569 research outputs found
Do Fermi-LAT observations really imply very large Lorentz factors in GRB outflows ?
Recent detections of GeV photons in a few GRBs by Fermi-LAT have led to
strong constraints on the bulk Lorentz factor in GRB outflows. To avoid a large
gamma gamma optical depth, minimum values of the Lorentz factor are estimated
to be as high as 800-1200 in some bursts. Here we present a detailed
calculation of the gamma gamma optical depth taking into account both the
geometry and the dynamics of the jet. In the framework of the internal shock
model, we compute lightcurves in different energy bands and the corresponding
spectrum and we show how the limits on the Lorentz factor can be significantly
lowered compared to previous estimates.Comment: 4 pages, 3 figures, proceedings of the Gamma Ray Bursts 2010
Conference, Annapolis, USA. Editors: McEnery, Racusin, Gehrel
A Correlation Between the Intrinsic Brightness and Average Decay Rate of Gamma-ray Burst X-ray Afterglow Light Curves
We present a correlation between the average temporal decay
({\alpha}X,avg,>200s) and early-time luminosity (LX,200s) of X-ray afterglows
of gamma-ray bursts as observed by Swift-XRT. Both quantities are measured
relative to a rest frame time of 200 s after the {\gamma}-ray trigger. The
luminosity average decay correlation does not depend on specific temporal
behavior and contains one scale independent quantity minimizing the role of
selection effects. This is a complementary correlation to that discovered by
Oates et al. (2012) in the optical light curves observed by Swift-UVOT. The
correlation indicates that on average, more luminous X-ray afterglows decay
faster than less luminous ones, indicating some relative mechanism for energy
dissipation. The X-ray and optical correlations are entirely consistent once
corrections are applied and contamination is removed. We explore the possible
biases introduced by different light curve morphologies and observational
selection effects, and how either geometrical effects or intrinsic properties
of the central engine and jet could explain the observed correlation.Comment: Accepted for Publication in ApJ; 16 pages, 15 figures, 2 table
Optical Classification of Gamma-Ray Bursts in the Swift Era
We propose a new method for the classification of optically dark gamma-ray
bursts (GRBs), based on the X-ray and optical-to-X-ray spectral indices of GRB
afterglows, and utilizing the spectral capabilities of Swift. This method
depends less on model assumptions than previous methods, and can be used as a
quick diagnostic tool to identify optically sub-luminous bursts. With this
method we can also find GRBs that are extremely bright at optical wavelengths.
We show that the previously suggested correlation between the optical darkness
and the X-ray/gamma-ray brightness is merely an observational selection effect.Comment: 6 pages, 3 figures; accepted for publication in Ap
Optimisation of the Swift X-ray follow-up of Advanced LIGO and Virgo gravitational wave triggers in 2015--16
One of the most exciting near-term prospects in physics is the potential
discovery of gravitational waves by the advanced LIGO and Virgo detectors. To
maximise both the confidence of the detection and the science return, it is
essential to identify an electromagnetic counterpart. This is not trivial, as
the events are expected to be poorly localised, particularly in the near-term,
with error regions covering hundreds or even thousands of square degrees. In
this paper we discuss the prospects for finding an X-ray counterpart to a
gravitational wave trigger with the Swift X-ray Telescope, using the assumption
that the trigger is caused by a binary neutron star merger which also produces
a short gamma-ray burst. We show that it is beneficial to target galaxies
within the GW error region, highlighting the need for substantially complete
galaxy catalogues out to distances of 300 Mpc. We also show that nearby,
on-axis short GRBs are either extremely rare, or are systematically less
luminous than those detected to date. We consider the prospects for detecting
afterglow emission from an an off-axis GRB which triggered the GW facilities,
finding that the detectability, and the best time to look, are strongly
dependent on the characteristics of the burst such as circumburst density and
our viewing angle.Comment: 17 pages, 14 figures. Accepted for publication in MNRA
Backlash against Gender Stereotype-violating Preschool Children
While there is substantial evidence that adults who violate gender stereotypes often face backlash (i.e. social and economic penalties), less is known about the nature of gender stereotypes for young children, and the penalties that children may face for violating them. We conducted three experiments, with over 2000 adults from the US, to better understand the content and consequences of adultsā gender stereotypes for young children. In Experiment 1, we tested which characteristics adults (N = 635) believed to be descriptive (i.e. typical), prescriptive (i.e. required), and proscriptive (i.e. forbidden) for preschool-aged boys and girls. Using the characteristics that were rated in Experiment 1, we then constructed vignettes that were either āmasculineā or āfeminineā, and manipulated whether the vignettes were said to describe a boy or a girl. Experiment 2 (N = 697) revealed that adults rated stereotype-violating children as less likeable than their stereotype-conforming peers, and that this difference was more robust for boys than girls. Experiment 3 (N = 731) was a direct replication of Experiment 2, and revealed converging evidence of backlash against stereotype-violating children. In sum, our results suggest that even young children encounter backlash from adults for stereotype violations, and that these effects may be strongest for boys
Exploring the Extreme Universe with the Fermi Gamma-Ray Space Telescope
In ways similar to experiments in nuclear and particle physics, high-energy astrophysics usesgamma rays and energetic charged particles toprobe processes that involve large energy transfers.Since its launch in 2008, the international Fermi Gamma-Ray Space Telescope has been exploringnatural particle accelerators and the interactionsof high-energy particles in the universe. Withsources ranging from thunderstorms on Earth to galaxies and exploding stars in distant parts of the cosmos, the telescopes subjects of study are almostas diverse as were those of the scientist whose name it bears
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