Neutron Star Swaps Lead to Short Gamma-Ray Bursts (Forwarded)

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For Release: Tuesday, January 31, 2006

Release No.: 06-12

Neutron Star Swaps Lead to Short Gamma-Ray Bursts

Cambridge, MA -- 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.

"We see these precursor systems, containing one neutron star in the form
of a millisecond pulsar, all over the place in globular clusters," stated
Grindlay. "Plus, globular clusters are so closely packed that you have a
lot of interactions. It's a natural way to make double neutron-star
systems."

The astronomers performed about 3 million computer simulations to
calculate the frequency with which double neutron-star systems can form in
globular clusters. Knowing how many have formed over the galaxy's history,
and approximately how long it takes for a system to merge, they then
determined the frequency of short gamma-ray bursts expected from globular
cluster binaries. They estimate that between 10 and 30 percent of all
short gamma-ray bursts that we observe may result from such systems.

This estimate takes into account a curious trend uncovered by recent GRB
observations. Mergers and thus bursts from so-called "disk" neutron-star
binaries -- systems created from two massive stars that formed together
and died together -- are estimated to occur 100 times more frequently than
bursts from globular cluster binaries. Yet the handful of short GRBs that
have been precisely located tend to come from galactic halos and very old
stars, as expected for globular clusters.

"There's a big bookkeeping problem here," said Grindlay.

To explain the discrepancy, Grindlay suggests that bursts from disk
binaries are likely to be harder to spot because they tend to emit
radiation in narrower blasts visible from fewer directions. Narrower
"beaming" might result from colliding stars whose spins are aligned with
their orbit, as expected for binaries that have been together from the
moment of their birth. Newly joined stars, with their random orientations,
might emit wider bursts when they merge.

"More short GRBs probably come from disk systems -- we just don't see them
all," explained Grindlay.

Only about a half dozen short GRBs have been precisely located by
gamma-ray satellites recently, making thorough studies difficult. As more
examples are gathered, the sources of short GRBs should become much better
understood.

The paper announcing this finding was published in the January 29 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

Headquartered in Cambridge, Mass., the Harvard-Smithsonian Center for
Astrophysics (CfA) is a joint collaboration between the Smithsonian
Astrophysical Observatory and the Harvard College Observatory. CfA
scientists, organized into six research divisions, study the origin,
evolution and ultimate fate of the universe.

Note to editors: Images to accompany this release are online at
http://www.cfa.harvard.edu/press/pr0612image.html