150 research outputs found
Off-Axis Afterglow Light Curves from High-Resolution Hydrodynamical Jet Simulations
Numerical jet simulations serve a valuable role in calculating gamma-ray
burst afterglow emission beyond analytical approximations. Here we present the
results of high resolution 2D simulations of decelerating relativistic jets
performed using the RAM adaptive mesh refinement relativistic hydrodynamics
code. We have applied a separate synchrotron radiation code to the simulation
results in order to calculate light curves at frequencies varying from radio to
X-ray for observers at various angles from the jet axis. We provide a
confirmation from radio light curves from simulations rather than from a
simplified jet model for earlier results in the literature finding that only a
very small number of local Ibc supernovae can possibly harbor an orphan
afterglow.
Also, recent studies have noted an unexpected lack of observed jet breaks in
the Swift sample. Using a jet simulation with physical parameters
representative for an average Swift sample burst, such as a jet half opening
angle of 0.1 rad and a source redshift of z = 2.23, we have created synthetic
light curves at 1.5 keV with artificial errors while accounting for Swift
instrument biases as well. A large set of these light curves have been
generated and analyzed using a Monte Carlo approach. Single and broken power
law fits are compared. We find that for increasing observer angle, the jet
break quickly becomes hard to detect. This holds true even when the observer
remains well within the jet opening angle. We find that the odds that a Swift
light curve from a randomly oriented 0.1 radians jet at z = 2.23 will exhibit a
jet break at the 3 sigma level are only 12 percent. The observer angle
therefore provides a natural explanation for the lack of perceived jet breaks
in the Swift sample.Comment: 4 pages, 3 figures. First of two contributions to proceedings GRB2010
Maryland conference. Editors: McEnery, Racusin and Gehrels. The data from
this paper is publicly available from http://cosmo.nyu.edu/afterglowlibrary
An on-line library of afterglow light curves
Numerical studies of gamma-ray burst afterglow jets reveal significant
qualitative differences with simplified analytical models. We present an
on-line library of synthetic afterglow light curves and broadband spectra for
use in interpreting observational data. Light curves have been calculated for
various physics settings such as explosion energy and circumburst structure, as
well as differing jet parameters and observer angle and redshift. Calculations
gave been done for observer frequencies ranging from low radio to X-ray and for
observer times from hours to decades after the burst. The light curves have
been calculated from high-resolution 2D hydrodynamical simulations performed
with the RAM adaptive-mesh refinement code and a detailed synchrotron radiation
code.
The library will contain both generic afterglow simulations as well as
specific case studies and will be freely accessible at
http://cosmo.nyu.edu/afterglowlibrary . The synthetic light curves can be used
as a check on the accuracy of physical parameters derived from analytical model
fits to afterglow data, to quantitatively explore the consequences of varying
parameters such as observer angle and for accurate predictions of future
telescope data.Comment: 4 pages, 2 figures. Second of two contributions to proceedings
GRB2010 Maryland conference. Editors: McEnery, Racusin and Gehrels. The data
from this paper is publicly available from
http://cosmo.nyu.edu/afterglowlibrary
Gamma-ray burst afterglow theory
It is by now fairly well established that gamma-ray burst afterglows result
from initially relativistic outflows interacting with the medium surrounding
the burster and emitting non-thermal radiation ranging from radio to X-rays.
However, beyond that, many big and small questions remain about afterglows,
with the accumulating amount of observational data at the various frequencies
raising as many questions as they answer. In this review I highlight a number
of current theoretical issues and how they fit or do not fit within our basic
theoretical framework. In addition to theoretical progress I will also
emphasize the increasing role and usefulness of numerical studies of afterglow
blast waves and their radiation.Comment: Invited review for 7th Huntsville Gamma-Ray Burst Symposium, GRB
2013: paper 24 in eConf Proceedings C130414
From blast wave to observation
Gamma-ray burst (GRB) afterglows are well described by synchrotron emission
originating from the interaction between a relativistic blast wave and the
external medium surrounding the GRB progenitor. We introduce a code to
reconstruct spectra and light curves from arbitrary fluid configurations,
making it especially suited to study the effects of fluid flows beyond those
that can be described using analytical approximations. As a check and first
application of our code we use it to fit the scaling coefficients of
theoretical models of afterglow spectra. We extend earlier results of other
authors to general circumburst density profiles. We rederive the physical
parameters of GRB 970508 and compare with other authors.
We also show the light curves resulting from a relativistic blast wave
encountering a wind termination shock. From high resolution calculations we
find that the observed transition from a stellar wind type light curve to an
interstellar medium type light curve is smooth and without short-time
transitory features.Comment: conference proceedings 6th Huntsville symposium, 20-23 October 2008.
Editors: C.A. Meegan, N. Gehrels, and C. Kouvelioto
No visible optical variability from a relativistic blast wave encountering a wind-termination shock
Gamma-ray burst afterglow flares and rebrightenings of the optical and X-ray
light curve have been attributed to both late time inner engine activity and
density changes in the medium surrounding the burster. To test the latter, we
study the encounter between the relativistic blast wave from a gamma-ray
burster and a stellar wind termination shock. The blast wave is simulated using
a high performance adaptive mesh relativistic hydrodynamics code, AMRVAC, and
the synchrotron emission is analyzed in detail with a separate radiation code.
We find no bump in the resulting light curve, not even for very high density
jumps. Furthermore, by analyzing the contributions from the different shock
wave regions we are able to establish that it is essential to resolve the blast
wave structure in order to make qualitatively correct predictions on the
observed output and that the contribution from the reverse shock region will
not stand out, even when the magnetic field is increased in this region by
repeated shocks. This study resolves a controversy in recent literature.Comment: 4 figures, submitted to MNRAS letter
A year in the life of GW170817: the rise and fall of a structured jet from a binary neutron star merger
We present the results of our year-long afterglow monitoring of GW170817, the
first binary neutron star (NS) merger detected by advanced LIGO and advanced
Virgo. New observations with the Australian Telescope Compact Array (ATCA) and
the Chandra X-ray Telescope were used to constrain its late-time behavior. The
broadband emission, from radio to X-rays, is well-described by a simple
power-law spectrum with index ~0.585 at all epochs. After an initial shallow
rise ~t^0.9, the afterglow displayed a smooth turn-over, reaching a peak X-ray
luminosity of ~5e39 erg/s at 160 d, and has now entered a phase of rapid
decline ~t^(-2). The latest temporal trend challenges most models of choked
jet/cocoon systems, and is instead consistent with the emergence of a
relativistic structured jet seen at an angle of ~22 deg from its axis. Within
such model, the properties of the explosion (such as its blastwave energy
E_K~2E50 erg, jet width theta_c~4 deg, and ambient density n~3E-3 cm^(-3)) fit
well within the range of properties of cosmological short GRBs.Comment: 11 pages, 8 figures, 2 tables, MNRAS, in press. Final version, minor
changes only relative to original submission dated 21 August 201
Gamma-ray burst afterglows from trans-relativistic blast wave simulations
We present a study of the intermediate regime between ultra-relativistic and
nonrelativistic flow for gamma-ray burst afterglows. The hydrodynamics of
spherically symmetric blast waves is numerically calculated using the AMRVAC
adaptive mesh refinement code. Spectra and light curves are calculated using a
separate radiation code that, for the first time, links a parametrisation of
the microphysics of shock acceleration, synchrotron self-absorption and
electron cooling to a high-performance hydrodynamics simulation. For the
dynamics we find that the transition to the nonrelativistic regime generally
occurs later than expected, that the Sedov-Taylor solution overpredicts the
late time blast wave radius and that the analytical formula for the blast wave
velocity from Huang (1999) overpredicts the late time velocity by a factor 4/3.
For the radiation we find that the flux may differ up to an order of magnitude
depending on the equation of state that is used for the fluid and that the
counterjet leads to a clear rebrightening at late times for hard-edged jets.
Simulating GRB030329 using predictions for its physical parameters from the
literature leads to spectra and light curves that may differ significantly from
the actual data, emphasizing the need for very accurate modelling. Predicted
light curves at low radio frequencies for a hard-edged jet model of GRB030329
with opening angle 22 degrees show typically two distinct peaks, due to the
combined effect of jet break, non relativistic break and counterjet.Comment: 18 pages, 19 figures. Replaced with MNRAS accepted version, minor
changes onl
A luminous blue kilonova and an off-axis jet from a compact binary merger at z=0.1341
The recent discovery of a faint gamma-ray burst (GRB) coincident with the
gravitational wave (GW) event GW 170817 revealed the existence of a population
of low-luminosity short duration gamma-ray transients produced by neutron star
mergers in the nearby Universe. These events could be routinely detected by
existing gamma-ray monitors, yet previous observations failed to identify them
without the aid of GW triggers. Here we show that GRB150101B was an analogue of
GRB170817A located at a cosmological distance. GRB 150101B was a faint short
duration GRB characterized by a bright optical counterpart and a long-lived
X-ray afterglow. These properties are unusual for standard short GRBs and are
instead consistent with an explosion viewed off-axis: the optical light is
produced by a luminous kilonova component, while the observed X-rays trace the
GRB afterglow viewed at an angle of ~13 degrees. Our findings suggest that
these properties could be common among future electromagnetic counterparts of
GW sources.Comment: 28 pages, 8 figures, 2 tables. Accepted for publicatio
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