54 research outputs found
Gamma-ray burst afterglow plateau break time - luminosity correlations favour thick shell models over thin shell models
A number of correlations between observables have been found to exist for
gamma-ray burst (GRB) afterglows, linking ejecta energy to prompt and afterglow
energy release and linking early stage optical and X-ray luminosity to the end
times of these stages. Here, these correlations are compared to thick and thin
shell models for GRB afterglows. In the thick shell model, the time evolution
of the underlying relativistic blast wave is still influenced by the original
ejecta, while in the thin shell model most energy in the explosion has been
transferred to the external medium. It is shown here that the observed
correlations rule out basic thin shell models but not the basic thick shell
model. In the thick shell case, both forward shock and reverse shock dominated
outflows are shown to be consistent with the correlations, using randomly
generated samples of thick shell model afterglows.Comment: 11 pages, 4 figures. Expanded version, now matches MNRAS accepted
versio
Self-similar relativistic blast waves with energy injection
A sufficiently powerful astrophysical source with power law luminosity in
time will give rise to a self-similar relativistic blast wave with a reverse
shock traveling into the ejecta and a forward shock moving into the surrounding
medium. Once energy injection ceases and the last energy is delivered to the
shock front, the blast wave will transit into another self-similar stage
depending only on the total amount of energy injected.
I describe the effect of limited duration energy injection into environments
with density depending on radius as a power law, emphasizing optical / X-ray
Gamma-ray Burst afterglows as applications. The blast wave during injection is
treated analytically, the transition following last energy injection with
one-dimensional simulations. Flux equations for synchrotron emission from the
forward and reverse shock regions are provided. The reverse shock emission can
easily dominate, especially with different magnetizations for both regions.
Reverse shock emission is shown to support both the reported X-ray and optical
correlations between afterglow plateau duration and end time flux,
independently of the luminosity power law slope. The model is demonstrated by
application to bursts 120521A and 090515, and can accommodate their steep
post-plateau light curve slopes.Comment: Revised and corrected. Matches MNRAS accepted versio
Simulation and physical model based gamma-ray burst afterglow analysis
Advances in our numerical and theoretical understanding of gamma-ray burst
afterglow processes allow us to construct models capable of dealing with
complex relativistic jet dynamics and non-thermal emission, that can be
compared directly to data from instruments such as Swift. Because afterglow
blast waves and power law spectra are intrinsically scale-invariant under
changes of explosion energy and medium density, templates can be generated from
large-scale hydrodynamics simulations. This allows for iterative template-based
model fitting using the physical model parameters (quantifying the properties
of the burster, emission and observer) directly as fit variables. Here I review
how such an approach to afterglow analysis works in practice, paying special
attention to the underlying model assumptions, possibilities, caveats and
limitations of this type of analysis. Because some model parameters can be
degenerate in certain regions of parameter space, or unconstrained if data in a
limited number of a bands is available, a Bayesian approach is a natural fit.
The main features of the standard afterglow model are reviewed in detail.Comment: Invited contribution to Journal of High Energy Astrophysics special
issue "Swift: 10 years of discovery". Replaced with expanded version matching
JHEAP publicatio
Extensive ground state entropy in supersymmetric lattice models
We present the result of calculations of the Witten index for a
supersymmetric lattice model on lattices of various type and size. Because the
model remains supersymmetric at finite lattice size, the Witten index can be
calculated using row-to-row transfer matrices and the calculations are similar
to calculations of the partition function at negative activity -1. The Witten
index provides a lower bound on the number of ground states. We find strong
numerical evidence that the Witten index grows exponentially with the number of
sites of the lattice, implying that the model has extensive entropy in the
ground state.Comment: 7 figure
Extensive ground state entropy in supersymmetric lattice models
We present the result of calculations of the Witten index for a supersymmetric lattice model on lattices of various type and size. Because the model remains supersymmetric at finite lattice size, the Witten index can be calculated using row-to-row transfer matrices and the calculations are similar to calculations of the partition function at negative activity −1 . The Witten index provides a lower bound on the number of ground states. We find strong numerical evidence that the Witten index grows exponentially with the number of sites of the lattice, implying that the model has extensive entropy in the ground stat
Recent developments in gamma-ray burst afterglow theory
Gamma-ray burst (GRB) afterglows have long played a key role in our understanding of the physics of GRBs. The recent detection of the electro-magnetic counterparts including an afterglow jet to a neutron star merger indicate that this will remain so in the future. With the detection of GRB 170817A, afterglow observations have fully caught up again with theory and we have been provided with an opportunity to discard old jet models, refine alternative jet geometry models already in the literature and to think hard about future predictions. The GRB community has admirably stepped up to the plate and observational, theoretical and computational progress has been very rapid over the past years. Additionally, large-scale electro-magnetic surveys, observations at extremely high frequencies and an increasing number of gravitational-wave detections of merging neutron stars offer tantalizing prospects of further upheavals in afterglow and GRB theory. In these proceedings, I will take stock of some theoretical progress on afterglow theory made in the past few years
Fitting Afterglows With Multi-Dimensional Simulations
We present preliminary data fit results of synthetic light curves computed
from numerical afterglow blast wave simulations. Our technique uses Markov
chain Monte Carlo (MCMC) in a new data analysis tool, ScaleFit. Scaling
relations in both the hydrodynamics and radiation equations allow light curves
to be parameterized by a small set of scale-invariant characteristic
quantities. These quantities have been calculated and tabulated from high
resolution two-dimensional hydrodynamic simulations. Producing a light curve
from the characteristics takes only a millisecond, allowing for the use of MCMC
data fitting techniques which can require millions of iterations. ScaleFit is a
portable, lightweight, python package which performs this analysis on afterglow
light curves. Using the set of Swift-XRT light curves from 2011 & 2012 with
known redshifts, we find ScaleFit can measure the jet opening angle, observer
angle, and spectral index of most afterglows. Globally we find gamma-ray burst
afterglows tend to be observed off axis, at a significant fraction of the jet
opening angle.Comment: 8 pages, 3 figures. 7th Huntsville Gamma-Ray Burst Symposium, GRB
2013: paper #30 in eConf Proceedings C130414
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