all Jonathan E. Grindlay stories

Neutron star swaps lead to short gamma-ray bursts

70652986 — Mon, 26 Mar 2007 06:24:31 -0400

Gamma-ray bursts are the most powerful explosions in the universe, emitting huge amounts of high-energy radiation. For decades their origin was a mystery. Scientists now believe they understand the processes that produce gamma-ray bursts. However, a new study by Jonathan Grindlay of the Harvard- Smithsonian Center for Astrophysics (CfA) and his colleagues, Simon Portegies Zwart (Astronomical Institute, The Netherlands) and Stephen McMillan (Drexel University), suggests a previously overlooked source for some gamma-ray bursts: stellar encounters within globular clusters.

"As many as one-third of all short gamma-ray bursts that we observe may come from merging neutron stars in globular clusters," said Grindlay.

Gamma-ray bursts (GRBs) come in two distinct "flavors." Some last up to a minute, or even longer. Astronomers believe those long GRBs are generated when a massive star explodes in a hypernova. Other bursts last for only a fraction of a second. Astronomers theorize that short GRBs originate from the collision of two neutrons stars, or a neutron star and a black hole.

Most double neutron star systems result from the evolution of two massive stars already orbiting each other. The natural aging process will cause both to become neutron stars (if they start with a given mass), which then spiral together over millions or billions of years until they merge and release a gamma-ray burst.

Grindlay's research points to another potential source of short GRBs - globular clusters. Globular clusters contain some of the oldest stars in the universe crammed into a tight space only a few light-years across. Such tight quarters provoke many close stellar encounters, some of which lead to star swaps. If a neutron star with a stellar companion (such as a white dwarf or main-sequence star) exchanges its partner with another neutron star, the resulting pair of neutron stars will eventually spiral together and collide explosively, creating a gamma-ray burst.

The paper announcing this finding was published in the Jan. 29, 2006 online issue of Nature Physics. It is available online at http: //www.nature.com/nphys/index.html and in preprint form at http://arxiv.org/abs/astro-ph/0512654.

See-through galaxy

70652986 — Mon, 26 Mar 2007 05:35:59 -0400

To peer into the galactic center of our own Milky Way galaxy, astronomers Silas Laycock and Josh Grindlay used the unique capabilities of the 6.5-meter-diameter Magellan Telescope in Chile. By gathering infrared light that more easily penetrates dust, the astronomers were able to detect thousands of stars that otherwise would have remained hidden. Their goal was to identify stars that orbit, and feed, X-ray-emitting white dwarfs, neutron stars or black holes — any of which could yield the faint X-ray sources discovered originally with NASA's Chandra X-ray Observatory. Chandra previously detected more than 2,000 X-ray sources in the central 75 light-years of our galaxy. About four-fifths of the sources emitted mostly hard (high-energy) X-rays. The precise nature of those hard X-ray sources remained a mystery. Determining the nature of the sources can teach us about the star formation history and dynamical evolution of the region near the galactic center. "If we found that most of the hard X-ray sources were high-mass X-ray binaries, it would tell us that there had been a lot of recent star formation because massive stars don't live long," says Laycock. "Instead, we found that most of the X-ray sources are likely to be older systems associated with low-mass stars." Their study was presented at the 205th meeting of the American Astronomical Society in San Diego, Calif.

Chandra reveals nest of tight binaries in dense cluster

70652986 — Mon, 26 Mar 2007 05:13:48 -0400

Observations from a scientific team at the Harvard-Smithsonian Center for Astrophysics have revealed that an incredibly dense star cluster known as 47 Tucanae includes many binary stars. Most of the binary star systems in 47 Tucanae are systems in which a normal, Sun-like companion orbits a collapsed star, either a white dwarf or a neutron star. 47 Tucanae is about 12 billion years old, making it one of the oldest structures in our own Milky Way galaxy. The observations, made with NASA's Chandra X-ray Observatory, and using data from the Hubble Space Telescope and ground-based radio data, may help to explain how this ancient cluster of stars evolved. NASA's Marshall Space Flight Center in Huntsville, AL, manages the Chandra program.