1,469 research outputs found
The Brightness Distribution of Bursting Sources in Relativistic Cosmologies
We present analytical solutions for the integral distribution of arbitrary
bursting or steady source counts as a function of peak photon count rate within
Friedmann cosmological models. We discuss both the standard candle and
truncated power-law luminosity function cases with a power-law density
evolution. While the analysis is quite general, the specific example discussed
here is that of a cosmological gamma-ray burst distribution. These solutions
show quantitatively the degree of dependence of the counts on the density and
luminosity function parameters, as well as the the weak dependence on the
closure parameter and the maximum redshift. An approximate comparison with the
publicly available Compton Gamma Ray Observatory data gives an estimate of the
maximum source luminosity and an upper limit to the minimum luminosity. We
discuss possible ways of further constraining the various parameters.Comment: 16 pages plus one figure, uuencoded postscript file. to appear in
Ap.J
Cosmological Brightness Distribution Fits of Gamma Ray Burst Sources
We discuss detailed fits of the BATSE and PVO gamma-ray burst peak-flux
distributions with Friedman models taking into account possible density
evolution and standard candle or power law luminosity functions. A chi-square
analysis is used to estimate the goodness of the fits and we derive the
significance level of limits on the density evolution and luminosity function
parameters. Cosmological models provide a good fit over a range of parameter
space which is physically reasonable.Comment: Ap.J. in press, uuencoded .ps file, 9 pages manuscript plus 5 figure
What is the Astrophysical Meaning of the Intermediate Subgroup of GRBs?
Published articles concerning the intermediate (third) subgroup of GRBs are
surveyed. From a statistical perspective this subgroup may exist, however its
significance depends on which data set is used. Its astrophysical meaning is
unclear because the occurrence of this subgroup can also be an artificial
selection effect. Hence, GRBs from this subgroup need not be given by a
physically different phenomenon. The aim of this contribution is to search for
the answer to the question in the title.Comment: journal: Proceedings of Science, Swift: 10 Years of Discovery;
conference date: 2-5 December 2014; location: La Sapienza University, Rome,
Italy; 6 pages, 4 figures, 1 table; accepted for publication in July 9 201
A Principal Component Analysis of the 3B Gamma-Ray Burst Data
We have carried out a principal component analysis for 625 gamma-ray bursts
in the BATSE 3B catalog for which non-zero values exist for the nine measured
variables. This shows that only two out of the three basic quantities of
duration, peak flux and fluence are independent, even if this relation is
strongly affected by instrumental effects, and these two account for 91.6% of
the total information content. The next most important variable is the fluence
in the fourth energy channel (at energies above 320 keV). This has a larger
variance and is less correlated with the fluences in the remaining three
channels than the latter correlate among themselves. Thus a separate
consideration of the fourth channel, and increased attention on the related
hardness ratio appears useful for future studies. The analysis gives the
weights for the individual measurements needed to define a single duration,
peak flux and fluence. It also shows that, in logarithmic variables, the
hardness ratio is significantly correlated with peak flux, while is
significantly anticorrelated with peak flux. The principal component analysis
provides a potentially useful tool for estimating the improvement in
information content to be achieved by considering alternative variables or
performing various corrections on available measurementsComment: Ap.J., accepted 12/9/97; revised version contains a new appendix,
somewhat expanded discussion; latex, aaspp4, 15 page
A curious relation between the flat cosmological model and the elliptic integral of the first kind
The dependence of the luminosity distance on the redshift has a key
importance in the cosmology. This dependence can well be given by standard
functions for the zero cosmological constant. The purpose of this article is to
present such a relation also for the non-zero cosmological constant, if the
universe is spatially flat. A definite integral is used. The integration ends
in the elliptic integral of the first kind. The result shows that no numerical
integration is needed for the non-zero cosmological constant, if the universe
is spatially flat.Comment: Accepted for publication in Astronomy & Astrophysics. 2 page
Cosmological Evolution and Luminosity Function Effects on Number Counts, Redshift and Time Dilation of Bursting Sources
We present analytic formulae for the integral number count distribution of
cosmological bursting or steady sources valid over the entire range of fluxes,
including density evolution and either standard candle or a power law
luminosity function. These are used to derive analytic formulae for the mean
redshift, the time dilations and the dispersion of these quantities for sources
within a given flux range for Friedmann models with
without K-corrections, and we discuss the extension to cases with
and inclusion of K-corrections. Applications to the spatial distribution of
cosmological gamma ray burst sources are discussed, both with and without an
intrinsic energy stretching of the burst time profiles, and the implied ranges
of redshift are considered for a specific time dilation signal value. The
simultaneous consideration of time dilation information and of fits of the
number distribution versus peak flux breaks the degeneracy inherent in the
latter alone, allowing a unique determination of the density evolution index
and the characteristic luminosity of the sources. For a reported time dilation
signal of 2.25 and neglecting [including] energy stretching we find that the
proper density should evolve more steeply with redshift than comoving constant,
and the redshifts of the dimmest sources with stretching would be very large.
However, the expected statistical dispersion in the redshifts is large,
especially for power law luminosity functions, and remains compatible with that
of distant quasars. For smaller time dilation values of 1.75 and 1.35 the
redshifts are more compatible with conventional ideas about galaxy formation,
and the evolution is closer to a comoving constant or a slower evolution. More
generally, we have considered a wide range of possible measured time dilationComment: Ap.J., in press, uuencoded .ps file, 33 pages plus 5 figure
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