NASA's Swift finds a gamma-ray burst with a dual personality

ScienceDaily (Nov. 30, 2011) — A peculiar cosmic explosion first detected by NASA's Swift observatory on Christmas Day 2010 was caused either by a novel type of supernova located billions of light-years away or an unusual collision much closer to home, within our own galaxy. Papers describing both interpretations appear in the Dec. 1 issue of the journal Nature.

Gamma-ray bursts (GRBs) are the universe's most luminous explosions, emitting more energy in a few seconds than our sun will during its entire energy-producing lifetime. What astronomers are calling the "Christmas burst" is so unusual that it can be modeled in such radically different ways.

"What the Christmas burst seems to be telling us is that the family of gamma-ray bursts is more diverse than we fully appreciate," said Christina Thoene, the supernova study's lead author, at the Institute of Astrophysics of Andalusia in Granada, Spain. It's only by rapidly detecting hundreds of them, as Swift is doing, that we can catch some of the more eccentric siblings."

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 a half-million times Earth's mass is squeezed into a sphere no larger than a city.

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. EST 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.

Thoene'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 and the production of oppositely directed jets of particles moving at nearly the speed of light, followed by a weak supernova.

The particle jets produced gamma rays. Jet 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, and 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," said Sergio Campana, who led the collision study at Brera Observatory in Merate, Italy.

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.

Campana's team supports an alternative model that 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. The 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 that lasted several hours may have resulted 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, astronomers have settled into two basic scenarios, either the collapse of a massive star or the merger of a compact binary system.

"The beauty of the 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 in Huntsville, Ala.

NASA's Swift 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.

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The above story is reprinted from materials provided by NASA/Goddard Space Flight Center.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal References:

C. C. Thöne, A. de Ugarte Postigo, C. L. Fryer, K. L. Page, J. Gorosabel, M. A. Aloy, D. A. Perley, C. Kouveliotou, H. T. Janka, P. Mimica, J. L. Racusin, H. Krimm, J. Cummings, S. R. Oates, S. T. Holland, M. H. Siegel, M. De Pasquale, E. Sonbas, M. Im, W.-K. Park, D. A. Kann, S. Guziy, L. Hernández García, A. Llorente, K. Bundy, C. Choi, H. Jeong, H. Korhonen, P. Kubànek, J. Lim, A. Moskvitin, T. Muñoz-Darias, S. Pak, I. Parrish. The unusual ?-ray burst GRB 101225A from a helium star/neutron star merger at redshift 0.33. Nature, 2011; 480 (7375): 72 DOI: 10.1038/nature10611S. Campana, G. Lodato, P. D’Avanzo, N. Panagia, E. M. Rossi, M. Della Valle, G. Tagliaferri, L. A. Antonelli, S. Covino, G. Ghirlanda, G. Ghisellini, A. Melandri, E. Pian, R. Salvaterra, G. Cusumano, V. D’Elia, D. Fugazza, E. Palazzi, B. Sbarufatti, S. D.Vergani. The unusual gamma-ray burst GRB 101225A explained as a minor body falling onto a neutron star. Nature, 2011; 480 (7375): 69 DOI: 10.1038/nature10592

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Galaxies are the ultimate recyclers, NASA's Hubble confirms

ScienceDaily (Nov. 17, 2011) — Galaxies learned to "go green" early in the history of the universe, continuously recycling immense volumes of hydrogen gas and heavy elements to build successive generations of stars stretching over billions of years.

This ongoing recycling keeps galaxies from emptying their "fuel tanks" and therefore stretches out their star-forming epoch to over 10 billion years. However, galaxies that ignite a rapid firestorm of star birth can blow away their remaining fuel, essentially turning off further star-birth activity.

This conclusion is based on a series of Hubble Space Telescope observations that flexed the special capabilities of its comparatively new Cosmic Origins Spectrograph (COS) to detect otherwise invisible mass in the halo of our Milky Way and a sample of more than 40 other galaxies. Data from large ground-based telescopes in Hawaii, Arizona, and Chile also contributed to the studies by measuring the properties of the galaxies.

This invisible mass is made up of normal matter -- hydrogen, helium, and heavier elements such as carbon, oxygen, nitrogen, and neon -- as opposed to dark matter that is an unknown exotic particle pervading space.

The results are being published in three papers in the November 18 issue of Science magazine. The leaders of the three studies are Nicolas Lehner of the University of Notre Dame in South Bend, Ind.; Jason Tumlinson of the Space Telescope Science Institute in Baltimore, Md.; and Todd Tripp of the University of Massachusetts at Amherst.

The Key Findings

The color and shape of a galaxy is largely controlled by gas flowing through an extended halo around it. All modern simulations of galaxy formation find that they cannot explain the observed properties of galaxies without modeling the complex accretion and "feedback" processes by which galaxies acquire gas and then later expel it after processing by stars. The three studies investigated different aspects of the gas-recycling phenomenon.

"Our results confirm a theoretical suspicion that galaxies expel and can recycle their gas, but they also present a fresh challenge to theoretical models to understand these gas flows and integrate them with the overall picture of galaxy formation," Tumlinson says.

The team used COS observations of distant stars to demonstrate that a large mass of clouds is falling through the giant corona halo of our Milky Way, fueling its ongoing star formation. These clouds of ionized hydrogen reside within 20,000 light-years of the Milky Way disk and contain enough material to make 100 million suns. Some of this gas is recycled material that is continually being replenished by star formation and the explosive energy of novae and supernovae, which kicks chemically enriched gas back into the halo; the remainder is gas being accreted for the first time. The infalling gas from this vast reservoir fuels the Milky Way with the equivalent of about a solar mass per year, which is comparable to the rate at which our galaxy makes stars. At this rate the Milky Way will continue making stars for another billion years by recycling gas into the halo and back onto the galaxy. "We now know where is the missing fuel for galactic star formation," Lehner concludes. "We now have to find out its birthplace."

One goal of the studies was to study how other galaxies like our Milky Way accrete mass for star making. But instead of widespread accretion, the team found nearly ubiquitous halos of hot gas surrounding vigorous star-forming galaxies. These galaxy halos, rich in heavy elements, extend as much as 450,000 light-years beyond the visible portions of their galactic disks. The surprise was discovering how much mass in heavy elements is far outside a galaxy. COS measured 10 million solar masses of oxygen in a galaxy's halo, which corresponds to about 1 billion solar masses of gas -- as much as in the entire interstellar medium between stars in a galaxy's disk. They also found that this gas is nearly absent from galaxies that have stopped forming stars. This is evidence that widespread outflows, rather than accretion, determine a galaxy's fate. "We didn't know how much mass was there in these gas halos, because we couldn't do these observations until we had COS," Tumlinson says. "This stuff is a huge component of galaxies but can't be seen in any images."

He points out that because so much of the heavy elements has been ejected into the halos instead of sticking around in the galaxies, the formation of planets, life, and other things requiring heavy elements could have been delayed in these galaxies.

The COS data also demonstrate that those galaxies forming stars at a very rapid rate, perhaps a hundred solar masses per year, can drive 2-million-degree gas very far out into intergalactic space at speeds of up to 2 million miles per hour. That's fast enough for the gas to escape forever and never refuel the parent galaxy. While hot plasma "winds" from galaxies have been known for some time, the new COS observations reveal that hot outflows extend to much greater distances than previously thought and can carry a tremendous amount of mass out of a galaxy. Some of the hot gas is moving more slowly and could eventually be recycled. The Hubble observations show how gas-rich star-forming spiral galaxies can evolve to quiescent elliptical galaxies that no longer have star formation. "So not only have we found that star-forming galaxies are pervasively surrounded by large halos of hot gas," says Tripp, "we have also observed that hot gas in transit -- we have caught the stuff in the process of moving out of a galaxy and into intergalactic space."

The light emitted by this hot plasma is invisible, so the researchers used COS to detect the presence of the gas by the way it absorbs certain colors of light from background quasars. The brightest objects in the universe, quasars are the brilliant cores of active galaxies that contain rapidly accreting supermassive black holes. The quasars serve as distant lighthouse beacons that shine through the gas-rich "fog" of hot plasma encircling galaxies. At ultraviolet wavelengths, COS is sensitive to absorption from many ionized heavy elements, such as nitrogen, oxygen, and neon. COS's high sensitivity allows many galaxies that happen to lie in front of the much more distant quasars to be studied. The ionized heavy elements serve as proxies for estimating how much mass is in a galaxy's halo.

"Only with COS can we now address some of the most crucial questions that are at the forefront of extragalactic astrophysics," Tumlinson says.

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The above story is reprinted from materials provided by NASA/Goddard Space Flight Center.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal References:

T. M. Tripp, J. D. Meiring, J. X. Prochaska, C. N. A. Willmer, J. C. Howk, J. K. Werk, E. B. Jenkins, D. V. Bowen, N. Lehner, K. R. Sembach, C. Thom, J. Tumlinson. The Hidden Mass and Large Spatial Extent of a Post-Starburst Galaxy Outflow. Science, 2011; 334 (6058): 952 DOI: 10.1126/science.1209850N. Lehner, J. C. Howk. A Reservoir of Ionized Gas in the Galactic Halo to Sustain Star Formation in the Milky Way. Science, 2011; 334 (6058): 955 DOI: 10.1126/science.1209069J. Tumlinson, C. Thom, J. K. Werk, J. X. Prochaska, T. M. Tripp, D. H. Weinberg, M. S. Peeples, J. M. O'Meara, B. D. Oppenheimer, J. D. Meiring, N. S. Katz, R. Dave, A. B. Ford, K. R. Sembach. The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are a Major Reservoir of Galactic Metals. Science, 2011; 334 (6058): 948 DOI: 10.1126/science.1209840

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