One thousand of the most powerful explosions in the universe -- gamma-ray bursts -- now have been detected by NASA's Swift Gamma-ray-Burst Explorer satellite, report scientists at Penn State's Mission Operations Center, which controls the science and flight operations for the satellite.
The Swift mission is named for its rapid-response capability that allows it to swiftly detect and study gamma-ray bursts (GRBs), which typically last less than a minute and are associated with the collapse of a massive star and the birth of a black hole. "Swift has the special capability to rapidly rotate in order to make rapid-response observations of fast-breaking events like GRBs and other kinds of high-energy eruptions throughout the universe," said John Nousek, Swift’s director of mission operations and a professor of astronomy and astrophysics at Penn State.
Swift carries two telescopes whose lead scientists are Penn State astronomers and a third telescope led by a NASA scientist. "The spacecraft remains in great shape after nearly 11 years in space, and we expect it to see many more GRBs, said Neil Gehrels, the Swift principal investigator at NASA's Goddard Space Flight Center in Maryland.
When Swift detects a GRB eruption, it automatically determines the blast's location, broadcasts the position to the astronomical community, and points its sensitive telescopes toward the explosion to investigate the burst. “This process can take as little as 40 seconds, which is so quick we sometimes can capture the tail end of the GRB itself before the high-energy gamma rays fade to afterglows of lower-energy X-ray, ultraviolet, and optical wavelengths,” Nousek, said. "Because Swift autonomously responds to sudden bursts of high-energy light, it also provides us with data on a wide range of short-lived events, such as X-ray flares from stars and other objects."
Swift is recognized as one of the most versatile astrophysics missions ever flown. NASA gave its top ranking to the Swift observatory last year for astronomy satellites in instrument classes other than its “great observatories,” the Hubble Space Telescope and Chandra X-ray Observatory. Swift is the only satellite that can precisely locate gamma-ray bursts. It also is the only satellite that can monitor the explosions in space across a wavelength range from visible light to X-rays using multiple instruments before these powerful bursts fade from view.
Once a GRB is identified, the race is on to observe its fading light with as many instruments as possible. Based on alerts from Swift, robotic observatories and human-operated telescopes turn to the blast site to measure its rapidly fading afterglow, which emits X-rays, ultraviolet, visible and infrared light, and radio waves. "Over the years, astronomers have constantly refined their techniques to get their telescopes onto the burst site in the shortest possible time," Nousek said."
The 1,000th burst, detected in the southern-hemisphere sky on October 27 and named GRB 151027B, is a good example. Swift automatically determined its location, broadcast the position to astronomers around the world, and turned to investigate the source with its own sensitive X-ray, ultraviolet and optical telescopes. A team at the Chinese National Astronomical Observatories in Beijing quickly captured the afterglow's visible light using the Very Large Telescope's X-shooter spectrograph. Five hours after the Swift alert, the burst location first became visible from the European Southern Observatory (ESO) in Chile. The ESO observations show that light from the burst had been traveling to us for more than 12 billion years, placing it in the most distant few percent of GRBs that Swift has recorded.
“Because Swift autonomously responds to sudden bursts of high-energy light, it also provides us with data on a wide range of short-lived events that last less than two seconds -- and sometimes just thousandths of a second -- as well bursts of the "long" variety, with a gamma-ray pulse that lasts more than two seconds,” Nousek said.
The 1,000th burst was of the "long" variety, like roughly 90 percent of GRBs. Long bursts are believed to occur in a massive star whose core has run out of fuel and then collapses into a black hole. As matter falls toward the newly formed black hole, it launches jets of subatomic particles that move out through the star's outer layers at nearly the speed of light. When the particle jets reach the star' surface, they emit gamma rays, the most energetic form of light. In many cases, the star is later seen to explode as a supernova.
Swift's discoveries include a new ultra-long class of gamma-ray bursts, whose high-energy emissions endure for hours. Swift's discoveries also include the farthest GRB, whose light took more than 13 billion years to reach us; and the "naked-eye" GRB, which for about a minute was bright enough to see with the naked eye, despite the fact that its light had traveled 7.5 billion years. Early in the mission, Swift observations provided the "smoking gun" that validated long-standing theoretical models suggesting that GRBs with durations under two seconds come from mergers of two neutron stars, objects with the mass of the Sun that have been crushed to the size of a city.
In addition to its GRB studies, Swift conducts multiwavelength observations of a wide array of astrophysical phenomena, from nearby comets and asteroids to faraway quasars and blazars -- galaxies where supermassive black holes produce unpredictable high-energy flares. With new types of observatories ramping up, Swift is poised to take on another new role to search for astronomical events that produce ghostly particles called neutrinos, as well as for the high-energy light associated with sources of gravitational waves -- ripples in space-time predicted by Einstein's relativity theory but never yet detected by scientific instruments.
Swift rocketed into orbit on November 20, 2004. The lead scientist for Swift’s X-ray Telescope is David Burrows, a Penn State professor of astronomy and astrophysics. The lead scientist for Swift’s Ultraviolet/Optical Telescope is Michael Siegel, a Penn State senior research scientist. The lead scientist for Swift's Burst-Alert Telescope is Scott Barthelmy at NASA Goddard Space Flight Center. The Swift mission is managed by NASA Goddard and operated in collaboration with Penn State University, the Los Alamos National Laboratory in New Mexico, and Orbital Sciences Corporation in Virginia. Other partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, Brera Observatory and the Italian Space Agency in Italy, with additional collaborators in Germany and Japan.