3,766 research outputs found
Testing Isotropic Universe Using the Gamma-Ray Burst Data of Fermi / GBM
The sky distribution of Gamma-Ray Bursts (GRBs) has been intensively studied
by various groups for more than two decades. Most of these studies test the
isotropy of GRBs based on their sky number density distribution. In this work
we propose an approach to test the isotropy of the Universe through inspecting
the isotropy of the properties of GRBs such as their duration, fluences and
peak fluxes at various energy bands and different time scales. We apply this
method on the {\em Fermi} / Gamma-ray Burst Monitor (GBM) data sample
containing 1591 GRBs. The most noticeable feature we found is near the Galactic
coordinates , and radius . The inferred probability for the occurrence of such an
anisotropic signal (in a random isotropic sample) is derived to be less than a
percent in some of the tests while the other tests give results consistent with
isotropy. These are based on the comparison of the results from the real data
with the randomly shuffled data samples. Considering large number of statistics
we used in this work (which some of them are correlated to each other) we can
anticipate that the detected feature could be result of statistical
fluctuations. Moreover, we noticed a considerably low number of GRBs in this
particular patch which might be due to some instrumentation or observational
effects that can consequently affect our statistics through some systematics.
Further investigation is highly desirable in order clarify about this result,
e.g. utilizing a larger future {\em Fermi} / GBM data sample as well as data
samples of other GRB missions and also looking for possible systematics.Comment: 17 pages, 10 figures, 4 tables, accepted for publication in The
Astrophysical Journa
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 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
On the properties of the RHESSI intermediate-duration gamma-ray bursts
The intermediate-duration gamma-ray bursts (GRBs) identified in the data of
the RHESSI satellite are investigated with respect to their spectral lags, peak
count rates, redshifts, supernova observations, and star formation rates of
their host galaxies. Standard statistical tests like Kolmogorov-Smirnov and
Student t-test are used. It is discussed whether these bursts belong to the
group of so-called short or long GRBs, or if they significantly differ from
both groups.Comment: 4 pages, 4 figures, 2 table
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