Re: Short Gamma-Ray Bursts: New Models Shed Light on Enigmatic Explosions (Forwarded)
From: Aladar (alistolmar_at_3dresearch.com)
Date: 09/07/04
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Date: 7 Sep 2004 06:53:22 -0700
Just two comments: the energy released is predicted to be 2-4 solar
masses in the supernovae - gamma ray burst initial event (followed by
further decay of super-heavy nuclear layers) (see Daisy-petal graph).
The small - short bursts could be just results of closing of two
neutron stars, without merger. It should be detected from the galactic
center, hence it is a multitude of neutron stars, moving in a well
defined system...
Cheers!
Aladar
http://stolmarphysics.com
Andrew Yee <ayee@nova.astro.utoronto.ca> wrote in message news:<lrP_c.3376$Nd6.133229@news20.bellglobal.com>...
> Max-Planck-Institute for Astrophysics
> Garching, Germany
>
> September 1st, 2004
>
> Short Gamma-Ray Bursts
>
> New Models Shed Light on Enigmatic Explosions
>
> H.-Thomas Janka, M.A. Aloy, E. Müller
>
> Researchers at the Max-Planck-Institute for Astrophysics have developed new
> relativistic models which allow predictions of so far unknown properties of
> short gamma-ray bursts. Their simulations will come under scrutiny by the Swift
> Gamma-Ray Burst Explorer, a NASA mission that is scheduled for launch in the
> fall of 2004.
>
> Gamma-ray bursts are among the most energetic and most luminous explosions in
> the Universe. They occur roughly once a day, last from a few thousandths of a
> second to a few hundred seconds, and come from all different directions of the
> sky. Their gamma radiation is more energetic than visible light and can be
> measured by satellites orbiting the Earth in space. The energy set free by the
> bursts in just one second is comparable to the energy production of the Sun
> during its whole life.
>
> The more than 2700 observed bursts are grouped into two distinct classes, one of
> which are the so-called long bursts that emit gamma radiation for more than two
> seconds, and the other one are the short bursts with durations up to two seconds.
>
> So far only long bursts could be observed in much detail. The detection of
> associated afterglows in X-rays, visible light and at radio wavelengths allowed
> the determination of their distances and confirmed their origin from host
> galaxies at large redshifts, i.e., typically hundreds of millions to billions of
> light years away. Until recently the source of these bursts was a mystery. But
> evidence has accumulated that they are death throes that accompany the
> catastrophic explosions which end the lives of very massive stars. A final
> confirmation of this conjecture was provided by GRB030329, a gamma-ray burst
> which was detected on March 29, 2003, by HETE, NASA's High-Energy Transient
> Explorer satellite. For the first time this burst could unambiguously be
> identified as linked to a peculiar supernova named SN 2003dh at a distance of
> about two billion light years.
>
> But where does the gigantic energy come from which powers the gamma-ray burst?
> Scientists have coined the theory that the "engine" is a rapidly spinning black
> hole which forms when the central core of a dying star becomes unstable and
> collapses under its own gravity. This newly formed black hole then swallows much
> of the infalling stellar matter and thereby releases enormous amounts of energy
> in two "jets". These expand "highly relativistically", i.e. with almost the
> speed of light, along the rotation axis of the star. Before they break out from
> the stellar surface, they have to drill their way through thick layers of
> stellar material, thus getting collimated into very narrow beams with an opening
> angle of only a few degrees (see Current Research -- March 2000). Indeed,
> observations not only confirm the origin of long gamma-ray bursts from exploding
> massive stars, but also provide evidence that the gamma emission comes from
> narrowly collimated, ultrarelativistic jets with velocities of more than 99.995
> per cent of the speed of light.
>
> Rotating, growing stellar mass black holes are also born in other cosmic events,
> for example in the violent mergers encountered by binary neutron stars (Fig. 1)
> or a neutron star and a black hole (Fig. 1) after hundreds of millions of years
> of inspiral, driven by the emission of gravitational waves. The remnant of such
> a catastrophy is a stellar-mass black hole sucking matter from a girding, thick
> torus of gas (Fig. 2). Such events have long been considered as possible sources
> of gamma-ray bursts, and they are still hot candidates for bursts of the short
> type, which so far could not be studied by observations in the same way as
> bursts from dying stars.
>
> Researchers at the Max-Planck-Institute for Astrophysics have now developed
> better computer models that take into account effects due to Einstein's theory
> of relativity. Their simulations can follow the highly relativistic ejection of
> matter that is caused by energy release (e.g., due to particle reactions) in the
> close vicinity of the black hole. The calculations confirm that short bursts
> have properties that are distinctively different from those of long bursts.
> Since the black hole -- torus system is not buried inside of many solar masses
> of stellar material as in case of dying stars, the polar jets do not have to
> make their ways through dense stellar layers and quickly reach extremely high
> velocities (Fig. 3). As a consequence, they are strongly collimated by the
> presence of the accretion torus, but their opening angles are somewhat larger
> than those measured for long bursts, typically around 5 to 10 degrees (Fig. 3).
> The models predict that outside of these polar cones gamma emission should
> become very weak (Fig. 4) so that a gamma-ray burst will be observable only from
> one out of hundred mergers when the ultrarelativistic jet is sent towards Earth.
> The models also suggest that short bursts can be nearly as bright as long
> bursts, although their total energy release is 100 times lower.
>
> Previous gamma-ray satellites were unable to make precise measurements for short
> bursts, but there is hope that these predictions can be tested soon. The Swift
> Gamma-Ray Burst Explorer, a NASA mission with international participation, is
> scheduled for launch in the fall of 2004. One of its prime goals is to unravel
> the mysteries of the short bursts.
>
> Literature:
>
> S. Setiawan. M. Ruffert und H.-Th. Janka, Monthly Not. R. Astron. Soc., 352,
> 753--758 (2004)
>
> M.A. Aloy, H.-Th. Janka und E. Müller (2004), Astron. Astrophys., submitted
> (astro-ph/0408291).
>
> [NOTE: Images supporting this release are available at
> http://www.mpa-garching.mpg.de/mpa/research/current_research/hl2004-9/hl2004-9-en.html
> ]
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