798 research outputs found
Synchrotron Radiation as the Source of GRB Spectra, Part I: Theory
We investigate synchrotron emission models as the source of gamma ray burst
spectra. We show that allowing for synchrotron self absorption and a ``smooth
cutoff'' to the electron energy distribution produces a wide range of low
energy spectral behavior. We show that there exists a correlation between the
value of the peak of the spectrum, , and the low energy
spectral index as determined by spectral fits over a finite bandwidth.
Finally, we discuss the implications of synchrotron emission from internal
shocks for GRB spectral evolution.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma
Ray Burst
On the Kinetic Energy and Radiative Efficiency of Gamma-Ray Bursts
Using measured X-ray luminosities to 17 Gamma-Ray Bursts (GRBs) during the
afterglow phase and accounting for radiative losses, we calculate the kinetic
energy of these bursts and investigate its relation to other GRB properties. We
then use the observed radiated energy during the prompt phase to determine the
radiative efficiency of these bursts, and explore how the efficiency relates to
other GRB observables. We find that the kinetic energy in the afterglow phase
is directly correlated with the radiated energy, total energy as well as
possibly the jet opening angle and spectral peak energy. More importantly, we
find the intriguing fact that the efficiency is correlated with the radiated
energy, and mildly with the total energy, jet opening angle and spectral peak
energy. XRF020903 also seems to follow the trends we find for our GRB sample.
We discuss the implications of these results for the GRB radiation and jet
models.Comment: 9 pages, 7 figures; Revised version, accepted to Ap
Synchrotron Emission as the Source of GRB Spectra, Part II: Observations
We test the models of synchrotron emission presented in Part I of this series
(Lloyd & Petrosian, these proceedings) against the distributions and evolution
of GRB spectral parameters (particularly the low energy index, ). With
knowledge of the distribution and the correlation between and
presented in Part I, we show how to derive the expected distribution of
from fits to optically thin synchrotron spectra, and compare this with
the observed distribution. We show that there is no difficulty explaining
bursts below the ``line of death'', , and that these bursts
indicate that the spectrum of accelerated electrons must flatten or decline at
low energies. Bursts with low energy spectral indices that fall above this
limit are explained by the synchrotron self-absorption frequency entering the
lower end of the BATSE window. Finally, we discuss a variety of spectral
evolution behavior seen in GRBs and explain this behavior in the context of
synchrotron emission from internal shocks.Comment: To appear in the proceedings of the 5th Huntsville Symposium on Gamma
Ray Burst
Towards an Understanding of GRB Prompt Emission
We discuss the prompt emission of Gamma-Ray Bursts in different spectral
energy bands. First, we suggest that a three-part synchrotron emission model is
a good description of the ~20 keV - 1 MeV gamma-ray emission of GRBs. We show
that this model provides excellent fits to the data and naturally explains the
observed global correlations between spectral parameters. In particular, we
show there exists a negative correlation between between the peak of the nu-Fnu
spectrum, Ep, and the low energy photon index \alpha for bursts with -2/3 <
\alpha < 0, and suggest that this correlation is due to the mechanism
responsible for producing \alpha's above the value of -2/3 - namely, a
decreasing mean pitch angle of the electrons. We then discuss the physical
origin of the increasing number of GRBs that are observed to peak in the X-ray
energy band (~5-40 keV). Although either a cosmological (i.e. high redshift) or
intrinsic interpretation for the low values of Ep is viable at this point, the
data appear to suggest that intrinsic effects are playing the dominant role.
Finally, we briefly comment on the prompt GRB optical emission (~ eV) and very
high energy emission (>10 MeV), and how these spectral bands may be used to
place additional constraints on the physics of gamma-ray bursts.Comment: Invited talk at the 2001 Woodshole meeting,"Gamma-Ray Bursts and
Afterglow Astronomy"; 8 pages including 8 postscript figure
Distribution of Spectral Characteristics and the Cosmological Evolution of GRBs
We investigate the cosmological evolution of GRBs, using the total gamma ray
fluence as a measure of the burst strength. This involves an understanding of
the distributions of the spectral parameters of GRBs as well as the total
fluence distribution - both of which are subject to detector selection effects.
We present new non-parametric statistical techniques to account for these
effects, and use these methods to estimate the true distribution of the peak of
the nu F_nu spectrum, E_p, from the raw distribution. The distributions are
obtained from four channel data and therefore are rough estimates. Here, we
emphasize the methods and present qualitative results. Given its spectral
parameters, we then calculate the total fluence for each burst, and compute its
cumulative and differential distributions. We use these distributions to
estimate the cosmological rate evolution of GRBs, for three cosmological
models. Our two main conclusions are the following: 1) Given our estimates of
the spectral parameters, we find that there may exist a significant population
of high E_p bursts that are not detected by BATSE, 2) We find a GRB co-moving
rate density quite different from that of other extragalactic objects; in
particular, it is different from the recently determined star formation rate.Comment: 20 pages, including 10 postscript figures. Submitted to Ap
Interpreting the Behavior of Time Resolved Gamma-Ray Burst Spectra
In this paper, we explore time resolved Gamma-Ray Burst (GRB) spectra in the
context of the synchrotron emission model presented in Lloyd and Petrosian
(2000; LP00). First, we show that our model - which involves three distinct
emission regimes - can provide excellent fits to the time resolved spectra of
GRBs, and we present these results for a few bursts. We then describe how the
phenomenological Band spectrum (Band et al., 1993) can be interpreted in the
context of our models based on the value of the low energy photon index
. We discuss the types of correlations one would expect to observe
among the Band parameters if these models are correct. We then compare these
predictions to the existing data, combining a sample of 2,026 time resolved
spectra (from approximately 80 bursts). We show that the correlations found in
the data are consistent with the models, and discuss the constraints they place
on the emission physics. In particular, we find a 4-sigma negative correlation
between the peak of the nu-Fnu spectrum, Ep, and the low energy photon index
for bursts with , in contrast to what is predicted
by the instrumental effect discussed in LP00. We suggest that this correlation
is simply due to the mechanism responsible for producing 's above the
value of -2/3 - namely, a decreasing mean pitch angle of the electrons. We also
show that Ep is correlated with the photon flux, and interpret this as a result
of changing magnetic field or characteristic electron energy between emission
episodes. Finally, we discuss the implications our results have on particle
acceleration in GRBs, and prospects for further testing these models with the
anticipated data from HETE-2, Swift and GLAST.Comment: 17 pages, including 13 figures. Accepted to Ap
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