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New Gamma-Ray Burst Smashes Cosmic Distance Record

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28 April 2009
Click on image for high-resolution file.

Credit: D. Fox, A. Cucchiara (Penn State Univ.), E. Berger (Harvard Univ.) and Gemini Observatory/NSF/AURA

The fading infrared afterglow of GRB 090423 appears in the center of this false-color image taken with the Gemini North Telescope in Hawaii. The burst is the farthest cosmic explosion yet seen.The most distant cosmic explosion ever seen has been discovered by an international team, which includes astronomers at Penn State, using NASA's Swift satellite and several large telescopes at sites around the globe. The explosion, dubbed GRB 090423, is a gamma-ray burst from a star that died when the universe was 630 million years old -- less than 5 percent of its present age.

"The burst most likely arose from the explosion of a massive star," said Derek Fox, an assistant professor of astronomy and astrophysics at Penn State. "We're seeing the demise of a star -- and probably the birth of a black hole -- in one of the universe's earliest stellar generations."

"Swift was designed to catch these very distant bursts," said Swift lead scientist Neil Gehrels at NASA's Goddard Space Flight Center. Penn State controls the Swift satellite from the Penn State Mission Operations Center at University Park. "The incredible distance to this burst exceeded our expectations -- it was a true blast from the past."

At 3:55 a.m. EDT on 23 April, the Swift satellite detected a ten-second-long gamma-ray burst of modest brightness. Swift quickly pivoted so its Ultraviolet/Optical and X-Ray telescopes could focus directly on the burst location. Swift saw a fading afterglow in X-ray light but saw nothing in visible light. "The X-ray afterglow looks just like hundreds of others that we have seen," said David Burrows, leader of the Swift X-ray Telescope team and a senior scientist/professor of astronomy and astrophysics at Penn State. "Because we saw nothing in visible light but infrared telescopes did see it, we had our first clue that this was a very distant object," explained Pete Roming, leader of the Swift Ultraviolet/Optical Telescope team and a senior research associate at Penn State.

Click on image for high-resolution file.

Credit: NASA/Swift/Stefan Immler

This image merges data from Swift's Ultraviolet/Optical (blue, green) and X-Ray (orange, red) telescopes. No visible light accompanied the burst, which hints at great distance. The image is 6.3 arcminutes wide.

Past a certain distance, the expansion of the universe shifts all visible photons into longer infrared wavelengths. Similarly, ultraviolet (UV) light is shifted into the visible. However, at these large distances, neutral hydrogen, which readily absorbs UV photons, is much more abundant, which is why no visible light from this very distant explosion was able to reach Earth. "We would never have known that this was such a distant object without this confirmation from infrared followup observations," Burrows said.

Within three hours of the burst, Nial Tanvir at the University of Leicester in the United Kingdom, and his colleagues using the United Kingdom Infrared Telescope on Mauna Kea, Hawaii, reported detection of an infrared source at the position Swift had pinpointed. "Burst afterglows provide us with the most information about the exploded star and its environs," Tanvir said. "But because afterglows fade out so fast, we must target them quickly."

At the same time, Penn State's Derek Fox led an effort to obtain infrared images of the afterglow using the Gemini North Telescope on Mauna Kea. The explosion source appeared in longer-wavelength images, but was absent in an image taken at the shortest wavelength (1 micron). This "drop out" corresponded to a jaw-dropping distance of about 13-billion light-years. The previous distance-record holder was a burst seen in September 2008, which showed a redshift of 6.7, placing it 190 million light-years nearer to Earth than the April 23 burst. As Fox spread the word about the record distance, telescopes around the world slewed toward GRB 090423 to observe the afterglow before it faded away.

At the Galileo National Telescope on La Palma in the Canary Islands, a team including Guido Chincarini at the University of Milan-Bicocca in Italy determined that the afterglow's so-called redshift was 8.2. Tanvir's team, gathering nearly simultaneous observations using one of the European Southern Observatory's Very Large Telescopes on Cerro Paranal, Chile, arrived at the same number. This redshift value indicates that the burst exploded 13.035 billion light-years away.

"It's an incredible find," Chincarini said. "What makes it even better is that a telescope named for Galileo made this measurement during the year in which we celebrate the 400th anniversary of Galileo's first astronomical use of the telescope."

Gamma-ray bursts are the universe's most luminous explosions. Most occur when massive stars run out of nuclear fuel. As their cores collapse into a black hole or neutron star, gas jets -- driven by processes not fully understood -- punch through the star and blast into space. There, they strike gas previously shed by the star and heat it, which generates short-lived afterglows in many wavelengths.

Swift is managed by NASA's Goddard Space Flight Center and is controlled from Penn State's Mission Operations Center at University Park. It was built and is being operated in collaboration with Penn State University, University Park, Pa., the Los Alamos National Laboratory in New Mexico, and General Dynamics of Gilbert, Arizona, in the United States, with international collaborators including the University of Leicester and Mullard Space Sciences Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, and additional partners in Germany and Japan.

[ Francis Reddy / Barbara K. Kennedy ]

Derek Fox (Penn State): (+1)814-863-4989, dfox@astro.psu.edu
David Burrows: (+1)814 571-4554, burrows@astro.psu.edu
Peter Roming: (+1) 814-777-7745, roming@astro.psu.edu
Barbara K. Kennedy (Penn State PIO): (+1)814-863-4682, science@psu.edu
Lynn Cominsky (Swift PIO): (+1)707-664-2655, lynnc@universe.sonoma.edu

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