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Supernovae Explosions
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Scientists have made the astonishing discovery that sound might drive supernovae explosions. Their computer simulations say that dying stars pulse at audible frequencies -- for instance, at about the F-note above middle C -- for a split second before they blow up.

Researchers in the 1960s began using computer models to test ideas about what, exactly, causes stars to explode. But mathematical simulations have so far failed to satisfactorily explain the inner workings of nature's most spectacular blasts.
Neutrinos -- subatomic particles widely thought to power supernovae explosions -- don't seem to be energetic enough to do the job, especially for more massive stars. More sophisticated models that include convective motion work a bit better, but not well enough.

Adam Burrows of The University of Arizona and colleagues at UA's Steward Observatory, Hebrew University, and Germany's Max Planck Institute (Potsdam) have developed computer models that simulate the full second or more of star death, from the dynamics of core collapse through supernova explosion. Their two-dimensional computer models allow for the fact that supernovae outbursts are not spherical, symmetrical events.

A supernova is a massive star that has burned for 10 million to 20 million years and developed a hot, dense 'white dwarf' star about the size of Earth at its core. When the white dwarf reaches a critical mass (about 1.5 times the mass of the sun), it collapses and creates a spherical shock wave, all within less than half a second before the star would explode as a supernova.
However, in all the best recent simulations, the shock wave stalls. So theorists have focused their work on what might revive the shock wave into becoming a supernova explosion.
According to Burrow's new results, part of the problem is that other computer models don't run long enough. His team's detailed models involve a million steps, or about five times as many as typical models that calculate only the first few hundred milliseconds of supernovae events. Burrows team's simulations also characterize the natural motion of a supernova core, something that other detailed models do not.

Isodensity contours for the core of an 11-solar-mass star in explosion. The remnant neutron star is the green "dot" in the centre and the outer shells are just interior to the blast wave that has been launched. The scale is ~2000 kilometres on a side and the time is ~800 milliseconds after the bounce at nuclear densities of the collapsed core. The colours reflect the entropies on the shells, where entropy is a measure of heat content.

"Our simulations show that the inner core starts to execute pulsations. And they allow us to follow the development to explosion for a longer time than other models do. They show that after about 500 milliseconds, the inner core begins to vibrate wildly. And after 600, 700 or 800 milliseconds, this oscillation becomes so vigorous that it sends out sound waves. In these computer runs, it's these sound waves that actually cause the star to explode, not the neutrinos. We were quite sure when we started seeing this phenomenon that we were seeing sound waves, but it was so unexpected that we kept rechecking and retesting our results" - Adam Burrows.

The team has used their models to make billions of calculations on computer clusters in the UA astronomy department, at Berkeley's supercomputer centre and elsewhere, checking their analysis for the past year. They are publishing the research in the Astrophysical Journal. Their research is funded by the National Science Foundation, the Department of Energy, and the Joint Institute for Nuclear Astrophysics.

The team got a clear picture of what likely happens by making movies from their simulations.
Collapsing material falls lopsidedly onto the inner core and soon excites oscillations at specific frequencies in the simulations. Within hundreds of milliseconds, the inner core vibrations become so intense that they actually generate sound waves. Typical sound frequencies are about 200 to 400 hertz, in the audible range bracketing middle C.

"Sound also generates pressure, which pushes the exciting streams of infalling matter to the opposite side of the core, further driving the core oscillations in a runaway process. The sound waves reinforce the shock wave (created by the collapsed star) until it finally explodes aspherically" - Adam Burrows.

Burrows said that others who study supernova explosions in computer experiments will be skeptical of his team's results -- and should be.

"This is such a break from 40 years of traditional thinking that one should be cautious trumpeting it. Nevertheless, this is provocative and interesting. It would open up many new possibilities and perhaps solve a long-standing problem of what triggers supernovae explosions" - Adam Burrows.

http://zenith.as.arizona.edu/~burrows/

-- Edited by Blobrana at 04:08, 2006-01-26

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L

Posts: 131433
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Elliptical Galaxies
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Except for rare cases, most violent activity in isolated elliptical galaxies was thought to have stopped long ago. Elliptical galaxies contain very little cool gas and dust, and far fewer massive young stars which explode as supernovas. Thus it was expected that the hot interstellar gas would have settled into an equilibrium shape similar to, but rounder than that of the stars.

Surprisingly, this study of elliptical galaxies shows that the distribution of hot gas has no correlation with the optical shape. A powerful source of energy must be pushing the hot gas around and stirring it up every hundred million years or so.



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Chandra x-ray telescope images of 56 elliptical galaxies have revealed evidence for unsuspected turmoil. As this sample gallery of X-ray (blue & white) and optical (grey & white) images shows, the shapes of the massive clouds of hot gas that produce X-ray light in these galaxies differ markedly from the distribution of stars that produce the optical light.

Although supernovas are a possible energy source, a more probable cause has been identified. The scientists detected a correlation between the shape of the hot gas clouds and the power produced at radio wavelengths by high-energy electrons. This power source can be traced back to the supermassive black hole in the galaxies' central regions.

Repetitive explosive activity fuelled by the infall of gas into the central supermassive black hole is known to occur in giant elliptical galaxies located in galaxy clusters. Scientists' analysis of the Chandra data indicates that the same phenomena are occurring in isolated elliptical galaxies as well.

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L

Posts: 131433
Date:
Neutron stars collide
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Chandra image.
Coordinates (2000): RA = 23 01 30.00 Dec= -38 ° 58' 33.00

The most likely explanation for GRB 050709 is that it was produced by a collision of two neutron stars, or a neutron star and a black hole 2 billion light years away. Such a collision would result in the formation of a black hole (or a larger black hole), and could generate a beam of high-energy particles that could account for the powerful gamma-ray pulse as well as observed radio, optical and X-ray afterglows.

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RE: Birth of a black hole
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Scientists have solved a 35-year-old mystery of the origin of powerful, split-second flashes of light called short gamma-ray bursts. These flashes, brighter than a billion suns yet lasting only a few milliseconds, have been simply too fast to catch... until now.

If you guessed that a black hole is involved, you are at least half right. Short gamma-ray bursts arise from collisions between a black hole and a neutron star or between two neutron stars. In the first scenario, the black hole gulps down the neutron star and grows bigger. In the second scenario, the two neutron stars create a black hole.

Gamma-ray bursts, the most powerful explosions known, were first detected in the late 1960s. They are random, fleeting, and can occur from any region of the sky. Try finding the location of a camera flash somewhere in a vast sports stadium and you'll have a sense of the challenge facing gamma-ray burst hunters. Solving this mystery took unprecedented coordination among scientists using a multitude of ground-based telescopes and NASA satellites.

Two years ago scientists discovered that longer bursts, lasting over two seconds, arise from the explosion of very massive stars. About 30 percent of bursts, however, are short and under two seconds.

Four short gamma-ray bursts have been detected since May. Two of these are featured in four papers in the October 6 issue of Nature. One burst from July provides the "smoking gun" evidence to support the collision theory. Another burst goes a step further by providing tantalizing, first-time evidence of a black hole eating a neutron star---first stretching the neutron star into a crescent, swallowing it, and then gulping up crumbs of the broken star in the minutes and hours that followed.

These discoveries might also aid in the direct detection of gravitational waves, never before seen. Such mergers create gravitational waves, or ripples in space-time. Short gamma-ray bursts could tell scientists when and where to look for the ripples.

"Gamma-ray bursts in general are notoriously difficult to study, but the shortest ones have been next to impossible to pin down. All that has changed. We now have the tools in place to study these events" - Dr. Neil Gehrels, NASA Goddard Space Flight Centre in Greenbelt, principal investigator of NASA's Swift satellite and lead author on one of the Nature reports.

"We had a hunch that short gamma-ray bursts came from a neutron star crashing into a black hole or another neutron star, but these new detections leave no doubt" - Dr. Derek Fox of Penn State, lead author on one Nature report detailing a multi-wavelength observation.

Fox's team discovered the X-ray afterglow of the July 9 burst with NASA's Chandra X-ray Observatory. A team led by Prof. Jens Hjorth of the University of Copenhagen then identified the optical afterglow using the Danish 1.5-meter telescope at the La Silla Observatory in Chile. Fox's team then continued its studies of the afterglow with NASA's Hubble Space Telescope; the du Pont and Swope telescopes at Las Campanas, Chile, funded by the Carnegie Institution; the Subaru telescope on Mauna Kea, Hawaii, operated by the National Astronomical Observatory of Japan; and the Very Large Array, a stretch of 27 radio telescopes near Socorro, N.M., operated by the National Radio Astronomy Observatory.

The multi-wavelength observation of the July 9 burst, called GRB 050709, provided all the pieces of the puzzle to solve the short burst mystery.

"Powerful telescopes detected no supernova as the gamma-ray burst faded, arguing against the explosion of a massive star" - Dr. George Ricker of MIT, HETE Principal Investigator.


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Hubble Space Telescope image of the sky surrounding the afterglow and host galaxy of the HETE short burst of July 9, 2005. The circle indicates the region of sky that HETE saw the burst from; according to the HETE team we would find the burst within this region. The box, inset, indicates where the X-ray and optical afterglow of the burst was ultimately found. The colours indicate the intensity of red light (814 nm) as seen by the Advanced Camera for Surveys instrument on HST.


Ricker added that the July 9 burst and probably the May 9 burst are located in the outskirts of their host galaxies, where old merging binaries are expected to be. Short gamma-ray bursts are not expected in young, star-forming galaxies. It takes billions of years for two massive stars, coupled in a binary system, to first evolve to the black hole or neutron star phase and then to merge. The transition of a star to a black hole or neutron star involves an explosion (supernova) that can kick the binary system far from its origin and out towards the edge of its host galaxy.

This July 9 burst and a later one on July 24 showed unique signals that point to not just any old merger but, more specifically, a black hole - neutron star merger. Scientists saw spikes of X-ray light after the initial gamma-ray burst. The quick gamma-ray portion is likely a signal of the black hole swallowing most of the neutron star. The X-ray signals, in the minutes to hours that followed, could be crumbs of neutron star material falling into the black hole, a bit like dessert.

And there's more. Mergers create gravitational waves, ripples in space-time predicted by Einstein but never detected directly. The July 9 burst was about two billion light years away. A big merger closer to the Earth could be detected by the National Science Foundation's Laser Interferometer Gravitational-Wave Observatory (LIGO). If Swift detects a nearby short burst, LIGO scientists could go back and check the data with a precise time and location in mind.

"This is good news for LIGO. The connection between short bursts and mergers firms up projected rates for LIGO, and they appear to be at the high end of previous estimates. Also, observations provide tantalizing hints of black hole - neutron star mergers, which have not been detected before. During LIGO's upcoming yearlong observation we may detect gravitational waves from such an event" - Dr. Albert Lazzarini, of LIGO Laboratory at Caltech.

A black hole - neutron star merger would generate stronger gravitational waves than two merging neutron stars. The question now is how common and how close these mergers are. Swift, launched in November 2004, can provide that answer.

source

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L

Posts: 131433
Date:
Short gamma-ray bursts
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Short gamma-ray bursts are the result of a merger of two neutron stars, or a neutron star with a black hole.
Theory predicts that Short-duration GRBs shoot out two jets of material as they merge.
And because it is only by chance that a jet would point in the same direction as the Earth; this would mean that there would be more than thirty times more or these peculiar event happening than is currently observed by the swift satellite.


Two events were studied.
The Swift satellite detected a Short gamma-ray burst on May 9, and another burst was recorded by the HETE-2 satellite on July 9.

The HETE-2 observations lead to an accurate distance being determined.
The July event occurred only a billion light-years away, 10 times closer than most other GRBs. The event was also 1,000 times less energetic than other GBRs.
The energy levels observed did not fit the theory that it could have been caused by the cataclysmic death of a massive star.
It seems that the most likely candidate involves the merger of two old stars.

"Our observations do not prove the coalescence model, but we surely have found a lady with a smoking gun next to a dead body" - Shri Kulkarni, Caltech scientist.

The discovery results are detailed in the October 6 issue of the journal Nature.

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Posts: 131433
Date:
NASA Solves Cosmic Mystery
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The Progenitors of Short-Hard Gamma-Ray Bursts from an Extended Sample of Events
Authors: Avishay Gal-Yam, Ehud Nakar, Eran Ofek, D. B. Fox, S. B. Cenko, S. R. Kulkarni, A. M. Soderberg, F. Harrison, P. A. Price, B. E. Penprase, E. Berger, M. Gladders, J. Mulchaey

The discovery and characterization of the afterglow emission and host galaxies of short-hard gamma-ray bursts (short-hard GRBs, or SHBs), following accurate localization by the SWIFT and HETE-2 spacecraft, is one of the most exciting recent astronomical discoveries.
In particular, indications that SHB progenitors belong to old stellar populations, in contrast to those of the long-soft GRBs, provide a strong clue for the physical nature of these systems. Definitive conclusions are currently limited by the small number of SHBs with known hosts available for study.

Here, researchers present their investigation of SHBs previously localized by the interplanetary network (IPN) using new and archival optical and X-ray observations. They show that they can likely identify the host galaxies/clusters for additional three bursts, doubling the sample of SHBs with known hosts and/or distances.
In particular, they determined at a very high probability that the bright SHB 790613 occurred within the rich galaxy cluster Abell 1892, making it probably the nearest SHB currently known.

They show that the brightest galaxy within the error box of SHB 000607, at z=0.1405, is most likely the host galaxy of this event. They revisit X-ray and optical observations of the SHB 020531, and suggest that for this burst both the X-ray afterglow and host galaxy may have been detected. Additionally, they rule out the existence of galaxy over densities (down to ~21 mag) near the locations of two other SHBs, and set a lower limit on the their probable redshift.

They discuss their results in the context of the recent advances made possible by the accurate localizations of SHBs by SWIFT and HETE-2 missions during the last few months.

Read More (PDF)

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L

Posts: 131433
Date:
Short GRBs
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NASA Solves Cosmic Mystery

NASA is hosting a news conference at 17:00 GMT, Wednesday, October 5th, to announce scientists have solved a 35-year-old mystery. The press conference is in NASA's auditorium, 300 E Street S.W., Washington.

Scientists solved the mystery of the origin of powerful, split-second flashes of light called short gamma ray bursts. The flashes, brighter than a billion suns and lasting only a few milliseconds, had previously been too fast to catch.

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Posts: 131433
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RE: Birth of a black hole
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Title: Hyperstars - Main Origin of Short Gamma Ray Bursts?
Authors: Arnon Dar

The first well-localized short-duration gamma ray bursts (GRBs), GRB 050509b, GRB 050709 and GRB 050724, could have been the narrowly beamed initial spike of a burst/hyper flare of soft gamma ray repeaters (SGRs) in host galaxies at cosmological distances.
Such bursts are expected if SGRs are young hyperstars, i.e. neutron stars where a considerable fraction of their neutrons have converted to hyperons and/or strange quark matter.

Read More (PDF)

-- Edited by Blobrana at 02:09, 2005-09-28

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Scientists using NASA's Swift satellite say they have found newborn black holes, just seconds old, in a confused state of existence. The holes are consuming material falling into them while somehow propelling other material away at great speeds.

These black holes are born in massive star explosions. An initial blast obliterates the star, yet the chaotic black hole activity appears to re-energize the explosion several times in just a few minutes. This is a dramatically different view of star death, one that entails multiple explosive outbursts and not just a single bang, as previously thought.

"Stars are exploding two, three and sometimes four times in the first minutes following the initial explosion. First comes a blast of gamma rays followed by intense pulses of X-rays. The energies involved are much greater than anyone expected" - Prof. David Burrows of Penn State, University Park.

Scientists have seen this phenomenon in nearly half of the longer gamma-ray bursts detected by Swift. These gamma-ray bursts are the most powerful explosions known. They are forerunners of a massive star explosion called a hypernova, which is bigger than a supernova. Using Swift, scientists are finally able to see gamma-ray bursts within minutes after the trigger, instead of hours or days, and are privy to newborn black hole activity.

Until this latest Swift discovery, scientists assumed a simple scenario of a single explosion followed by a graceful afterglow of the dying embers. The new scenario of a blast followed by a series of powerful "hiccups" is particularly evident in a gamma-ray burst from May 2, 2005, named GRB 050502B. This burst lasted 17 seconds during the early morning hours in the constellation Leo. About 500 seconds later, Swift detected a spike in X-ray light about 100 times brighter than anything seen before.

Position(2000): RA 09:30:10.024 DEC +16:59:48.07

Previously there had been hints of an "X-ray bump" between the burst and afterglow in previous gamma-ray bursts, coming a minute or so after the burst. Swift has seen more than one dozen clear cases of multiple explosions. There are several theories to describe this newly discovered phenomenon and most point to the presence of a newborn black hole.

"The newly formed black hole immediately gets to work. We aren't clear on the details yet, but it appears to be messy. Matter is falling into the black hole, which releases a great amount of energy. Other matter gets blasted away from the black hole and flies out into the interstellar medium. This is by no means a smooth operation" - Prof. Peter Meszaros of Penn State, head of the Swift theory team.

Another theory is the jet of material shooting away from the dead star starts to fall back onto itself, creating shockwaves in the jet core that ram together blobs of gas and produce X-ray light.

"None of this was realized before simply because we couldn't get to the scene of the explosion fast enough. Swift has the unique ability to detect bursts and turn its X-ray and ultraviolet-optical telescopes to the explosion's embers within minutes. As such, Swift is detecting new burst details that might rewrite theory" - Dr. Neil Gehrels of NASA Goddard Space Flight Center, Greenbelt, Md., Swift principal investigator.

Swift carries three main instruments: the Burst Alert Telescope (BAT); X-ray Telescope (XRT); and the Ultraviolet/Optical Telescope (UVOT). Today's announcement is based largely on XRT data. The XRT was built at Penn State with partners at the Brera Astronomical Observatory in Italy and the University of Leicester in England.

A paper discussing these findings appears online today on Science Express and in the September 9 issue of Science. Prof. David Burrows is lead author of the paper.

source

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Posts: 131433
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RE: GRBs
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Scientists at the Istituto Nazionale di Astrofisica (INAF) in Italy have used NASA’s Swift satellite to show that Gamma Ray Bursts (GRBs) are a two-step explosion.

"The first burst of energy, lasting less than a few minutes, is produced by shockwaves within the collapsing star. Whereas the longer, less energetic afterglow is produced by collisions between ejected matter and the material around the star, witnessing the X-ray light curves in the transition period from prompt emission to afterglow" - Sergio Campana, study author.

The soft X-ray afterglow following the initial GRB can last for periods ranging from hours to weeks. This afterglow was initially thought to be the slow fading of the initial burst.
But after observing five GRBs and measuring the patterns of X-ray emission, the INAF scientists determined that the afterglow was instead caused by violent shock interactions caused by the initial high-intensity blast.

"The duration of GRBs, together with their spectral properties, suggest a classification into short and long bursts" - Physicist Dieter Hartmann, Clemson University.

Short Gamma Ray Bursts are believed to be caused by the merging of compact binary stars, such as two neutron stars. However, scientists are still not sure that this is the true explanation and plan to continue to use the Swift satellite to investigate further the nature of these bursts.
To explain long Gamma ray bursts, scientists favour the "collapsar" model; where a rapidly spinning massive star has undergone extreme gravitational collapse and created a black hole. A disk of material from the collapsed star forms around the outer rim of the black hole. Through complex processes, a jet of high-energy radiation shoots from this disk, bursting through the surface of the star at almost the speed of light.

Intense bursts of high-frequency gamma ray and X-ray radiation from exploding stars were discovered nearly four decades ago. Since NASA launched the Swift satellite in Nov. 2004, its detectors have picked up a burst every couple of days.

This research is detailed in the August 18 2005 issue of the journal Nature.

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