* Astronomy

Blobrana · L

RE: Long Gamma Ray Bursts

Blobrana · L

Title: The Dark Bursts population in a complete sample of bright Swift Long Gamma-Ray Bursts
Authors: A. Melandri, B. Sbarufatti, P. D'Avanzo, R. Salvaterra, S. Campana, S. Covino, S. D. Vergani, L. Nava, G. Ghisellini, G. Ghirlanda, D. Fugazza, V. Mangano, M. Capalbi, G. Tagliaferri

We study the properties of the population of optically dark events present in a carefully selected complete sample of bright Swift long gamma-ray bursts. The high level of completeness in redshift of our sample (53 objects out of 58) allow us to establish the existence of a genuine dark population and we are able to estimate the maximum fraction of dark burst events (~30%) expected for the whole class of long gamma-ray burst. The redshift distribution of this population of dark bursts is similar to the one of the whole sample. Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux) of the sub-class of dark bursts is slightly higher than the average luminosity of the non-dark events. At the same time the prompt properties do not differ and the optical flux of dark events is at the lower tail of the optical flux distribution, corrected for Galactic absorption. All these properties suggest that dark bursts events generate in much denser environments with respect to normal bright events. We can therefore exclude the high-z and the low-density scenarios and conclude that the major cause of the origin of optically dark events is the dust extinction.

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Blobrana · L

Title: Dynamics and Gravitational Wave Signature of Collapsar Formation
Authors: C. D. Ott (1), C. Reisswig (1), E. Schnetter (2), E. O'Connor (1), U. Sperhake (3), F. Loeffler (2), P. Diener (2), E. Abdikamalov (2), I. Hawke (4), A. Burrows (5) ((1) Caltech, (2) LSU, (3) CSIC-IEEC Barcelona, (4) Southampton, (5) Princeton)

We perform 3+1 general relativistic simulations of rotating core collapse in the context of the collapsar model for long gamma-ray bursts. We employ a realistic progenitor, rotation based on results of stellar evolution calculations, and a simplified equation of state. Our simulations track self-consistently collapse, bounce, the postbounce phase, black hole formation, and the subsequent early hyperaccretion phase. We extract gravitational waves from the spacetime curvature and identify a unique gravitational wave signature associated with the early phase of collapsar formation.

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Blobrana · L

Black hole parasites explain cosmic flashes
Some of the brightest flashes in the universe may be the result of black holes burrowing into stars and devouring them from inside.
The flashes are known as gamma-ray bursts because most of their energy is in the form of high-energy radiation, including gamma rays and X-rays. The longer flashes, lasting at least a few seconds, have long been thought to signal the deaths of massive stars that have run out of fuel, causing them to collapse to form black holes, unleashing powerful jets of radiation in the process.

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Blobrana · L

Title: Close Binary Progenitors of Long Gamma Ray Bursts
Authors: M.V. Barkov, S.S. Komissarov

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the LGRBs with duration in excess of 100 seconds as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 10,000 seconds. These data suggest that some other, most likely magnetic mechanisms have to be considered. The magnetic models do not require the development of accretion disk within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion disks in the collapsar model and investigate the available range of mass accretion rates, black hole masses, and spins. A particularly interesting version of the binary progenitor involves merger of a WR star with an ultra-compact companion, neutron star or black hole. In this case we expect the formation of very long-lived accretion disks, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation.

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