829 research outputs found
Gamma-Ray Bursts from Neutron Star Mergers
Binary neutron stars merger (NSM) at cosmological distances is probably
the only -ray bursts model based on an independently observed
phenomenon which is known to be taking place at a comparable rate. We describe
this model, its predictions and some open questions.Comment: 4 pages, 1 Figure can be obtained on request by e-mail from the
Autho
Reverse Shock Emission as a Probe of GRB Ejecta
We calculate the reverse shock (RS) synchrotron emission in the optical and
the radio wavelength bands from electron-positron pair enriched gamma-ray burst
ejecta with the goal of determining the pair content of GRBs using early time
observations. We take into account an extensive number of physical effects that
influence radiation from the reverse-shock heated GRB ejecta. We find that
optical/IR flux depends very weakly on the number of pairs in the ejecta, and
there is no unique signature of ejecta pair enrichment if observations are
confined to a single wavelength band. It may be possible to determine if the
number of pairs per proton in the ejecta is > 100 by using observations in
optical and radio bands; the ratio of flux in the optical and radio at the peak
of each respective reverse-shock light curve is dependent on the number of
pairs per proton. We also find that over a large parameter space, RS emission
is expected to be very weak; GRB 990123 seems to have been an exceptional burst
in that only a very small fraction of the parameter space produces optical
flashes this bright. Also, it is often the case that the optical flux from the
forward shock is brighter than the reverse shock flux at deceleration. This
could be another possible reason for the paucity of prompt optical flashes with
a rapidly declining light curve at early times as was seen in 990123 and
021211. Some of these results are a generalization of similar results reported
in Nakar & Piran (2004).Comment: 12 pages, 6 figures, 2 tables, accepted to MNRA
Towards understanding gamma-ray bursts
\gamma-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the sixties. The BATSE detector on COMPTON-GRO satellite has been detecting GRBs for the last four years at a rate of one burst per day. Its findings has revolutionized our ideas about the nature of these objects. In this lecture I show that the simplest, most conventional and practically inevitable, interpretation of the observations is that GRBs form during the conversion of the kinetic energy of ultra-relativistic particles to radiation. The inner ``engine" that accelerates these particles is well hidden from direct observations and its origin might remain mysterious for a long time
Pure and loaded fireballs in SGR giant flares
On December 27, 2004, a giant flare from SGR 180620 was detected on earth.
Its thermal spectrum and temperature suggest that the flare resulted from an
energy release of about erg/sec close to the surface of a neutron
star in the form of radiation and/or pairs. This plasma expanded under its own
pressure producing a fireball and the observed gamma-rays escaped once the
fireball became optically thin. The giant flare was followed by a bright radio
afterglow, with an observable extended size, implying an energetic relativistic
outflow. We revisit here the evolution of relativistic fireballs and we
calculate the Lorentz factor and energy remaining in relativistic outflow once
the radiation escapes. We show that pairs that arise naturally in a pure
pairs-radiation fireball do not carry enough energy to account for the observed
afterglow. We consider various alternatives and we show that if the
relativistic outflow that causes the afterglow is related directly to the
prompt flare, then the initial fireball must be loaded by baryons or Poynting
flux. While we focus on parameters applicable to the giant flare and the radio
afterglow of SGR 180620 the calculations presented here might be also
applicable to GRBs
Neutrinos from the Propagation of a Relativistic Jet Through a Star
We discuss the neutrino signature of a relativistic jet propagating through a
stellar envelope, a scenario realized in the collapsar model for Gamma Ray
Bursts (GRBs). It is shown that the dramatic slowing of the jet deep within the
star is accompanied by inelastic neutron-nucleon collisions and the conversion
of a substantial fraction of the jet kinetic energy to neutrinos. These
neutrinos have observed energies in the range two to tens of GeV and an
estimated detection rate comparable to or larger than the detection rate of GeV
neutrinos from other GRB-related processes. The time delay between the arrival
of these neutrinos and the GRB photons is tens of seconds. An observation of
this delay would provide an indication that the GRB jet originated in a massive
star.Comment: To appear in Ap
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