The mysterious origin of powerful, split-second flashes of light known as short gamma-ray bursts will be revealed with the publication of four papers in the 6 October issue of the scientific journal Nature, including a comprehensive multi-wavelength study lead by Derek Fox, assistant professor of astronomy and astrophysics at Penn State. Fox and his collaborators are the first research team to pinpoint the origin of a short gamma-ray burst, to gauge its distance from Earth, to measure the shape of the jet of energy it beamed toward Earth, and to calculate how often such bursts are likely to occur within our galaxy.
“Gamma-ray bursts in general are notoriously difficult to study, but the shortest ones have been next to impossible to pin down,” said Dr. Neil Gehrels of NASA Goddard Space Flight Center in Greenbelt, Maryland, principal investigator of NASA’s Swift satellite and lead author on one of the Nature reports. “Now, all that has changed. We now have the tools in place to study these events.”
Through unprecedented coordination, Fox and other scientists used a multitude of ground-based telescopes and NASA satellites to determine that the flashes arise from violent collisions, either between a black hole and a neutron star or between two neutron stars. “A neutron star is the mass of the Sun extremely compressed into a chunk of rock the size of Manhattan,” Fox explains. “The collision of two objects that dense results in the formation of a new black hole and creates short, powerful bursts of gamma rays that last only a few milliseconds but are a trillion times brighter than our Sun,” Fox explains.
Long and short gamma-ray bursts were discovered in the 1970s and have been detected by many satellites over the years; however no distance to any short burt could be determined until 9 May this year, when the Swift satellite discovered its first short burst and observed its fading afterglow. NASA’s High-Energy Transient Explorer (HETE) then detected a second, and much more powerful, short burst on 9 July. Swift and HETE quickly and autonomously relayed the burst coordinates to scientists and observatories via cell phone, beepers, and e-mail so other telescopes could search for and study the burst’s afterglow. The Fox team’s multi-wavelength observations of the 9 July burst provided strong evidence to support the collision theory by putting together all the pieces of the puzzle to solve the short-burst mystery. The papers published in 6 October issue of Nature represent thorough analyses of these two burst afterglows, which clinch the case for the origin of short bursts.
“We decided to mobilize all the telescopes at our disposal to go after the second short burst, because it was was 100 times brighter in gamma rays than the first short burst,” Fox said. His team reasoned that the bright burst’s afterglow in other wavelengths might be much brighter, as well, giving his team precious additional time to study it. “We figured the 9 July burst was giving us a good chance to, in effect, catch lightning in a bottle,” Fox said.
Fox’s team first discovered the X-ray afterglow of the 9 July burst with NASA’s Chandra X-ray Observatory, using Chandra’s Chandra’s Advanced CCD Imaging Spectrometer (ACIS), which was conceived and developed for NASA by Penn State and the Massachusetts Institute of Technology under the leadership of Gordon Garmire, Evan Pugh Professor of Astronomy and Astrophysics at Penn State. Chandra played an important role because it sees the sky with X-ray vision, ignoring all the lower-energy light and pinpointing only those objects with conditions extreme enough to produce heat in millions of degrees and emit bright X-rays. “To hunt for an afterglow in order to pinpoint the origin of the now-invisible brief burst of gamma rays, we search in the general location of the burst to find an object whose emissions are changing in certain characteristic ways,” Fox explains. “Chandra helped us to quickly narrow the search for the afterglow of this burst to one particular galaxy.” Fox and his team then quickly gauged the distance to the galaxy by obtaining the spectrum of its optical light with the Gemini observatory on Mauna Kea in Hawaii. “We discovered that the galaxy is 2 billion light years from Earth, is fairly young, somewhat like our Milky Way, and is still forming stars,” he said.
A team led by Jens Hjorth of the University of Copenhagen then identified the optical afterglow using the Danish 1.5-meter telescope at the La Silla Observatory in Chile. Fox’s team then continued its studies of the afterglow with NASA’s Hubble Space Telescope; the du Pont and Swope telescopes at Las Campanas in Chile, funded by the Carnegie Institution; the Subaru telescope on Mauna Kea, Hawaii, operated by the National Astronomical Observatory of Japan; and the Very Large Array, a stretch of 27 radio telescopes near Socorro, N.M., operated by the National Radio Astronomy Observatory.
“With Hubble, we could detect the afterglow until 18.6 days after the burst,” Fox says. The Hubble observations helped Fox and his team discover that the optical afterglow is associated with the galaxy, providing additional confirmation of its distance from Earth.
“Our observations show that the jet was about 15 degrees wide and illuminated only about 1/30th of the sky,” Fox said. “With this information, we now, for the first time, can calculate the expected rate of collisions between supermassive objects within a given distance from Earth. In the Milky Way galaxy, for example, we now can estimate with some confidence that one short gamma-ray burst–and the strong gravitational waves associated with it–will occur about every 500 thousand years.” One reason this new ability to predict short gamma-ray bursts is exciting is that it gives scientists a long-sought tool–based, for the first time on data from actual observations–for planning their observations of gravitational waves.
“One of the reasons our new ability to study these short bursts is extremely exciting is that we now have hope that the gravitational waves that form when the two stars pull each other apart and merge to form a black hole will tell us something about how a neutron star is constructed and how matter behaves under such extreme conditions of pressure and temperature,” Fox says. If Swift detects a nearby short burst, scientists working with the new National Science Foundation Laser Interferometer Gravitational-Wave Observatory (LIGO) would know to look for evidence of the gravitational ripples in the Ligo data taken at that precise time and location. “This is good news for LIGO,” said Dr. Albert Lazzarini, of the LIGO Laboratory at CalTech.
Fox adds, “Another thing that is really exciting is that within a decade Ligo will be able to detect the gravitational waves from merging neutron stars out to hundreds of millions of light years. Those observations will give us an unprecedented opportunity to test general relativity and to learn whether we really understand gravity–one of the fundamental forces that shape our universe.”
Gamma-ray bursts are random, fleeting, and can occur from any region of the sky. Two years ago scientists discovered that longer bursts, lasting over two seconds, arise from the explosion of very massive individual stars, whose remnants are visible as a supernova. “Powerful telescopes detected no supernova as the gamma-ray burst faded, arguing against the explosion of a massive star,” said Dr. George Ricker of MIT, HETE Principal Investigator and co-author of another Nature article about the origin of the short bursts.
Swift is managed by NASA Goddard. Penn State controls Swift’s science and flight operations from the Mission Operations Center at the University Park campus. Swift’s other national laboratories, universities, and international partners include the Los Alamos National Laboratory, Sonoma State University, the United Kingdom, and Italy. HETE was built by MIT as a mission of opportunity under the NASA Explorer Program, with collaboration among U.S. universities, Los Alamos National Laboratory, and scientists and organizations in Brazil, France, India, Italy and Japan.
SCIENCE CONTACTS AT PENN STATE:
— Derek Fox, assistant professor of astronomy and astrophysics: 814-863-4989, dfox@astro.psu.edu
— Peter Mészáros, head of the Swift science team and Holder of the Eberly Family Chair in Astronomy and Astrophysics: 814-865-0418, pmeszaros@astro.psu.edu
— John Nousek, director of the Swift Mission Operations Center and professor of astronomy and astrophysics: 814-863-1937, nousek@astro.psu.edu
–David Burrows, senior scientist, professor of astronomy and astrophysics, and lead scientist for Swift’s X-ray telescope: 814 863-2466, burrows@astro.psu.edu
P.I.O. CONTACTS:
Barbara K. Kennedy (Penn State PIO): 814-863-4682, science@psu.edu Lynn
Cominsky (Swift PIO): 707-664-2655, lynnc@universe.sonoma.edu
MORE INFORMATION AND IMAGES:
Video and still images associated with this research are available on teh web at http://www.nasa.gov/mission_pages/swift/bursts/short_burst_oct5.html
Information about each Swift-detected gamma-ray burst is available at http://grb.sonoma.edu. Additional information about Swift is available at http://swift.nasa.gov and http://www.science.psu.edu/alert/Swift.htm .
For HETE, refer to http://space.mit.edu/HETE/.
For more information about other observatories, refer to the following links:
— Chandra X-ray Observatory — http://chandra.harvard.edu
— Danish 1.5-meter, La Silla Observatory — http://www.ls.eso.org/index.html
— The du Pont and Swope telescopes, Las Campanas – http://www.ociw.edu/lco/
— Gemini Observatory — http://www.gemini.edu/
— Hubble Space Telescope — http://hubblesite.org
— LIGO — http://www.ligo.caltech.edu
— Subaru Observatory, Mauna Kea — http://www.naoj.org/
— Very Large Array — http://www.vla.nrao.edu/
