Scientists using the NASA Swift satellite and several ground-based telescopes have detected the most distant explosion yet, a gamma-ray burst from the edge of the visible Universe.
This powerful burst, likely marking the death of a massive star as it collapsed into a black hole, was detected on September 4. It comes from an era soon after stars and galaxies first formed, about 500 million to 1 billion years after the Big Bang. The science team cannot yet determine the nature of the exploded star; a detailed analysis is forthcoming.
“This is uncharted territory,” said Dr. Daniel Reichart of the University of North Carolina at Chapel Hill, who spearheaded the distance measurement. “This burst smashes the old distance record by 500 million light years. We are finally starting to see the remnants of some of the oldest objects in the Universe.”
Only one quasar has been discovered at a greater distance. Yet whereas quasars are supermassive black holes containing the mass of billions of stars, this burst comes from a single star. Scientists say that it is in fact puzzling how a single star could generate so much energy as to be seen across the entire Universe. This early star is perhaps physically different from the kinds that exist today.
Scientists on four continents using a multitude of telescopes tracked the burst and its afterglow for days as the burst gradually faded, a true worldwide effort. The discovery is being heralded as a major breakthrough in the study of the early Universe. Few quasars are seen at exceedingly far distances, or the earliest epoch of star and galaxy formation, despite exhaustive searches. Yet gamma-ray bursts might be plentiful, according to Prof. Donald Lamb of the University of Chicago, who with Reichart had predicted the detection of very far gamma-ray bursts.
“This is what we’ve been hoping and waiting for,” Lamb said. Lamb anticipates the discovery of scores, if not hundreds, of very distant gamma-ray bursts in the coming years. These bursts would provide information about where and when the earliest stars formed and what chemical elements they produce when they explode. Traveling across the entire Universe, the light from the bursts would also contain information about all the material it passed through on its long journey towards us. “The fun is just beginning,” Lamb said.
Scientists measure cosmic distances via redshift, the extent to which light is “shifted” towards the red (lower energy) part of the electromagnetic spectrum during its long journey across the universe. The greater the distance, the higher the redshift.
The September 4 burst, named GRB 050904 for the date it was detected, had a redshift of 6.29, which translates to a distance of about 13 billion light years from Earth. (The Universe is thought to be 13.7 billion years old.) The previous most distant gamma-ray burst had a redshift of 4.5. The most distant quasar known is at redshift 6.4. GRB 050904 was also very long, lasting over 200 seconds. Most bursts last only about 10 seconds.
Swift detected GRB 050904 and relayed its coordinates to scientists around the world within minutes. Gamma-ray bursts disappear quickly, which is why Swift was designed to autonomously detect and locate bursts and notify the science community via e-mail, Web sites and even cell phone.
Reichart’s team at UNC was one of the first groups to respond to the alert. The team discovered the afterglow with the SOAR (Southern Observatory for Astrophysical Research) telescope atop Cerro Pachon, Chile. Over the next several nights, the team used SOAR and the Gemini South telescope, also on Cerro Pachon, to calculate a redshift of greater than 6 via a light filtering technique.
Reichart searched for the afterglow with PROMPT (six Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes), located near SOAR. PROMPT didn’t see the afterglow, providing further evidence that this was a very distant burst redshifted to an infrared energy below PROMPT’s range in visible light. Similarly, Dr. Derek Fox of Caltech searched with the Palomar 60-inch in southern California and saw nothing. The PROMPT and Palomar “non-detections” were key to deciding how to interpret SOAR and Gemini data.
Building upon all this information, a team led by Nobuyuki Kawai of the Tokyo Institute of Technology used the Subaru Observatory on Mauna Kea, Hawaii, to confirm the distance and fine-tune the redshift measurement to 6.29 via a technique called spectroscopy.
“We designed Swift to look for faint bursts coming from the edge of the Universe,” said Dr. Neil Gehrels of NASA Goddard Space Flight Center, Greenbelt, Md., Swift principal investigator. “Now we’ve got one and it’s fascinating. For the first time we can learn about individual stars from near the beginning of time. There are surely many more out there.”
Quasars, the famous distance record holder in years past, are actually most common around redshift 2 and teeter out at about redshift 5, according to Lamb. Only a handful of quasars have been detected beyond redshift 6. Gamma-ray bursts might be detectable as far as redshift 20, Lamb said. This makes gamma-ray bursts, in Lamb’s opinion, the premier method to probe the early Universe.
(Redshift 2 is about 10 billion light years; redshift 5 is about 12 billion light years. Star formation began about 200 million years after the Big Bang, at a redshift between 20 and 10.)
“The earliest stars exploded eons ago; we know very little about them,” said Josh Haislip, a UNC undergraduate who analyzed data from SOAR. “One of the best ways we can study them is by watching for their explosions. Swift can pinpoint the location of the explosions, and telescopes such as SOAR can study the composition of the debris to understand where and when these stars formed and what they were made of.”
Dedicated in April 2004, the 4.1-meter SOAR telescope is funded by the U.S. National Optical Astronomy Observatory (through the National Science Foundation), the Ministry of Science of Brazil, Michigan State University and UNC. The twin 8.2-meter Gemini Observatory telescopes and PROMPT represent international partnerships funded in part by the NSF. The 8.2-meter Subaru telescope is operated by the National Astronomical Observatory of Japan.
Swift is a NASA mission managed by Goddard Space Flight Center in Greenbelt, Md. Mission operations are conducted by Penn State University. Swift’s other national laboratories, universities, and international partners include the Los Alamos National Laboratory, Sonoma State University, the United Kingdom, and Italy. Refer to http://grb.sonoma.edu or http://swift.gsfc.nasa.gov.