128 research outputs found
GRB Spectral Hardness and Afterglow Properties
A possible relationship between the presence of a radio afterglow and
gamma-ray burst spectral hardness is discussed. The correlation is marginally
significant; the spectral hardness of the bursts with radio afterglows
apparently results from a combination of the break energy Ebreak and the
high-energy spectral index beta. If valid, this relationship would indicate
that the afterglow does carry information pertaining to the GRB central engine.Comment: 5 pages, 3 figures, presented at the 5th Huntsville Gamma-Ray Burst
Symposiu
A Simple BATSE Measure of GRB Duty Cycle
We introduce a definition of gamma-ray burst (GRB) duty cycle that describes
the GRB's efficiency as an emitter; it is the GRB's average flux relative to
the peak flux. This GRB duty cycle is easily described in terms of measured
BATSE parameters; it is essentially fluence divided by the quantity peak flux
times duration. Since fluence and duration are two of the three defining
characteristics of the GRB classes identified by statistical clustering
techniques (the other is spectral hardness), duty cycle is a potentially
valuable probe for studying properties of these classes.Comment: 4 pages, 1 figure, presented at the 5th Huntsville Gamma-Ray Burst
Symposiu
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
Testing the Gamma-Ray Burst Energy Relationships
Building on Nakar & Piran's analysis of the Amati relation relating gamma-ray
burst peak energies E_p and isotropic energies E_iso, we test the consistency
of a large sample of BATSE bursts with the Amati and Ghirlanda (which relates
peak energies and actual gamma-ray energies E_gamma) relations. Each of these
relations can be expressed as a ratio of the different energies that is a
function of redshift (for both the Amati and Ghirlanda relations) and beaming
fraction f_B (for the Ghirlanda relation). The most rigorous test, which allows
bursts to be at any redshift, corroborates Nakar & Piran's result--88% of the
BATSE bursts are inconsistent with the Amati relation--while only 1.6% of the
bursts are inconsistent with the Ghirlanda relation if f_B=1. Even when we
allow for a real dispersion in the Amati relation we find an inconsistency.
Modelling the redshift distribution results in an energy ratio distribution for
the Amati relation that is shifted by an order of magnitude relative to the
observed distribution; any sub-population satisfying the Amati relation can
comprise at most ~18% of our burst sample. A similar analysis of the Ghirlanda
relation depends sensitively on the beaming fraction distribution for small
values of f_B; for reasonable estimates of this distribution about a third of
the burst sample is inconsistent with the Ghirlanda relation. Our results
indicate that these relations are an artifact of the selection effects of the
burst sample in which they were found; these selection effects may favor
sub-populations for which these relations are valid.Comment: 17 pages, 4 figures. To appear in ApJ, 627, #2 (10 July 2005
A significant problem with using the Amati relation for cosmological purposes
We consider the distribution of many samples of gamma-ray bursts when plotted in a diagram with their bolometric fluence (Sbolo) versus the observed photon energy of peak spectral flux (E peak, obs). In this diagram, all bursts that obey the Amati relation (a luminosity relation where the total burst energy has a power-law relation to E peak, obs) must lie above some limiting line, although observational scatter is expected to be substantial. We confirm that early bursts with spectroscopic redshifts are consistent with this Amati limit. But we find that the bursts from BATSE, Swift, Suzaku, and Konus are all greatly in violation of the Amati limit, and this is true whether or not the bursts have measured spectroscopic redshifts. That is, the Amati relation has definitely failed. In the S bolo-E peak, obs diagram, wefind that every satellite has a greatly different distribution. This requires that selection effects are dominating these distributions, which we quantitatively identify. For detector selections, the trigger threshold and the threshold for the burst to obtain a measured E peak, obs combine to make a diagonal cutoff with the position of this cutoff varying greatly detector to detector. For selection effects due to the intrinsic properties of the burst population, the distribution of E peak, obs makes bursts with low and high values rare, while the fluence distribution makes bright bursts relatively uncommon. For a detector with a high threshold, the combination of these selection effects serves to allow only bursts within a region along the Amati limit line to be measured, and these bursts will then appear to follow an Amati relation. Therefore, the Amati relation is an artifact of selection effects within the burst population and the detector. As such, the Amati relation should not be used for cosmological tasks. This failure of the Amati relation is in no way prejudicial against the other luminosity relations. © 2012. The American Astronomical Society. All rights reserved
The sharpness of gamma-ray burst prompt emission spectra
We aim to obtain a measure of the curvature of time-resolved spectra that can
be compared directly to theory. This tests the ability of models such as
synchrotron emission to explain the peaks or breaks of GBM prompt emission
spectra. We take the burst sample from the official Fermi GBM GRB time-resolved
spectral catalog. We re-fit all spectra with a measured peak or break energy in
the catalog best-fit models in various energy ranges, which cover the curvature
around the spectral peak or break, resulting in a total of 1,113 spectra being
analysed. We compute the sharpness angles under the peak or break of the
triangle constructed under the model fit curves and compare to the values
obtained from various representative emission models: blackbody,
single-electron synchrotron, synchrotron emission from a Maxwellian or
power-law electron distribution. We find that 35% of the time-resolved spectra
are inconsistent with the single-electron synchrotron function, and 91% are
inconsistent with the Maxwellian synchrotron function. The single temperature,
single emission time and location blackbody function is found to be sharper
than all the spectra. No general evolutionary trend of the sharpness angle is
observed, neither per burst nor for the whole population. It is found that the
limiting case, a single temperature Maxwellian synchrotron function, can only
contribute up to % of the peak flux. Our results show that even
the sharpest but non-realistic case, the single-electron synchrotron function,
cannot explain a large fraction of the observed GRB prompt spectra. Because of
the fact that any combination of physically possible synchrotron spectra added
together will always further broaden the spectrum, emission mechanisms other
than optically thin synchrotron radiation are likely required in a full
explanation of the spectral peaks or breaks of the GRB prompt emission phase.Comment: 16 pages, 13 figures, 2 tables, accepted for publication in A&
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