631 research outputs found
Probing cosmological parameters with GRBs
In light of the recent finding of the narrow clustering of the
geometrically-corrected gamma-ray energies emitted by Gamma Ray Bursts (GRBs),
we investigate the possibility to use these sources as standard candles to
probe cosmological parameters such as the matter density Omega_m and the
cosmological constant energy density Omega_Lambda. By simulating different
samples of gamma-ray bursts, based on recent observational results, we find
that Omega_m (with the prior Omega_m + Omega_Lambda = 1) can be determined with
accuracy ~7% with data from 300 GRBs, provided a local calibration of the
standard candles be achieved.Comment: 4 pages, 2 figures, to appear in the Proceedings of the Conference
"30 Years of GRB Discovery", Santa Fe, New Mexico, USA, September 8-12, 200
Iron line emission in X-ray afterglows
Recent observations of X-ray afterglows reveal the presence of a redshifted
Kalpha iron line in emission in four bursts. In GRB 991216, the line was
detected by the low energy grating of Chandra, which showed the line to be
broad, with a full width of ~15,000 km/s. These observations indicate the
presence of a >1 solar mass of iron rich material in the close vicinity of the
burst, most likely a supernova remnant. The fact that such strong lines are
observed less than a day after the trigger strongly limits the size of the
remnant, which must be very compact. If the remnant had the observed velocity
since the supernova explosion, its age would be less than a month. In this case
nickel and cobalt have not yet decayed into iron. We show how to solve this
paradox.Comment: 3 pages, to appear in the proceedings of the the 2nd Workshop on
Gamma-Ray Bursts in the Afterglow Era, Rome, Oct. 200
Ultra high energy neutrinos from gamma ray bursts
Protons accelerated to high energies in the relativistic shocks that generate
gamma ray bursts photoproduce pions, and then neutrinos in situ. I show that
ultra high energy neutrinos (> 10^19 eV) are produced during the burst and the
afterglow. A larger flux, also from bursts, is generated via photoproduction
off CMBR photons in flight but is not correlated with currently observable
bursts, appearing as a bright background. Adiabatic/synchrotron losses from
protons/pions/muons are negligible. Temporal and directional coincidences with
bursts detected by satellites can separate correlated neutrinos from the
background.Comment: Adiabatic/synchrotron losses from protons/pions/muons shown to be
negligible. Accepted for publication in Phys. Rev. Letters. RevTe
On Particle Acceleration around Shocks. III. Shock Waves Moving at Arbitrary Speed. The Case of Large-Scale Magnetic Field and Anisotropic Scattering
A mathematical approach to investigate particle acceleration at shock waves moving at arbitrary speed in a medium with arbitrary scattering properties was first discussed in work by Vietri and Blasi. We use this method and somewhat extend it in order to include the effect of a large-scale magnetic field in the upstream plasma, with arbitrary orientation with respect to the direction of motion of the shock. We also use this approach to investigate the effects of anisotropic scattering on spectra and anisotropies of the distribution function of the accelerated particles
Evolution of perturbed accelerating relativistic shock waves
We study the evolution of an accelerating hyperrelativistic shock under the
presence of upstream inhomogeneities wrinkling the discontinuity surface. The
investigation is conducted by means of numerical simulations using the PLUTO
code for astrophysical fluid dynamics. The reliability and robustness of the
code are demonstrated against well known results coming from the linear
perturbation theory. We then follow the nonlinear evolution of two classes of
perturbing upstream atmospheres and conclude that no lasting wrinkle can be
preserved indefinitely by the flow. Finally we derive analytically a
description of the geometrical effects of a turbulent upstream ambient on the
discontinuity surface.Comment: 54 Pages, 24 Figures. Accepted for Publication in the Astrophysical
Journa
GeV Photons from Ultra High Energy Cosmic Rays accelerated in Gamma Ray Bursts
Gamma-ray bursts are produced by the dissipation of the kinetic energy of a
highly relativistic fireball, via the formation of a collisionless shock. When
this happens, Ultra High Energy Cosmic Rays up to 10^20 eV are produced. I show
in this paper that these particles produce, via synchrotron emission as they
cross the acceleration region, photons up to 300 GeV which carry away a small,
~0.01, but non-negligible fraction of the total burst energy. I show that, when
the shock occurs with the interstellar medium, the optical depth to
photon-photon scattering, which might cause energy degradation of the photons,
is small. The burst thusly produced would be detected at Earth simultaneoulsy
with the parent gamma-ray burst, although its duration may differ significantly
from that of the lower energy photons. The expected fluences, ~10^{-5}-10^{-6}
erg/cm^2 are well within the range of planned detectors. A new explanation for
the exceptional burst GRB 940217 is discussed.Comment: Accepted for publication in The Physical Review Letters. 4 pages,
RevTeX needed, no figure
The rest-frame UV-to-optical spectroscopy of APM 08279+5255 - BAL classification and black hole mass estimates
We present the analysis of the rest-frame optical-to-UV spectrum of APM
08279+5255, a well-known lensed broad absorption line (BAL) quasar at . The spectroscopic data are taken with the optical DOLoRes and near-IR
NICS instruments at TNG, and include the previously unexplored range between C
III] 1910 and [O III] 4959,5007. We investigate the
possible presence of multiple BALs by computing "balnicity" and absorption
indexes (i.e. BI, BI and AI) for the transitions Si IV 1400, C IV
1549, Al III 1860 and Mg II 2800. No clear evidence
for the presence of absorption features is found in addition to the already
known, prominent BAL associated to C IV, which supports a high-ionization BAL
classification for APM 08279+5255. We also study the properties of the [O III],
H and Mg II emission lines. We find that [O III] is intrinsically weak
(), as it is typically found in
luminous quasars with a strongly blueshifted C IV emission line (2500 km
s for APM 08279+5255). We compute the single-epoch black hole mass based
on Mg II and H broad emission lines, finding M, with the magnification factor that can
vary between 4 and 100 according to CO and rest-frame UV-to-mid-IR imaging
respectively. Using a Mg II equivalent width (EW)-to-Eddington ratio relation,
the EW \AA\ measured for APM 08279+5255 translates into an
Eddington ratio of 0.4, which is more consistent with . This
magnification factor also provides a value of that is consistent
with recent reverberation-mapping measurements derived from C IV and Si IV.Comment: 10 pages, 4 figures, 4 tables, accepted for publication in A&
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