* Astronomy

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RE: Gamma-Ray Bursts

Blobrana · L

Title: The nature of "dark" gamma-ray bursts
Authors: J. Greiner, T. Krühler, S. Klose, P. Afonso, C. Clemens, R. Filgas, D.H. Hartmann, A. Küpcü Yolda, M. Nardini, F.Olivares Estay, A. Rau, A. Rossi, P. Schady, A. Updike

Context: Thirteen years after the discovery of the first afterglows, the nature of dark gamma-ray bursts (GRB) still eludes explanation: while each ng-duration GRB typically has an X-ray afterglow, optical/NIR emission is only seen for 40-60% of them.
Aim: Here we use the afterglow detection statistics of the systematic follow-up observations performed with GROND since mid-2007 in order to derive the fraction of "dark bursts" according to different methods, and to distinguish between various scenarios for "dark bursts".
Method: Observations were performed with the 7-channel "Gamma-Ray Optical and Near-infrared Detector" (GROND) at the 2.2m MPI/ESO telescope. We used the afterglow detection rate in dependence on the delay time between GRB and the first GROND exposure.
Results: For long-duration Swift bursts with a detected X-ray afterglow, we achieve a 90% (35/39) detection rate of optical/NIR afterglows whenever our observations started within less than 240\,min after the burst. Complementing our GROND data with Swift/XRT spectra we construct broad-band spectral energy distributions and derive rest-frame extinctions. e detect 25-40% "dark bursts", depending on the definition used. The faint optical afterglow emission of "dark bursts" is mainly due to a combination of two contributing factors: (i) moderate intrinsic extinction at moderate redshifts, and (ii) about 22% of "dark" bursts at redshift >5.

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Light Dawns on Dark Gamma-ray Bursts

Gamma-ray bursts are among the most energetic events in the Universe, but some appear curiously faint in visible light. The biggest study to date of these so-called dark gamma-ray bursts, using the GROND instrument on the 2.2-metre MPG/ESO telescope at La Silla in Chile, has found that these gigantic explosions don't require exotic explanations. Their faintness is now fully explained by a combination of causes, the most important of which is the presence of dust between the Earth and the explosion.
Gamma-ray bursts (GRBs), fleeting events that last from less than a second to several minutes, are detected by orbiting observatories that can pick up their high energy radiation. Thirteen years ago, however, astronomers discovered a longer-lasting stream of less energetic radiation coming from these violent outbursts, which can last for weeks or even years after the initial explosion. Astronomers call this the burst's afterglow.
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Most powerful gamma-ray bursts linked to black holes

The most powerful explosions in the universe are caused by the births of black holes rather than dense neutron stars called magnetars, new evidence confirms.
Gamma-ray bursts are blasts of high-energy radiation that arrive at Earth from all directions.
Bursts lasting longer than 2 seconds are thought to be related to the collapse of massive stars that fire out jets of matter at near light speed, emitting copious radiation in the process. When the jets happen to be pointed at Earth, they send a volley of gamma rays our way.
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Title: First Light on GRBs with Fermi
Authors: Charles D. Dermer (NRL)

Fermi LAT (Large Area Telescope) and GBM (Gamma ray Burst Monitor) observations of GRBs are briefly reviewed, keeping in mind EGRET expectations. Using gamma\gamma constraints on outflow Lorentz factors, leptonic models are pitted against hadronic models, and found to be energetically favoured. Interpretation of the Fermi data on GRBs helps establish whether GRBs accelerate cosmic rays, including those reaching \approx 10^{20} eV.

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The Subaru Telescope has detected clues crucial for understanding the origin of mysterious dark gamma-ray bursts (GRBs).
The research, led by astronomers from Kyoto University, Tokyo Institute of Technology, and the National Astronomical Observatory of Japan, is a very rare case of the detection of a dark GRBs host galaxy and afterglow in the near-infrared wavelength.
The researchers not only found that the host galaxy of this GRB is one of the most massive GRB host galaxies but also that a local dusty environment around the GRB significantly suppresses its afterglow.
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Space explosion from collapsing star blinds satellite in brightest ever X-ray blast

Scientists have identified a dazzling burst of X-rays so bright that they blinded a camera that had been set up to watch the event.
The X-ray blast is the brightest ever detected from an object outside our own galaxy.
The X-rays had been travelling for over 5 billion years before being detected by the X-ray telescope on board the Swift satellite: a joint NASA, UK, and Italian mission.
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Title: Gamma-Ray Burst Central Engines: Black Hole Vs. Magnetar
Authors: B.D. Metzger (Princeton University)

Discovered over forty years ago, Gamma-Ray Bursts (GRBs) remain a forefront topic in modern astrophysics. Perhaps the most fundamental question associated with GRBs is the nature of the astrophysical agent (or agents) that ultimately powers them: the central engine. In this review, I focus on the possible central engines of long-duration GRBs, and the constraints that present observations place on these models. Long GRBs are definitively associated with the deaths of massive stars, but whether the central engine is an accreting black hole or a rapidly-spinning, highly-magnetized neutron star (a "proto-magnetar") remains unsettled. This distinction has been brought into particular focus by recent MHD simulations of the core-collapse of massive, rotating "collapsar progenitors," which suggest that powerful magneto-centrifugal outflows from the proto-neutron star may stave off black hole formation entirely. Although both black hole and magnetar GRB models remain viable, I argue that the magnetar model is more mature in the sense that it provides quantitative explanations for the durations, energies, Lorentz factors, and collimation of long GRB outflows. Given these virtues, one promising strategy to break the present stalemate is to further develop the magnetar model until inescapable (and falsifiable) predictions emerge. This course of action signals a renewed challenge to translate time-dependent jet properties (power, magnetisation, and Lorentz factor) into observables (gamma-ray light curves and spectra).

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Title: Can Gamma Ray Bursts be Detected Using Infrasound
Authors: Palmer, Jahi; McGruder, C.; Hetzer, C.

Infrasound has been used to detect sonic disturbances in earth's atmosphere caused by terrestrial events such as earthquakes and lightning. It may be possible to detect celestial events such as Gamma Ray Bursts (GRB's) through this method. We have searched for GRB's which are known to have caused ionospheric disturbances in infrasonic data. None of the selected GRB's were found to be associated with infrasonic disturbances.

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Magnetic Power Revealed in Gamma-Ray Burst Jet

A specialised camera on a telescope operated by U.K. astronomers from Liverpool has made the first measurement of magnetic fields in the afterglow of a gamma-ray burst (GRB). The result is reported in the Dec.10 issue of Nature magazine by the team of Liverpool John Moores University (LJMU) astronomers who built and operate the telescope and its unique scientific camera, named RINGO.
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