526 research outputs found
Polarization lightcurves and position angle variation of beamed gamma-ray bursts
The recently detected linear polarization in the optical lightcurve of GRB
990510 renewed the interest on how polarization can be produced in gamma-ray
burst fireballs. Here we present a model based on the assumption that we are
seeing a collimated fireball, observed slightly off-axis. This introduces some
degree of anisotropy, and makes it possible to observe a linearly polarized
flux even if the magnetic field is completely tangled in the plane orthogonal
to the line of sight. We construct the lightcurve of the polarization flux,
showing that it is always characterized by two maxima, with the polarization
position angle changing by 90 deg. between the first and the second maximum.
The very same geometry here assumed implies that the total flux initially
decays in time as a power law, but gradually steepens as the bulk Lorentz
factor of the fireball decreases.Comment: 5 pages, 4 postscript figures, submitted to MNRAS letter
Constraints on the bulk Lorentz factor in the internal shock scenario for gamma-ray bursts
We investigate, independently of specific emission models, the constraints on
the value of the bulk Lorentz factor Gamma of a fireball. We assume that the
burst emission comes from internal shocks in a region transparent to Thomson
scattering and before deceleration due to the swept up external matter is
effective. We consider the role of Compton drag in decelerating fast moving
shells before they interact with slower ones, thus limiting the possible
differences in bulk Lorentz factor of shells. Tighter constraints on the
possible range of Gamma are derived by requiring that the internal shocks
transform more than a few per cent of the bulk energy into radiation. Efficient
bursts may require a hierarchical scenario, where a shell undergoes multiple
interactions with other shells. We conclude that fireballs with average Lorentz
factors larger than 1000 are unlikely to give rise to the observed bursts.Comment: 5 pages, 3 figures, accepted for publication in MNRAS, pink page
Thermal components in the early X-ray afterglow of GRBs
The possible presence of thermal components in the early X-ray afterglows of gamma-ray bursts is investigated. We discuss both the presence of a thermal continuum and, in particular, of collisional X-ray emission lines. We compute the predicted luminosity by a thin plasma for a range of metallicities for the continuum and the K_alpha lines of the elements Mg, Si, S, Ar, Ca and Fe. We show that light travel effects are dominant in the determination of the thermal continuum and line luminosities, and derive the relevant equations. We conclude that thermal lines and continua are unlikely to dominate the early afterglow of GRBs, unless the explosion site is surrounded by a very massive and extremely clumped shell of material. Such conditions are difficult to envisage in the close environment of GRB progenitor, unless they are excited by some strong precursor activity, like in the Supranova scenario
Afterglow lightcurves, viewing angle and the jet structure of gamma-ray bursts
Gamma ray bursts are often modelled as jet-like outflows directed towards the
observer; the cone angle of the jet is then commonly inferred from the time at
which there is a steepening in the power-law decay of the afterglow. We
consider an alternative model in which the jet has a beam pattern where the
luminosity per unit solid angle (and perhaps also the initial Lorentz factor)
decreases smoothly away from the axis, rather than having a well-defined cone
angle within which the flow is uniform. We show that the break in the afterglow
light curve then occurs at a time that depends on the viewing angle. Instead of
implying a range of intrinsically different jets - some very narrow, and others
with similar power spread over a wider cone - the data on afterglow breaks
could be consistent with a standardized jet, viewed from different angles. We
discuss the implication of this model for the luminosity function.Comment: Corrected typo in Eq. 1
The Brera Multi-scale Wavelet (BMW) ROSAT HRI source catalog. I: the algorithm
We present a new detection algorithm based on the wavelet transform for the
analysis of high energy astronomical images. The wavelet transform, due to its
multi-scale structure, is suited for the optimal detection of point-like as
well as extended sources, regardless of any loss of resolution with the
off-axis angle. Sources are detected as significant enhancements in the wavelet
space, after the subtraction of the non-flat components of the background.
Detection thresholds are computed through Monte Carlo simulations in order to
establish the expected number of spurious sources per field. The source
characterization is performed through a multi-source fitting in the wavelet
space. The procedure is designed to correctly deal with very crowded fields,
allowing for the simultaneous characterization of nearby sources. To obtain a
fast and reliable estimate of the source parameters and related errors, we
apply a novel decimation technique which, taking into account the correlation
properties of the wavelet transform, extracts a subset of almost independent
coefficients. We test the performance of this algorithm on synthetic fields,
analyzing with particular care the characterization of sources in poor
background situations, where the assumption of Gaussian statistics does not
hold. For these cases, where standard wavelet algorithms generally provide
underestimated errors, we infer errors through a procedure which relies on
robust basic statistics. Our algorithm is well suited for the analysis of
images taken with the new generation of X-ray instruments equipped with CCD
technology which will produce images with very low background and/or high
source density.Comment: 8 pages, 6 figures, ApJ in pres
The updated E_peak - E_gamma correlation in GRBs
The recently discovered correlation between the rest frame GRB peak spectral
energy and the collimation corrected energy in long
GRBs is potentially very important, yet awaits confirmation from an independent
sample. It may help to shed light on the radiation mechanism of the prompt GRB
phase and on the way -- and in which form -- the energy is released from the
central engine. We here present some additional evidence for the correlation
(two new bursts) and re-derive the best-fit parameters. The tightness of the
correlation is confirmed (sigma=0.1 dex). We show that this correlation allows
us, for the first time, to use GRBs as cosmological probes to constrain the
expansion history of the universe.Comment: 4 pages, 1 figure, submitted to Il Nuovo Cimento (4th Workshop
Gamma-Ray Bursts in the Afterglow Era, Rome, 18-22 October 2004). Additional
material at http://www.merate.mi.astro.it/~ghirla/deep/blink.ht
X-ray flares from propagation instabilities in long Gamma-Ray Burst jets
We present a numerical simulation of a gamma-ray burst jet from a
long-lasting engine in the core of a 16 solar mass Wolf-Rayet star. The engine
is kept active for 6000 s with a luminosity that decays in time as a power-law
with index -5/3. Even though there is no short time-scale variability in the
injected engine luminosity, we find that the jet's kinetic luminosity outside
the progenitor star is characterized by fluctuations with relatively short time
scale. We analyze the temporal characteristics of those fluctuations and we
find that they are consistent with the properties of observed flares in X-ray
afterglows. The peak to continuum flux ratio of the flares in the simulation is
consistent with some, but not all, the observed flares. We propose that
propagation instabilities, rather than variability in the engine luminosity,
are responsible for the X-ray flares with moderate contrast. Strong flares such
as the one detected in GRB 050502B, instead, cannot be reproduced by this model
and require strong variability in the engine activity.Comment: 6 pages, MNRAS in pres
Gamma Ray Bursts: new rulers to measure the Universe
The best measure of the Universe should be done using a standard "ruler" at
any redshift. Type Ia Supernovae (SN Ia) probe the universe up to z1.5,
while the Cosmic Microwave Background (CMB) primary anisotropies concern
basically 1000. Apparently, Gamma--Ray Bursts (GRBs) are all but
standard candles. However, their emission is collimated and the
collimation--corrected energy correlates tightly with the frequency at which
most of the radiation of the prompt is emitted, as found by Ghirlanda et al.
(2004). Through this correlation we can infer the burst energy accurately
enough to probe the intermediate redshift () Universe. Using the best
known 15 GRBs we find very encouraging results that emphasize the cosmological
GRB role. A combined fit with SN Ia yields and
. Assuming in addition a flat Universe, the
parameters are constrained to be and
. GRBs accomplish the role of "missing link"
between SN Ia and CMB primary anisotropies. They can provide a new insight on
the cosmic effects of dark energy, complementary to the one supplied by CMB
secondary anisotropies through the Integrated Sachs Wolfe effect. The
unexpected Standard Candle cosmological role of GRBs motivates the most
optimistic hopes for what can be obtained when the GRB-dedicated satellite,
Swift, will be launched.Comment: 11 pages, 4 color figures, ApJ Letters (vol. 613) in pres
Reconsidering the origin of the X-ray emission lines in GRB 011211
We reanalyze the XMM--Newton data of GRB 011211 showing that the spectral
features, interpreted by Reeves et al. (2002, 2003) as due thermal emission
from a collisionally ionized plasma, can be also reproduced by a reflection
model (with ionization parameter ). We discuss the implications
of this interpretation, estimating the total mass required in the simplified
case of a funnel geometry. We conclude that a moderate clumping of the
reprocessing material (corresponding to a filling factor of the order of ) is required. Finally we show that, if this interpretation is correct,
a bright quasi--thermal component is expected in the optical--UV band
(containing about 90% of the luminosity of the illuminating continuum), whose
presence can be used to test the reflection model.Comment: revised version accepted for publication by A&
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