333 research outputs found
Applying an accurate spherical model to gamma-ray burst afterglow observations
We present results of model fits to afterglow data sets of GRB970508,
GRB980703 and GRB070125, characterized by long and broadband coverage. The
model assumes synchrotron radiation (including self-absorption) from a
spherical adiabatic blast wave and consists of analytic flux prescriptions
based on numerical results. For the first time it combines the accuracy of
hydrodynamic simulations through different stages of the outflow dynamics with
the flexibility of simple heuristic formulas. The prescriptions are especially
geared towards accurate description of the dynamical transition of the outflow
from relativistic to Newtonian velocities in an arbitrary power-law density
environment. We show that the spherical model can accurately describe the data
only in the case of GRB970508, for which we find a circumburst medium density
consistent with a stellar wind. We investigate in detail the implied spectra
and physical parameters of that burst. For the microphysics we show evidence
for equipartition between the fraction of energy density carried by
relativistic electrons and magnetic field. We also find that for the blast wave
to be adiabatic, the fraction of electrons accelerated at the shock has to be
smaller than 1. We present best-fit parameters for the afterglows of all three
bursts, including uncertainties in the parameters of GRB970508, and compare the
inferred values to those obtained by different authors
Transient Simulations for Radio Surveys
Several new radio facilities have a field of view and sensitivity well suited
for transient searches. This makes it more important than ever to accurately
determine transient rates in radio surveys. The work presented here seeks to do
this task by using Monte-Carlo simulations. In particular, the user inputs
either a real or simulated observational setup, and the simulations code
calculates transient rate as a function of transient duration and peak flux.
These simulations allow for simulating a wide variety of scenarios including
observations with varying sensitivities and durations, multiple overlapping
telescope pointings, and a wide variety of light curve shapes with the user
having the ability to easily add more. While the current scientific focus is on
the radio regime, with examples given here from the MeerKAT telescope in South
Africa, the simulations code can be easily adapted to other wavelength regimes.Comment: Minor updates to plot
A new approach to modelling γ-ray burst afterglows: Using Gaussian processes to account for the systematics
The afterglow emission from gamma-ray bursts (GRBs) is a valuable source of
information to understand the physics of these energetic explosions. The blast
wave model has become the standard to describe the evolution of the afterglow
emission over time and frequency. Thanks to recent developments in the theory
of afterglows and numerical simulations of relativistic outflows, we are able
to model the afterglow emission with realistic dynamics and radiative
processes. Although the models agree with observations remarkably well, the
afterglow emission still contains additional physics, instrumental systematics,
and propagation effects which make the modelling of these events challenging.
In this work, we present a new approach to modelling GRB afterglows, using
Gaussian processes (GPs) to take into account systematics in the afterglow
data. We show that, using this new approach, it is possible to obtain more
reliable estimates of the explosion and microphysical parameters of GRBs. We
present fit results for 5 long GRBs and find a preliminary correlation between
the isotropic energetics and opening angles of GRBs, which confirms the idea of
a common energy reservoir for the kinetic energy of long GRBs.Comment: Accepted to MNRAS. 10 pages, 5 figure
High Spatial Resolution X-Ray Spectroscopy of the IC443 Pulsar Wind Nebula and Environs
Deep Chandra ACIS observations of the region around the putative pulsar, CXOU
J061705.3+222127, in the supernova remnant IC443 reveal an
~5-radius ring-like structure surrounding the pulsar and a
jet-like feature oriented roughly north-south across the ring and through the
pulsar's location at 06175.200
+222127.52 (J2000.0 coordinates). The
observations further confirm that (1) the spectrum and flux of the central
object are consistent with a rotation-powered pulsar, (2) the non-thermal
spectrum and morphology of the surrounding nebula are consistent with a pulsar
wind and, (3) the spectrum at greater distances is consistent with thermal
emission from the supernova remnant. The cometary shape of the nebula,
suggesting motion towards the southwest, appears to be subsonic: There is no
evidence either spectrally or morphologically for a bow shock or contact
discontinuity; the nearly circular ring is not distorted by motion through the
ambient medium; and the shape near the apex of the nebula is narrow. Comparing
this observation with previous observations of the same target, we set a 99%
confidence upper limit to the proper motion of CXOU J061705.3+222127 to be less
than 44 mas/yr (310 km/s for a distance of 1.5 kpc), with the best-fit (but not
statistically significant) projected direction toward the west.Comment: Accepted for publication in the Astrophysical Journa
Fast Radio Burst Luminosity Function and Death Line in the Low-twist Magnetar Model
We explore the burst energy distribution of fast radio bursts (FRBs) in the low-twist magnetar model of Wadiasingh & Timokhin (WT19). Motivated by the power-law fluence distributions of FRB 121102, we propose an elementary model for the FRB luminosity function of individual repeaters with an inversion protocol that directly relates the power-law distribution index of magnetar short burst fluences to that for FRBs. The protocol indicates that the FRB energy scales virtually linearly with crust/field dislocation amplitude, if magnetar short bursts prevail in the magnetoelastic regime. Charge starvation in the magnetosphere during bursts (required in WT19) for individual repeaters implies the predicted burst fluence distribution is narrow, ≾3 decades for yielding strains and oscillation frequencies feasible in magnetar crusts. Requiring magnetic confinement and charge starvation, we obtain a death line for FRBs, which segregates magnetars from the normal pulsar population, suggesting only the former will host recurrent FRBs. We convolve the burst energy distribution for individual magnetars to define the distribution of luminosities in evolved magnetar populations. The broken power-law luminosity function's low-energy character depends on the population model, while the high-energy index traces that of individual repeaters. Independent of the evolved population, the broken power-law isotropic-equivalent energy/luminosity function peaks at ~10³⁷-10⁴⁰ erg with a low-energy cutoff at ~10³⁷ erg. Lastly, we consider the local fluence distribution of FRBs and find that it can constrain the subset of FRB-producing magnetar progenitors. Our model suggests that improvements in sensitivity may reveal a flattening of the global FRB fluence distribution and saturation in FRB rates
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