3,192 research outputs found
GLAST Prospects for Swift-Era Afterglows
We calculate the GeV spectra of GRB afterglows produced by inverse Compton
scattering of the sub-MeV emission of these objects. We improve on earlier
treatments by using refined afterglow parameters and new model developments
motivated by recent Swift observations. We present time-dependent GeV spectra
for standard, constant parameter models, as well as for models with energy
injection and with time-varying parameters, for a range of burst parameters. We
evaluate the limiting redshift to which such afterglows can be detected by the
GLAST LAT, as well as AGILE.Comment: 19 pages, 5 figures, ApJ, in pres
GRB Afterglows from Anisotropic Jets
Some progenitor models of gamma-ray bursts (GRBs) (e.g., collapsars) may
produce anisotropic jets in which the energy per unit solid angle is a
power-law function of the angle (). We calculate light
curves and spectra for GRB afterglows when such jets expand either in the
interstellar medium or in the wind medium. In particular, we take into account
two kinds of wind: one () possibly from a typical red
supergiant star and another () possibly from a Wolf-Rayet
star. We find that in each type of medium, one break appears in the late-time
afterglow light curve for small but becomes weaker and smoother as
increases. When , the break seems to disappear but the afterglow decays
rapidly. Thus, one expects that the emission from expanding, highly anisotropic
jets provides a plausible explanation for some rapidly fading afteglows whose
light curves have no break. We also present good fits to the optical afterglow
light curve of GRB 991208. Finally, we argue that this burst might arise from a
highly anisotropic jet expanding in the wind () from a red
supergiant to interpret the observed radio-to-optical-band afterglow data
(spectrum and light curve).Comment: 12 pages + 10 figures, accepted by Ap
Post density functional theoretical studies of highly polar semiconductive Pb(TiNi)O solid solutions: The effects of cation arrangement on band gap
We use a combination of conventional density functional theory (DFT) and
post-DFT methods, including the local density approximation plus Hubbard
(LDA+), PBE0, and self-consistent to study the electronic properties of
Ni-substituted PbTiO (Ni-PTO) solid solutions. We find that LDA
calculations yield unreasonable band structures, especially for Ni-PTO solid
solutions that contain an uninterrupted NiO layer. Accurate treatment of
localized states in transition-metal oxides like Ni-PTO requires post-DFT
methods. -site Ni/Ti cation ordering is also investigated. The -site
cation arrangement alters the bonding between Ni and O, and therefore strongly
affects the band gap () of Ni-PTO. We predict that Ni-PTO solid
solutions should have a direct band gap in the visible light energy range, with
polarization similar to the parent PbTiO. This combination of properties
make Ni-PTO solid solutions promising candidate materials for solar energy
conversion devices.Comment: 19 pages, 6 figure
Modeling GRB 050904: Autopsy of a Massive Stellar Explosion at z=6.29
GRB 050904 at redshift z=6.29, discovered and observed by Swift and with
spectroscopic redshift from the Subaru telescope, is the first gamma-ray burst
to be identified from beyond the epoch of reionization. Since the progenitors
of long gamma-ray bursts have been identified as massive stars, this event
offers a unique opportunity to investigate star formation environments at this
epoch. Apart from its record redshift, the burst is remarkable in two respects:
first, it exhibits fast-evolving X-ray and optical flares that peak
simultaneously at t~470 s in the observer frame, and may thus originate in the
same emission region; and second, its afterglow exhibits an accelerated decay
in the near-infrared (NIR) from t~10^4 s to t~3 10^4 s after the burst,
coincident with repeated and energetic X-ray flaring activity. We make a
complete analysis of available X-ray, NIR, and radio observations, utilizing
afterglow models that incorporate a range of physical effects not previously
considered for this or any other GRB afterglow, and quantifying our model
uncertainties in detail via Markov Chain Monte Carlo analysis. In the process,
we explore the possibility that the early optical and X-ray flare is due to
synchrotron and inverse Compton emission from the reverse shock regions of the
outflow. We suggest that the period of accelerated decay in the NIR may be due
to suppression of synchrotron radiation by inverse Compton interaction of X-ray
flare photons with electrons in the forward shock; a subsequent interval of
slow decay would then be due to a progressive decline in this suppression. The
range of acceptable models demonstrates that the kinetic energy and circumburst
density of GRB 050904 are well above the typical values found for low-redshift
GRBs.Comment: 45 pages, 7 figures, and ApJ accepted. Revised version, minor
modifications and 1 extra figur
Measuring the Spins of Stellar Black Holes: A Progress Report
We use the Novikov-Thorne thin disk model to fit the thermal continuum X-ray
spectra of black hole X-ray binaries, and thereby extract the dimensionless
spin parameter a* = a/M of the black hole as a parameter of the fit. We
summarize the results obtained to date for six systems and describe work in
progress on additional systems. We also describe recent methodological
advances, our current efforts to make our analysis software fully available to
others, and our theoretical efforts to validate the Novikov-Thorne model.Comment: 6 pages, conference proceedings, X-ray Astronomy 2009: Present
Status, Multi-Wavelength Approach and Future Perspectives, AIP, eds. A.
Comastri et al.; list of authors revise
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