more intense than anything we’ve ever seen before. It’s also much closer to us than all other observed bursts so we can study it in considerably more detail," said Carl W. Akerlof, an astrophysicist in the Physics Department at the University of Michigan.

Contrary to visible light, gamma rays are non-thermal meaning that they are not produced in hot celestial bodies like the sun. Gamma rays occur in exceptional circumstances such as in the aftermath of a stellar explosion, in the vicinity of black holes, or at the core of active galaxies.

Just recently, the ROTSE group commissioned two optical telescopes in Australia and Texas and were waiting for the first opportunities to use the new equipment. The burst was promptly detected by NASA’s Earth orbiting High-Energy Transient Explorer (HETE-2) but human intervention was required to find the exact location.

Despite sporadic clouds and rainstorms in Australia, the ROTSE instrument at Siding Spring Observatory in northern New South Wales was able to record the decaying light from the blast. Twelve hours later, the second ROTSE telescope in Fort Davis, Texas was picking up the job of monitoring this spectacular explosion.

"During the first minute after the explosion it emitted energy at a rate more than a million times the combined output of all the stars in the Milky Way. If you concentrated all the energy that the sun will put out over its entire 9 billion-year life into a tenth of a second, then you would have some idea of the brightness," said Michael Ashley, faculty member in the astrophysics and optics department at the University of New South Wales and a member of the ROTSE team.

Given that the history of astronomy goes back centuries, observations in the gamma spectrum are really among the newest areas in celestial research. The high-energy light is swallowed by the earth’s atmosphere yet the light cannot be captured with conventional lenses or mirrors. Special detectors in satellites and high altitude research rockets register gamma rays with energies of up to around ten billion electron volts.

M100 Spiral Galaxy
Gamma rays occur in exceptional circumstances such as in the aftermath of a stellar explosion, in the vicinity of black holes, or at the core of active galaxies. Credit: NASA

Fortunately for life on earth, a gamma particle from the universe does not penetrate to the earth’s surface, but if it flies past an atomic nucleus within the earth’s atmosphere, the gamma particle can transform itself into an electron and its (positive) antiparticle, a positron. During its journey through the air, this pair comes across more atomic nuclei and a gamma quantum is generated which then once again hits atomic nuclei. Thus, a single cosmic gamma particle creates roughly a thousand secondary particles in a cascade-like process or sub-atomic shower. Space radiation can be electromagnetic, like x-rays and gamma-rays, or particulate, like protons and electrons. Particulate radiation poses the greater threat to humans in space. Most charged particles in our solar system come from two sources: solar flares, which produce a rain of dangerous protons, and distant supernova explosions, which accelerate atomic nuclei –called "cosmic rays"– to nearly light speed. Solar systems nearer the galactic center would experience increased exposure to gamma rays, X-rays, and cosmic rays, which would destroy any life trying to evolve on a planet.

What’s Next

While they are the most powerful explosions in the universe, gamma-ray bursts are extremely hard to study because they are extremely distant, occur randomly in time and seldom last more than a minute. Small, fast, and relatively inexpensive robotic ground-based telescopes like ROTSE offer the best chance of catching early optical emissions from the bursts. ROTSE attracted national notice in 1999 when it captured the rise and fall of GRB990123, one of the brightest bursts prior to this latest event.

"The ROTSE equipment is quite modest by modern standards, but its wide field of view and fast response allow it to make measurements that more conventional instruments cannot," Akerlof said. "We have two telescopes online now, and installations in Namibia and Turkey will follow soon. Our goal is to have telescopes continuously trained on the night sky. Our motto is "The Sun never rises on the ROTSE array." That’s why we want them spread as widely as possible."

Another role for ROTSE and other small telescopes is to alert larger facilities about gamma ray bursts and other transient phenomena. "One of the most exciting things about an event like this is the way the global community of scientists pulls together, pooling their data and their different capabilities," Akerlof said.

Akerlof is the leader of ROTSE, an international collaboration of astrophysicists using a network of telescopes specially designed to capture just this sort of event. The collaboration is headquartered at U-M and funded by NASA and the National Science Foundation (NSF).

Related Web Pages

Robotic Optical Transient Search Experiment (ROTSE)
Univ. Michigan Physics
Carl Akerlof
Homing Signals
The Use of Gamma-ray Bursts as Direction and Time Markers in SETI Strategies: Corbet
Galactic Gammas from Ground