Astronomers Use MMT Telescope To Detect Origin of Gamma-Ray Bursts

April 17, 2003
The MMT atop Mount Hopkins, Santa Rita Mountains, Arizona. (Photo: Howard Lester)
The MMT atop Mount Hopkins, Santa Rita Mountains, Arizona. (Photo: Howard Lester)

Astronomers who include several from Arizona early this month used the 6.5-meter MMT telescope on Mount Hopkins near Tucson to get the first direct evidence that powerful gamma-ray bursts come from exploding stars, or supernovae.

NASA's High-Energy Transient Explorer satellite initially detected one of the brightest and closest gamma-ray bursts ever seen, on March 29. The burst, about 2 billion light years away in the constellation Leo, outshone the entire universe in gamma rays for a full 30 seconds. Its optical afterglow remained a trillion times brighter than the sun two hours later.

Astronomers at the MMT, which is jointly operated by the University of Arizona and the Smithsonian Institution, revised their early April observing schedules to catch the afterglow of the burst. They saw spectra of a supernova emerge as the afterglow faded, clinching the case that at least some gamma-ray bursts are triggered in star explosions.

The origin of gamma-ray bursts has been a long-standing mystery. Brief, incredibly intense bursts of gamma radiation coming from unknown sources beyond our solar system were first detected in the late 1960s by U.S. military satellites deployed for nuclear test surveillance, said UA astronomer Daniel J. Eisenstein.

Gamma-ray bursts are hard to study because they occur at random, and few last more than a minute. Great energy from the burst lights up the surrounding gas, creating the optical afterglow, but it is very faint because the bursts are so distant, typically 10 billion light years away. Earth's biggest, most powerful optical telescopes would be needed to monitor such distant afterglows, Eisenstein said.

Eisenstein, Edward W. Olszewski and graduate student Janice Lee of the UA Steward Observatory are among the astronomers who got data on the very recent, relatively close gamma-ray burst using the MMT.

The NASA satellite detected the burst early morning March 29, and that night Lee was observing with Steward Observatory's 2.3-meter Bok telescope on Kitt Peak. She was recruited to observe the burst afterglow, "by far the brightest afterglow ever detected at that late a time and nearly brighter than all events detected within seconds of a burst," she said.

"Not knowing what would develop, I put aside my own thesis project and gladly took an observation of the interesting-sounding object," she said. She also contributed some of her observing time at the MMT to study spectra of the gamma-ray burst afterglow on April 3.

"There have been very tantalizing studies over the past few years that suggest gamma-ray bursts and supernovae are related," Olszewski said.

Olszewski was at the telescope at the right time. He observed the event beginning around 8 p.m. for three nights, from Monday, April 7, through Wednesday, April 9. On Monday night, light from the afterglow had dimmed sufficiently that light from the brightening supernova began to shine through.

"We could see the supernova rising out of the gamma-ray burst. For this object, it was the smoking gun that the gamma-ray burst and supernova are intimately related," Olszewski said.

"For the first time, we were measuring an event no other human being had seen before," said Krzysztof Stanek of the Harvard-Smithsonian Center for Astrophysics, who recruited nearly two dozen scientists to observe the burst, primarily on the MMT. "The MMT was our magic time machine that we used to capture this catastrophic cosmic event," Stanek said.

Lee said that as the news spread that the team was detecting a supernova hidden in the afterglow, "I became extremely excited. I felt very fortunate to be involved in this major discovery. It's all complete serendipity."

Ironically, when Lee became a graduate student at Steward four years ago, she and her advisor during a brainstorming session briefly considered the origin of gamma-ray bursts as a possible dissertation topic.

"I felt it was an intractable question for one graduate student to tackle, since the project would require the cooperation of more than a dozen astronomers all over the country. So in the end, I took on a different thesis project."

"The really important thing is that the MMT got this campaign together. A whole group of people dropped what they were doing to monitor this event in the hopes that it would shed light on the gamma-ray burst – supernova link," Olszewski said. "It really has for this one. It is almost certain that whatever caused the gamma rayburst also led to the supernova."

Until 1997, most of what astronomers knew about gamma-ray bursts was to expect one such event per day from a random location in the sky, Eisenstein noted.

The BeppoSAX satellite launched in 1997 located gamma-ray bursts precisely enough for astronomers to point ground-based optical telescopes at these bursts and study the faint optical afterglows, he added. That's how astronomers learned they were seeing enormously powerful gamma-ray bursts coming from the far distant universe, not relatively less energetic bursts close by, he said.

Stanek and collaborators reported the new results last week onthe Astrophysics preprint server and submitted a paper toAstrophysical Journal Letters.

Arizona State University astronomer Rogier Windhorst, and Nimish Hathi, Rolf Jansen and Luis Echevarria of ASU also used the MMT in this campaign.