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TOPIC: Gamma-Ray Bursts


L

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RE: Gamma-Ray Bursts
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The most distant object ever observed in space has provided scientists with an unprecedented insight into the "cosmic dark ages" following the birth of the Universe some 13.7 billion years ago.
A gigantic explosion on the edge of the known Universe has been confirmed as the furthermost object in the cosmos. It occurred nearly 700 million years after the Big Bang and its radiation has taken some 13 billion years to reach Earth - making it 13 billion light years away.


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Highest Redshift Gamma-Ray Bursts
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Title: On The Origin Of The Highest Redshift Gamma-Ray Bursts
Authors: Krzysztof Belczynski, Daniel E. Holz, Chris L. Fryer, Edo Berger, Dieter H. Hartmann, Brian O'Shea
(Version v2)

GRB 080913 and GRB 090423 are the most distant gamma-ray bursts (GRBs) known to-date, with spectroscopically determined redshifts of z = 6.7 and z = 8.1, respectively. The detection of bursts at this early epoch of the Universe significantly constrains the nature of GRBs and their progenitors. We perform population synthesis studies of the formation and evolution of early stars, and calculate the resulting formation rates of short and long-duration GRBs at high redshift. The peak of the GRB rate from Population II stars occurs at z ~ 7 for a model with efficient/fast mixing of metals, while it is found at z ~ 3 for an inefficient/slow metallicity evolution model. We show that in the redshift range z = 6--10 essentially all GRBs originate from Population II stars, regardless of metallicity evolution model. These stars (having small, but non-zero metallicity) are the most likely progenitors for both long GRBs (collapsars) and short GRBs (NS-NS or BH-NS mergers) at this epoch. Although the predicted intrinsic rates of long and short GRBs are similar at these high redshifts, observational selection effects lead to higher (factor of ~ 10) observed rates for long GRBs. We conclude that the two recently observed high-z GRB events are most likely long GRBs originating from Population II collapsars.

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L

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RE: Gamma-Ray Bursts
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Invading black holes explain cosmic flashes
Black holes are invading stars, providing a radical explanation to bright flashes in the universe that are one of the biggest mysteries in astronomy today.
The flashes, known as gamma ray bursts, are beams of high energy radiation - similar to the radiation emitted by explosions of nuclear weapons - produced by jets of plasma from massive dying stars.
The orthodox model for this cosmic jet engine involves plasma being heated by neutrinos in a disk of matter that forms around a black hole, which is created when a star collapses.
But mathematicians at the University of Leeds have come up with a different explanation: the jets come directly from black holes, which can dive into nearby massive stars and devour them.

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L

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Dark Gamma-Ray Bursts
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New Light Shed On 'Dark' Gamma-ray Bursts
Gamma-ray bursts are the universe's biggest explosions, capable of producing so much light that ground-based telescopes easily detect it billions of light-years away. Yet, for more than a decade, astronomers have puzzled over the nature of so-called dark bursts, which produce gamma rays and X-rays but little or no visible light. They make up roughly half of the bursts detected by NASA's Swift satellite since its 2004 launch.

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Gamma-Ray Bursts
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Mysterious Gamma-Ray Bursts May Have Ties to Failed Black Holes
New research published last week on the arXiv website, and not yet peer reviewed, suggests that gamma-ray bursts may be the result of a strange effect that can stop a black hole from forming.
The current thinking is any star more than three times the size of the sun will eventually collapse into a black hole. But Ilya Rozen of the P.N. Lebedev Physical Institute of the Russian Academy of Sciences in Moscow thinks a phase change of matter into a very different form creates a vaccuum in an imploding star that results in a burning wall that he predicts would emit powerful gamma-ray bursts.

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Title: QCD against black holes?
Authors: Ilya I. Royzen

Along with compacting baryon (neutron) spacing, two very important factors come into play at once: the lack of self-stabilisation within a compact neutron star (NS) associated with possible black hole (BH) horizon appearance and the phase transition - colour deconfinement and QCD-vacuum reconstruction - within the nuclear matter. That is why both phenomena should be taken into account side by side, as the gravitational collapse is considered. Since, under the above transition, the hadronic-phase vacuum (filled up with gluon and chiral q\bar q-condensates) turns into the "empty" (perturbation) subhadronic-phase one and, thus, the corresponding (very high) pressure falls down rather abruptly, the formerly cold (degenerated) nuclear medium starts to implode into the new vacuum. If the mass of a star is sufficiently large, then this implosion produces an enormous heating, which stops only after quark-gluon plasma of a temperature about 100 MeV (or even higher) is formed to withstand the gravitational compression (whereas the highest temperatures of supernovae bursts are, at least, one order lower). As a consequence, a "burning wall" must be, most probably, erected on the way of further collapsing the matter towards a black hole formation.

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L

Posts: 131433
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Dark Gamma-Ray Bursts
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Astronomers using the Keck telescopes have solved the mystery of dark gamma ray bursts - intense flashes of X-ray and gamma-ray radiation that have little to no optical signature. The observations have allowed the astronomers to peer through celestial gas and dust to reveal star formation and stellar death in the dusty corners of otherwise dust-free galaxies.

"We have compelling evidence that a large percentage of star formation in the early Universe is actually hidden by dust, even inside galaxies that do not appear dusty" - astronomer Daniel Perley, University of California, Berkeley.


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Short- and Long-Duration Gamma-Ray Bursts
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Title: A Comparison of the Afterglows of Short- and Long-Duration Gamma-Ray Bursts
Authors: M. Nysewander, A.S. Fruchter, A. Pe'er
(Version v2)

We present a comparative study of the observed properties of the optical and X-ray afterglows of short- and long-duration gamma-ray bursts (GRBs). Using a large sample of 37 short and 421 long GRBs, we find a strong correlation between the afterglow brightness measured after 11 hours and the observed fluence of the prompt emission. Both the optical (R band) and X-ray flux densities (F_R and F_X) scale with the gamma-ray fluence, F_gamma. For bursts with a known redshift, a tight correlation exists between the afterglow flux densities at 11 hours (rest-frame) and the total isotropic gamma-ray energy, Egi: F_{R,X} ~ Egi^{alpha}, with alpha ~ 1. The constant of proportionality is nearly identical for long and short bursts, when Egi is obtained from the Swift data. Additionally, we find that for short busts with F_gamma >10^{-7} erg cm^{-2}, optical afterglows are nearly always detected by reasonably deep early observations. Finally, we show that the ratio F_R/F_X has very similar values for short and long bursts. These results are difficult to explain in the framework of the standard scenario, since they require that either (1) the number density of the surrounding medium of short bursts is typically comparable to, or even larger than the number density of long bursts; (2) short bursts explode into a density profile, n(r)\alpha r^{-2} or (3) the prompt gamma-ray fluence depends on the density of the external medium. We therefore find it likely that either basic assumptions on the properties of the circumburst environment of short GRBs or else the standard models of GRB emission must be re-examined. We believe that the most likely solution is that the ambient density surrounding typical short bursts is higher than has generally been expected: a typical value of ~1 per cm^{-3} is indicated.

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Posts: 131433
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RE: Gamma-Ray Bursts
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Last June astronomers at the Keck Observatory focused on a gamma burst that originated 11 5 billion light years away the most distant such burst of energy ever seen.

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L

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Bing Zhang takes the long view, peering back sometimes as far as a billion years after the birth of the universe and some eight billion years before our own sun and planets precipitated from a hydrogen fog drifting in the void.
From this perspective, the 100,000-year history of the human species is a minor rounding error.
Yet Zhang is concerned with events that last a fraction of a second or, at most, almost half an hour. He studies gamma ray bursts - the by-products of unimaginable violence, worse than an extinction-level asteroid impact or an all-Canadian hockey playoff series, calamities like the collapse of a star into a black hole.

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