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L

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RE:Short gamma-ray bursts
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Astronomers have taken a big leap towards solving the mystery of short gamma-ray bursts.
Researchers accurately pinpointed the locations of two recent short GRBs to relatively distant galaxies, and concluded that they arose from the collisions of dense neutron stars.
It had previously been thought that the bursts were produced by magnetars, highly magnetised neutron stars. But the observed bursts occurred at distances too far for that to be the source. The bursts would only get so strong before they destroy the stars, putting an upper limit on how far away a magnetar-based GRB can be observed. These bursts occurred about 10 times farther away than this limit, ruling out a magnetar source.
The new observations favour the popular "coalescence model", where short bursts can occur during mergers between two neutron stars, or between a neutron star and a black hole.

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L

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RE: GRB 050509b
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L

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RE: Birth of a black hole
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The brief burst came from the outskirts of an old elliptical galaxy - a galaxy with no new stars being formed.
"We have never seen a Gamma Ray Burst coming from an old galaxy like this before."



Astronomers divide Gamma Ray Bursts (GRBs) into two types. The long duration type seems to come from the collapse of young massive stars into black holes.
The short duration type - like GRB 050509b - appears to come from the collision of two neutron stars (which also result in black holes) or a neutron star and a black hole.
Currently, astronomers are scrutinising the outskirts of the elliptical galaxy to see if they can pinpoint the burst still further. They have found four, possibly five, faint smudges of light in the region but remain unconvinced that they are anything other than chance alignments.
Some scientists believe that the event could have been accompanied by a burst of neutrinos and the team examining the details of GRB 050509b are asking their colleagues monitoring neutrinos from the Sun and deep space to check their detectors at the time of the event.



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Anonymous

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Mergers of neutron stars with black holes have been suggested as candidates for short gamma-ray bursts. They have also been studied for their potential as gravitational wave sources observable with ground-based detectors.
For these purposes, it is important to know under what circumstances such a merger could leave an accretion disk or result in a period of stable mass transfer.
New research suggests that, consistent with recent numerical simulations, it is expected that mergers between neutron stars and black holes will be prompt, with no accretion disk and no stable mass transfer, if the black hole has a mass greater than that of the neutron star and is spinning slowly.
The reason is that for comparable masses, angular momentum loss to gravitational radiation starts a plunge orbit well outside the innermost stable circular orbit, causing direct merging rather than extended mass transfer. Even when the black hole is spinning rapidly and exactly prograde with respect to the orbit, we show that it is possible within current understanding that no accretion disk will form under any circumstances, but resolution of this will require full general relativistic numerical simulations with no approximations.

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L

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The birth of a black hole was witnessed this morning. (9th May 22:45:53.95 UT)
(Notified on Monday 09 May 01:46:45 UT)

The Gamma ray burst GRB050509b was detected by NASA's orbiting Swift telescope, and was most likely the merger of two dense neutron stars.

"A fraction of a second before contact, the lower mass neutron star is disrupted and forms a neutrino driven accretion disk around the higher mass neutron star. It implodes under the weight and forms a maximally spinning low-mass black hole." - Steinn Sigurdsson, Penn State University researcher.

The gamma ray burst signals the formation of a superheated jet of gas being shot out at a high speed from the region surrounding the newly formed black hole.
The merger occurred 2.2 billion light-years away.

IMAGE (Expand)
Position (2000): RA +12h 36m 13s Dec +29d 00' 01"

-- Edited by Blobrana at 18:33, 2005-08-17

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