301 research outputs found
Choosing a measure of GRB brightness that approaches a standard candle
Studies using the GRB brightness as a distance indicator require a measure of
brightness with a small intrinsic dispersion (close to a standard candle).
There is unfortunately no general agreement on the definition of such a
quantity. We show here that the comparison of the size-frequency curves
obtained with various measures of brightness can be used to select the quantity
which is closer to a standard candle. Our method relies on a few general
assumptions on the burster spatial distribution, namely that nearby bursters
are homogeneously distributed in an Euclidean space with no density or
luminosity evolution. We apply it to 5 measures of GRB brightness in the
Current BATSE Catalog and we find that the GRB size-frequency distribution
depends significantly on the energy window used to measure the GRB brightness.
The influence of the time window being, in comparison, negligible. Our method
suggests that the best distance indicator in this Catalog is the fluence
measured below 100 keV, indicating that GRB luminosities have a smaller
intrinsic dispersion below 100 keV than above.Comment: 5 pages (LateX), 2 Postscript figures, Proceedings of the 4th
Huntsville GRB Worksho
Gamma Ray Burst Host Galaxies Have `Normal' Luminosities
The galactic environment of Gamma Ray Bursts can provide good evidence about
the nature of the progenitor system, with two old arguments implying that the
burst host galaxies are significantly subluminous. New data and new analysis
have now reversed this picture: (A) Even though the first two known host
galaxies are indeed greatly subluminous, the next eight hosts have absolute
magnitudes typical for a population of field galaxies. A detailed analysis of
the 16 known hosts (ten with red shifts) shows them to be consistent with a
Schechter luminosity function with as expected for
normal galaxies. (B) Bright bursts from the Interplanetary Network are
typically 18 times brighter than the faint bursts with red shifts, however the
bright bursts do not have galaxies inside their error boxes to limits deeper
than expected based on the luminosities for the two samples being identical. A
new solution to this dilemma is that a broad burst luminosity function along
with a burst number density varying as the star formation rate will require the
average luminosity of the bright sample (
or ) to be much greater than the
average luminosity of the faint sample ( or ). This places the bright bursts at distances
for which host galaxies with a normal luminosity will not violate the observed
limits. In conclusion, all current evidence points to GRB host galaxies being
normal in luminosity.Comment: 18 pages, 3 figures, Submitted to ApJLet
Study of time lags in HETE-2 Gamma-Ray Bursts with redshift: search for astrophysical effects and Quantum Gravity signature
The study of time lags between spikes in Gamma-Ray Bursts light curves in
different energy bands as a function of redshift may lead to the detection of
effects due to Quantum Gravity. We present an analysis of 15 Gamma-Ray Bursts
with measured redshift, detected by the HETE-2 mission between 2001 and 2006 in
order to measure time lags related to astrophysical effects and search for
Quantum Gravity signature in the framework of an extra-dimension string model.
The use of photon-tagged data allows us to consider various energy ranges.
Systematic effects due to selection and cuts are evaluated. No significant
Quantum Gravity effect is detected from the study of the maxima of the light
curves and a lower limit at 95% Confidence Level on the Quantum Gravity scale
parameter of 3.2x10**15 GeV is set.Comment: 4 pages, 5 figures. v3: Error corrected in Eq. 1. Results updated.
Proceedings of the 30th ICRC, Merida, Mexico (2007
A simple empirical redshift indicator for gamma-ray bursts
We propose a new empirical redshift indicator for gamma-ray bursts. This
indicator is easily computed from the gamma-ray burst spectral parameters, and
its duration, and it provides ``pseudo-redshifts'' accurate to a factor two.
Possible applications of this redshift indicator are briefly discussed.Comment: Final version, 4 pages, 3 figures, A&A, vol. 407, L
Evidence for a Fast Decline in the Progenitor Population of Gamma Ray Bursts and the Nature of their Origin
We show that the source population of long gamma-ray bursts (GRBs) has
declined by at least a factor of 12 (at the 90% confidence level) since the
early stages of the Universe (). This result has been obtained
using the combined BATSE and \it Ulysses \rm GRB brightness distribution and
the detection of four GRBs with known redshifts brighter than 10 erg
s in the 50 - 300 keV range at their peak. The data indicate that the
decline of the GRB source population is as fast as, or even faster than, the
measured decline of the star formation rate. Models for the evolution of
neutron star binaries predict a significantly larger number of apparently
bright GRBs than observed. Thus our results give independent support to the
hypernova model, which naturally explains the fast decline in the progenitor
population.Comment: 7 pages, 6 figures, submitted to ApJ, added reference
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