1,094 research outputs found
Spectra of Unsteady Wind Models of Gamma-Ray Bursts
We calculate the spectra expected from unsteady relativistic wind models of
gamma-ray bursts, suitable for events of arbitrary duration. The spectral
energy distribution of the burst is calculated over photon energies spanning
from eV to TeV, for a range of event durations and variability timescales. The
relative strength of the emission at different wavelengths can provide valuable
information on the particle acceleration, radiation mechanisms and the possible
types of models.Comment: 10 pages, 2 postscript figures included, uses aaspp4.sty. Accepted
for publication in the Astrophysical Journal Letters. Also available at
http://www.astro.psu.edu/users/hara/Preprints/xxx_sub.p
BPMPD User's Manual, Version 2.20
The purpose of this document is to describe a software package, called BPMPD, which implements the infeasible primal-dual interior point method for linear and quadratic programming problems. This manual describes how to prepare data to solve with the package, how to use BPMPD as a callable solver library and which algorithmic options can be specified by the user
Steep Slopes and Preferred Breaks in GRB Spectra: the Role of Photospheres and Comptonization
The role of a photospheric component and of pair breakdown is examined in the
internal shock model of gamma-ray bursts. We discuss some of the mechanisms by
which they would produce anomalously steep low energy slopes, X-ray excesses
and preferred energy breaks. Sub-relativistic comptonization should dominate in
high comoving luminosity bursts with high baryon load, while synchrotron
radiation dominates the power law component in bursts which have lower comoving
luminosity or have moderate to low baryon loads. A photosphere leading to steep
low energy spectral slopes should be prominent in the lowest baryon loadComment: ApJ'00, in press; minor revs. 10/5/99; (uses aaspp4.sty), 15 pages, 3
figure
A repository of convex quadratic programming problems
The introduction of a standard set of linear programming problems, to be found in NETLIB/LP/DATA, had an important impact on measuring, comparing and reporting the performance of LP solvers Until recently the efficiency of new algorithmic developments has been measured using this important reference set Presently, we are witnessing an ever growing interest in the area of quadratic programming The research community is somewhat troubled by the lack of a standard format for defining a QP problem and also by the lack of a standard reference set of problems for purposes similar to that of LP In the paper we propose a standard format and announce the availability of a test set of 138 collected QP problem
Modelling of Centrally Planned Food and Agriculture Systems: A Framework for a National Policy Model for the Hungarian Food and Agriculture Sector
In this paper the general structure and mathematical description of the Hungarian Agricultural Model is presented. As an introduction the basic characteristics of food and agriculture systems in the centrally planned economies and IIASA's approach in their modelling and some features of Hungarian agriculture are discussed.
The Hungarian Agricultural Model has a descriptive and dynamic (recursive with a one year time increment) character. Besides the disaggregated food and agriculture (25 agricultural and 25 processed food commodities) the rest of the economy is also considered. The model is in fact a system of interconnected models. The economic management and planning submodel describes the decision making and control of socialist state following the idea of central planning of the economy. The desired structure of food production, export, import and investment targets are calculated by a linear programming model. The submodel of real sphere covers the whole national economy. The major blocks of the latter submodel are related to production (linear programming models for socialist agriculture and food processing sector, nonlinear optimization model for household and private agriculture), consumption and trade including nonlinear demand system as well as updating available resource and other model parameters
On the large-scale angular distribution of short-Gamma ray bursts
We investigate the large-scale angular distribution of the short-Gamma ray
bursts (SGRBs) from BATSE experiment, using a new coordinates-free method. The
analyses performed take into account the angular correlations induced by the
non-uniform sky exposure during the experiment, and the uncertainty in the
measured angular coordinates. Comparising the large-scale angular correlations
from the data with those expected from simulations using the exposure function
we find similar features. Additionally, confronting the large-angle
correlations computed from the data with those obtained from simulated maps
produced under the assumption of statistical isotropy we found that they are
incompatible at 95% confidence level. However, such differences are restricted
to the angular scales 36o - 45o, which are likely to be due to the non-uniform
sky exposure. This result strongly suggests that the set of SGRBs from BATSE
are intrinsically isotropic. Moreover, we also investigated a possible
large-angle correlation of these data with the supergalactic plane. No evidence
for such large-scale anisotropy was found.Comment: Accepted for publication in The Astrophysical Journal, 6 pages, 3
figure
Effect of Primordial Black Holes on the Cosmic Microwave Background and Cosmological Parameter Estimates
We investigate the effect of non-evaporating primordial black holes (PBHs) on
the ionization and thermal history of the universe. X-rays emitted by gas
accretion onto PBHs modify the cosmic recombination history, producing
measurable effects on the spectrum and anisotropies of the Cosmic Microwave
Background (CMB). Using the third-year WMAP data and FIRAS data we improve
existing upper limits on the abundance of PBHs with masses >0.1 Msun by several
orders of magnitude. Fitting WMAP3 data with cosmological models that do not
allow for non-standard recombination histories, as produced by PBHs or other
early energy sources, may lead to an underestimate of the best-fit values of
the amplitude of linear density fluctuations (sigma_8) and the scalar spectral
index (n_s). Cosmological parameter estimates are affected because models with
PBHs allow for larger values of the Thomson scattering optical depth, whose
correlation with other parameters may not be correctly taken into account when
PBHs are ignored. Values of tau_e=0.2, n_s=1 and sigma_8=0.9 are allowed at 95%
CF. This result that may relieve recent tension between WMAP3 data and clusters
data on the value of sigma_8. PBHs may increase the primordial molecular
hydrogen abundance by up to two orders of magnitude, this promoting cooling and
star formation. The suppression of galaxy formation due to X-ray heating is
negligible for models consistent with the CMB data. Thus, the formation rate of
the first galaxies and stars would be enhanced by a population of PBHs.Comment: 17 pages (Apj style), 9 figures, submitted to Ap
The role of shell crossing on the existence and stability of trapped matter shells in spherical inhomogeneous \Lambda-CDM models
We analyse the dynamics of trapped matter shells in spherically symmetric
inhomogeneous \Lambda-CDM models. The investigation uses a Generalised
Lema\^itre-Tolman-Bondi description with initial conditions subject to the
constraints of having spatially asymptotic cosmological expansion, initial
Hubble-type flow and a regular initial density distribution. We discuss the
effects of shell crossing and use a qualitative description of the local
trapped matter shells to explore global properties of the models. Once shell
crossing occurs, we find a splitting of the global shells separating expansion
from collapse into, at most, two global shells: an inner and an outer limit
trapped matter shell. In the case of expanding models, the outer limit trapped
matter shell necessarily exists. We also study the role of shear in this
process, compare our analysis with the Newtonian framework and give concrete
examples using density profile models of structure formation in cosmology.Comment: 17pp 12fig
- …