UNIVERSITY PARK, Pa. -- The nature of a peculiar cosmic explosion detected on Dec. 25, 2010, remains an intriguing question without a clear answer. The cause of the explosion, a gamma-ray burst that first was detected by NASA's Swift observatory, either was a novel type of supernova located billions of light-years from Earth or an unusual collision much closer to home inside our own galaxy, report astronomers in papers published in the Dec. 1 issue of Nature.
"It is nice to find that the universe can still surprise us, after seven years and 600 bursts since Swift was launched," said Michael Siegel, a research associate in astronomy and astrophysics at Penn State and the lead scientist for Swift's Ultraviolet/Optical Telescope (UVOT). Siegel analyzed the UVOT data as it came down to Penn State's Mission Operations Center from the spacecraft, and he also coordinated Swift's follow-up observations.
Gamma-ray bursts (GRBs) are the universe's most luminous explosions, emitting more energy in a few seconds than our Sun will produce during its entire energy-making lifetime. The fact that this "Christmas burst" can be modeled in such radically different ways indicates how odd this explosion is. Common to both scenarios is the presence of a neutron star, the crushed core that forms when a star many times the Sun's mass explodes. When the star's fuel is exhausted, it collapses under its own weight, compressing its core so much that about half-a-million times Earth's mass is squeezed into a sphere no larger than a city. Sometimes the collapse generates the largest-known type of explosion in the universe, a gamma-ray burst.
Siegel was one of the Swift scientists who received the automatic pager alert on Christmas afternoon, when the burst went off. He rushed to fire up his laptop over presents and cookies to assess the data from the UVOT and alert other astronomers as soon as possible before the explosion began fading away. "What was strange about this burst was its unusually long optical afterglow and its unusual profile as the afterglow faded. Usually we either don't detect an optical afterglow at all, or it disappears in a few days," Siegel said. "But this burst brightened again after its initial fading and had an unusual spectrum."
The X-ray and UV/optical observations, which are critically important to understanding the nature of this unusual object, were made by two instruments on Swift that were developed at Penn State. "It is very gratifying that both Swift's X-ray Telescope and its Ultraviolet/Optical Telescope continue to make dramatic new discoveries like the ones presented in this paper, said David Burrows, professor of astronomy and astrophysics at Penn State, who is the lead scientist for Swift's X-ray Telescope (XRT).
The supernova study's lead author, Christina Thöne at the Institute of Astrophysics of Andalusia in Spain, said "What the Christmas burst seems to be telling us is that the family of gamma-ray bursts is more diverse than we fully appreciate. It's only by rapidly detecting hundreds of them, as Swift is doing, that we can catch some of the more eccentric siblings.
The Christmas burst, also known as GRB 101225A, was discovered in the constellation Andromeda by Swift's Burst Alert Telescope at 1:38 p.m. Eastern Standard Time on Dec. 25, 2010. The gamma-ray emission lasted at least 28 minutes, which is unusually long. Follow-up observations of the burst's afterglow by the Hubble Space Telescope and ground-based observatories were unable to determine the object's distance.
Thöne's team proposes that the burst occurred in an exotic binary system where a neutron star orbited a normal star that had just entered its red-giant phase, enormously expanding its outer atmosphere. This expansion engulfed the neutron star, resulting in both the ejection of the giant's atmosphere and rapid tightening of the neutron star's orbit. Once the two stars became wrapped in a common envelope of gas, the neutron star may have merged with the giant's core after just five orbits, or about 18 months. The end result of the merger was the birth of a black hole, the production of oppositely directed jets of particles moving at nearly the speed of light, followed by a weak supernova.These particle jets produced gamma rays, and the jets' interactions with gas ejected before the merger explain many of the burst's signature oddities. Based on this interpretation, the event took place about 5.5 billion light-years away. The team has detected what may be a faint galaxy at the right location.
"Deep exposures using Hubble may settle the nature of this object. If it is indeed a galaxy, that would be evidence for the binary model. On the other hand, if NASA's Chandra X-ray Observatory finds an X-ray point source or if radio telescopes detect a pulsar, that goes against it," said Sergio Campana, who led the collision study at Brera Observatory in Merate, Italy.
Campana's team supports an alternative model, which involves the tidal disruption of a large comet-like object and the ensuing crash of debris onto a neutron star located only about 10,000 light-years away. This scenario requires the break-up of an object with about half the mass of the dwarf planet Ceres. While rare in the asteroid belt, such objects are thought to be common in the icy Kuiper belt beyond Neptune. Similar objects located far away from the neutron star may have survived the supernova that formed it. Gamma-ray emission occurred when debris fell onto the neutron star. Clumps of cometary material likely made a few orbits, with different clumps following different paths before settling into a disk around the neutron star. X-ray variations detected by Swift's X-Ray Telescope and lasting several hours may result from late-arriving clumps that struck the neutron star as the disk formed.
In the early years of studying GRBs, astronomers had very few events to study in detail and dozens of theories to explain them. "In the Swift era, we've settled into two basic scenarios, either the collapse of a massive star or the merger of a compact binary system. The beauty of this Christmas burst is that we must invoke two exotic scenarios to explain it, but such rare oddballs will help us advance the field," said Chryssa Kouveliotou, a co-author of the supernova study at NASA's Marshall Space Flight Center.
NASA's Swift observatory was launched in November 2004 and is managed by Goddard. It is operated in collaboration with several U.S. institutions and partners in the United Kingdom, Italy, Germany, and Japan.
For more information, contact Michael Siegel at email@example.com or 814-865-7748; Barbara Kennedy (Penn State Eberly College of Science PIO) at firstname.lastname@example.org or 814-863-4682; or Lynn Cominsky (Swift PIO): email@example.com or 707-664-2655.
An animation associated with this research is online at http://www.science.psu.edu/news-and-events/2011-news/Swift 11-2011-1.
More information about Swift:
The Swift observatory was launched in November 2004 and was fully operational by January 2005. Swift carries three main instruments: the Burst Alert Telescope, the X-ray Telescope, and the Ultraviolet/Optical Telescope. Its science and science and flight operations are controlled by Penn State from the Mission Operations Center in State College, Pennsylvania. Swift's gamma-ray detector, the Burst Alert Telescope, provides the rapid initial location and was built primarily by the NASA Goddard Space Flight Center in Greenbelt, Maryland, and Los Alamos National Laboratory in New Mexico and constructed at GSFC. Swift's X-Ray Telescope and UV/Optical Telescope were developed and built by international teams led by Penn State and drew heavily on each institution's experience with previous space missions. The X-ray Telescope resulted from Penn State's collaboration with the University of Leicester in the United Kingdom and the Brera Astronomical Observatory in Italy. The Ultraviolet/Optical Telescope resulted from Penn State's collaboration with the Mullard Space Science Laboratory of the University College-London. These three telescopes give Swift the ability to do almost immediate follow-up observations of most gamma-ray bursts because Swift can rotate so quickly to point toward the source of the gamma-ray signal. The spacecraft was built by Spectrum Astro, which became part of General Dynamics and then part of Orbital Sciences Corporation.