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TOPIC: Gamma-Ray Bursts


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RE: Gamma-Ray Bursts
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Astronomers are scrambling to understand what caused a fleeting but extremely powerful burst of radio waves that originated beyond the Milky Way. Initial suspects include the merger of two dense stellar corpses called neutron stars and the complete evaporation of a black hole.
Finding more such events in new or archived data could help astronomers make the first ever detection of ripples in spacetime which should occur when neutron stars merge, according to Einstein's theory of general relativity.
The burst was discovered by David Narkevic, an undergraduate student at West Virginia University in Morgantown, US, while he was searching through archived data taken in 2001 by the Parkes radio dish in Australia. He was looking for periodic signals from pulsars rotating neutron stars within our galaxy.
But he came upon a short burst of radio emission that appeared to come from outside the Milky Way. Because electrons in space cause different frequencies of radio waves to arrive at Earth at different times, the researchers used the delay to estimate that the source probably lay about 1.6 billion light years away.

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burst smc
A visible-light image of the Small Magellanic Cloud, combined with radio observation data (contoured lines). The blue circle marks where a strange 5-millisecond radio burst originated.
Credit: Lorimer et al., NRAO, AUI, NSF

"We're confused and excited, but it could open up a whole new research field. If we really go after these things, we expect to find out that a couple hundred of them occur each day" - Duncan Lorimer, an astrophysicist at West Virginia University in Morgantown and the National Radio Astronomy Observatory who led the discovery-making team.

The discovery is detailed in the Sept. 27 issue of the online journal Science Express.

-- Edited by Blobrana at 22:33, 2007-09-27

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Title: Spitzer Observations of Gamma-Ray Burst Host Galaxies: A Unique Window into High Redshift Chemical Evolution and Star-formation
Authors: Ranga-Ram Chary, Edo Berger, Len Cowie

We present deep Spitzer 3.6 micron observations of three z~5 GRB host galaxies. Our observations reveal that z~5 GRB hosts are a factor of 3 less luminous than the median rest-frame V-band luminosity of spectroscopically confirmed z~5 galaxies in the GOODS fields and the UDF. The strong connection between GRBs and massive star formation implies that not all star-forming galaxies at these redshifts are currently being accounted for in deep surveys and GRBs provide a unique way to measure the contribution to the star-formation rate density from galaxies at the faint end of the galaxy luminosity function. By correlating the co-moving star-formation rate density with co-moving GRB rates at lower redshifts, we estimate a lower limit to the star-formation rate density of 0.12 ±0.09 and 0.09 ±0.05 M_sun/yr/Mpc³ at z~4.5 and z~6, respectively. Finally, we provide evidence that the average metallicity of star-forming galaxies evolves as (stellar mass density)^(0.69 ±0.17) between z ~ 5 and z ~0, probably indicative of the loss of a significant fraction of metals to the intergalactic medium, particularly in low-mass galaxies.

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Title: The GRB afterglow onset observed by REM: fireball Lorentz factor and afterglow fluence
Authors: Daniele Malesani (DARK), Emilio Molinari (INAF/Brera), Susanna Vergani (DIAS and DCU), Stefano Covino (INAF/Brera), for the REM team

We report observations of the early light curves of GRB 060418 and GRB 060607A, carried out with the pink robotic telescope REM. A clear peak is detected for both events, which is interpreted as the onset of the afterglow, that is the time at which the fireball starts decelerating. This detection allows to directly measure the initial fireball Lorentz factor, which was found to be Gamma_0 ~ 400 for both events, fully confirming the ultrarelativistic nature of gamma-ray burst fireballs. Sampling the light curve before the peak also allows to compute the bolometric fluence of the afterglow, which is 16% of the prompt one in the case of GRB 060418.


X-ray and NIR/optical light curves of GRB 060418 and GRB060607A. The REM data have been complemented by GCN and VLT data.

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The fastest flows of matter in the universe shoot out of dying stars at more than 99.999% the speed of light, new observations reveal.
When a massive star runs out of fuel, it collapses to form a black hole or a neutron star. In the process, some of the matter from the star also explodes outwards at blistering speeds, producing an intense burst of gamma rays and other radiation.
Scientists had predicted that the matter expanding in these explosions would be propelled to very nearly the speed of light, but it has previously not been possible to clock them precisely.
Now, rapid follow-up measurements of two gamma-ray bursts have allowed a team of scientists to precisely measure the expansion speed of matter in these explosion to more than 99.999% the speed of light. The team was led by Emilio Molinari of the Osservatorio Astronomico di Brera in Merate, Italy.

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Matter Flashed at Ultra Speed
Robotic Telescope Measures Speed of Material Ejected in Cosmic Death
Using a robotic telescope at the ESO La Silla Observatory, astronomers have for the first time measured the velocity of the explosions known as gamma-ray bursts. The material is travelling at the extraordinary speed of more than 99.999% of the velocity of light, the maximum speed limit in the Universe.

"With the development of fast-slewing ground-based telescopes such as the 0.6-m REM telescope at ESO La Silla, we can now study in great detail the very first moments following these cosmic catastrophes" - Emilio Molinari, leader of the team that made the discovery.

Gamma-ray bursts (GRBs) are powerful explosions occurring in distant galaxies, that often signal the death of stars. They are so bright that, for a brief moment, they almost rival the whole Universe in luminosity. They last, however, for only a very short time, from less than a second to a few minutes. Astronomers have long known that, in order to emit such incredible power in so little time, the exploding material must be moving at a speed comparable with that of light, namely 300 000 km per second. By studying the temporal evolution of the burst luminosity, it has now been possible for the first time to precisely measure this velocity.
Gamma-ray bursts, which are unseen by our eyes, are discovered by artificial satellites. The collision of the gamma-ray burst jets into the surrounding gas generates an afterglow visible in the optical and near-infrared that can radiate for several weeks. An array of robotic telescopes were built on the ground, ready to catch this vanishing emission. On 18 April and 7 June 2006, the NASA/PPARC/ASI Swift satellite detected two bright gamma-ray bursts. In a matter of a few seconds, their position was transmitted to the ground, and the REM telescope began automatically to observe the two GRB fields, detecting the near-infrared afterglows, and monitored the evolution of their luminosity as a function of time (the light curve). The small size of the telescope is compensated by its rapidity of slewing, which allowed astronomers to begin observations very soon after each GRB's detection (39 and 41 seconds after the alert, respectively), and to monitor the very early stages of their light curve.
The two gamma-ray bursts were located 9.3 and 11.5 billion light-years away, respectively.

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GRB 060206
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Title: GRB 060206 and the quandary of achromatic breaks in afterglow light curves
Authors: P.A. Curran, A.J. van der Horst, R.A.M.J. Wijers, R.L.C. Starling, A.J. Castro-Tirado, J.P.U. Fynbo, J. Gorosabel, A.S. Jarvinen, D. Malesani, E. Rol, N.R. Tanvir, K. Wiersema, M.R. Burleigh, S.L. Casewell, P.D. Dobbie, S. Guziy, P. Jakobsson, M. Jelinek, P. Laursen, A.J. Levan, C.G. Mundell, J. Naranen, S. Piranomonte

Gamma-ray burst afterglow observations in the Swift era have a perceived lack of achromatic jet breaks compared to the BeppoSAX era. We examine the effect of the fact that pre-Swift afterglow observations tend to be dominated by the optical, whereas in the Swift era most well sampled light curves are in the X-ray. We present our multi-wavelength analysis of GRB 060206 as an illustrative example to demonstrate the possible effect of this change. The results of temporal and spectral analyses are compared, and attempts are made to fit the data within the context of the standard blast wave model. We find that while the break appears more pronounced in the optical and evidence for it from the X-ray alone is weak, the data are actually consistent with an achromatic break at about 16 hours. This break and the light curves fit standard blast wave models, either as a jet break or as an injection break. The Swift and pre-Swift samples of afterglows could well be consistent in their break behaviour, but the lack of accurate models for jet breaks and the dearth of well sampled late time optical light curves for Swift bursts preclude definitive statements on the matter.

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U.S. astronomers have determined flares seen after a gamma-ray burst are apparently a continuation of the burst itself.
Gamma-ray bursts release in seconds the same amount of energy the sun will emit during its expected 10 billion-year lifetime. That energy comes from the core of a massive star collapsing to form a black hole or neutron star.
Early in its mission, the National Aeronautics and Space Administration's Swift satellite's X-ray Telescope discovered the initial pulse of gamma-rays -- known as prompt emission -- is often followed by short-lived but powerful, X-ray flares, suggesting a GRB's central engines remain active long after the prompt emission.
In the latest study, Hans Krimm and colleagues at the Universities Space Research Association and the Goddard Space Flight Centre demonstrated X-ray flares are indeed a continuation of the prompt emission.

"This pattern points to a continuous injection of energy from the central engine, perhaps fuelled by sporadic in-fall of material onto a black hole. The black hole just keeps gobbling up gas and the engine keeps spewing out energy" - Hans Krimm.

The research is to appear in the Aug. 10 issue of the Astrophysical Journal.

Source UPI


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There is a certain type of cosmic explosion that becomes, in a flash, the brightest thing in the universe, emitting for a few seconds as much radiation as a million galaxies. Don't bother looking for one in the sky, though, since most of the light is in the gamma-ray part of the spectrum, a realm we can't see.
Astronomers observe these colossal gamma-ray bursts with space-based telescopes, however.
They generally agree that only the birth of a black hole could supply enough spark for one of these intense flashes, but there remains a great deal of uncertainty over what converts the newborn black hole's energy into the radiation that astronomers detect.

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VLT Automatically Takes Detailed Spectra of Gamma-Ray Burst Afterglows Only Minutes After Discovery
A time-series of high-resolution spectra in the optical and ultraviolet has twice been obtained just a few minutes after the detection of a gamma-ray bust explosion in a distant galaxy. The international team of astronomers responsible for these observations derived new conclusive evidence about the nature of the surroundings of these powerful explosions linked to the death of massive stars.
At 11:08 pm on 17 April 2006, an alarm rang in the Control Room of ESO's Very Large Telescope on Paranal, Chile. Fortunately, it did not announce any catastrophe on the mountain, nor with one of the world's largest telescopes. Instead, it signalled the doom of a massive star, 9.3 billion light-years away, whose final scream of agony - a powerful burst of gamma rays - had been recorded by the Swift satellite only two minutes earlier. The alarm was triggered by the activation of the VLT Rapid Response Mode, a novel system that allows for robotic observations without any human intervention, except for the alignment of the spectrograph slit.
Starting less than 10 minutes after the Swift detection, a series of spectra of increasing integration times (3, 5, 10, 20, 40 and 80 minutes) were taken with the Ultraviolet and Visual Echelle Spectrograph (UVES), mounted on Kueyen, the second Unit Telescope of the VLT.

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Long-duration Gamma-Ray Bursts
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Title: No supernovae detected in two long-duration Gamma-Ray Bursts
Authors: D. Watson (1), J. P. U. Fynbo (1), C. C. Thne (1), J. Sollerman (1,2) ((1) DARK, NBI, U. Copenhagen, (2) U. Stockholm)

There is strong evidence that long duration gamma-ray bursts (GRBs) are produced during the collapse of a massive star. In the standard version of the Collapsar model, a broad-lined and luminous Type Ic core-collapse supernova (SN) accompanies the GRB. This association has been confirmed in observations of several nearby GRBs. Recent observations show that some long duration GRBs are different. No SN emission accompanied the long duration GRBs 060505 and 060614 down to limits fainter than any known Type Ic SN and hundreds of times fainter than the archetypal SN1998bw that accompanied GRB980425. Multi-band observations of the early afterglows, as well as spectroscopy of the host galaxies, exclude the possibility of significant dust obscuration. Furthermore, the bursts originated in star-forming galaxies, and in the case of GRBs060505 the burst was localised to a compact star-forming knot in a spiral arm of its host galaxy. We find that the properties of the host galaxies, the long duration of the bursts and, in the case of GRB060505 the location of the burst within its host, all imply a massive stellar origin. The absence of a SN to such deep limits therefore suggests a new phenomenological type of massive stellar death.

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