966 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
Large magnetoresistance in the antiferromagnetic semi-metal NdSb
There has been considerable interest in topological semi-metals that exhibit
extreme magnetoresistance (XMR). These have included materials lacking
inversion symmetry such as TaAs, as well Dirac semi-metals such as Cd3As2.
However, it was reported recently that LaSb and LaBi also exhibit XMR, even
though the rock-salt structure of these materials has inversion symmetry, and
the band-structure calculations do not show a Dirac dispersion in the bulk.
Here, we present magnetoresistance and specific heat measurements on NdSb,
which is isostructural with LaSb. NdSb has an antiferromagnetic groundstate,
and in analogy with the lanthanum monopnictides, is expected to be a
topologically non-trivial semi-metal. We show that NdSb has an XMR of 10^4 %,
even within the AFM state, illustrating that XMR can occur independently of the
absence of time reversal symmetry breaking in zero magnetic field. The
persistence of XMR in a magnetic system offers promise of new functionality
when combining topological matter with electronic correlations. We also find
that in an applied magnetic field below the Neel temperature there is a first
order transition, consistent with evidence from previous neutron scattering
work.Comment: 5 pages, 6 figure
Are there cosmological evolution trends on Gamma-Ray Burst features?
The variability of gamma-ray burst (GRB) is thought to be correlated with its
absolute peak luminosity, and this relation had been used to derive an estimate
of the redshifts of GRBs. Recently Amati et al. presented the results of
spectral and energetic properties of several GRBs with known redshifts. Here we
analyse the properties of two group GRBs, one group with known redshift from
afterglow observation, and another group with redshift derived from the
luminosity - variability relation. We study the redshift dependence of various
GRBs features in their cosmological rest frames, including the burst duration,
the isotropic luminosity and radiated energy, and the peak energy of spectra. We find that, for these two group GRBs, their properties are
all redshift dependent, i.e. their intrinsic duration, luminosity, radiated
energy and peak energy , are all correlated with the redshift, which means
that there are cosmological evolution effects on gamma-ray bursts features, and
this can provide an interesting clue to the nature of GRBs. If this is true,
then the results also imply that the redshift derived from the luminosity -
variability relation may be reliable.Comment: Latex, 11 pages. Discussion of the selection effects have been added.
Accepted for publication in MNRA
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
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