64,572 research outputs found
An unexpectedly low-redshift excess of Swift gamma-ray burst rate
Gamma-ray bursts (GRBs) are the most violent explosions in the Universe and
can be used to explore the properties of high-redshift universe. It is believed
that the long GRBs are associated with the deaths of massive stars. So it is
possible to use GRBs to investigate the star formation rate (SFR). In this
paper, we use Lynden-Bell's method to study the luminosity function and
rate of \emph{Swift} long GRBs without any assumptions. We find that the
luminosity of GRBs evolves with redshift as with
. After correcting the redshift evolution through
, the luminosity function can be expressed as
for dim GRBs and for bright GRBs, with the break point
. We also find that the formation
rate of GRBs is almost constant at for the first time, which is
remarkably different from the SFR. At , the formation rate of GRB is
consistent with the SFR. Our results are dramatically different from previous
studies. Some possible reasons for this low-redshift excess are discussed. We
also test the robustness of our results with Monte Carlo simulations. The
distributions of mock data (i.e., luminosity-redshift distribution, luminosity
function, cumulative distribution and distribution) are in good
agreement with the observations. Besides, we also find that there are
remarkable difference between the mock data and the observations if long GRB
are unbiased tracers of SFR at .Comment: 33 pages, 10 figures, 1 table, accepted by ApJ
Beaming Effects in Gamma-Ray Bursts
Based on a refined generic dynamical model, we investigate afterglows from
jetted gamma-ray burst (GRB) remnants numerically. In the relativistic phase,
the light curve break could marginally be seen. However, an obvious break does
exist at the transition from the relativistic phase to the non-relativistic
phase, which typically occurs at time 10 to 30 days. It is very interesting
that the break is affected by many parameters, especially by the electron
energy fraction (xi_e), and the magnetic energy fraction (xi_B^2). Implication
of orphan afterglow surveys on GRB beaming is investigated. The possible
existence of a kind of cylindrical jets is also discussed.Comment: Minor changes; 10 pages, with 9 eps figures embedded. Talk given at
the Sixth Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July
11-17, 2002). A slightly revised version will appear in the proceeding
Measuring dark energy with the correlation of gamma-ray bursts using model-independent methods
In this paper, we use two model-independent methods to standardize long
gamma-ray bursts (GRBs) using the correlation, where
is the isotropic-equivalent gamma-ray energy and is
the spectral peak energy. We update 42 long GRBs and try to make constraint on
cosmological parameters. The full sample contains 151 long GRBs with redshifts
from 0.0331 to 8.2. The first method is the simultaneous fitting method. The
extrinsic scatter is taken into account and assigned to the
parameter . The best-fitting values are ,
, and in the flat
CDM model. The constraint on is at the
1 confidence level. If reduced method is used, the best-fit
results are , and . The
second method is using type Ia supernovae (SNe Ia) to calibrate the correlation. We calibrate 90 high-redshift GRBs in the redshift
range from 1.44 to 8.1. The cosmological constraints from these 90 GRBs are
for flat CDM, and
and for non-flat
CDM. For the combination of GRB and SNe Ia sample, we obtain
and for the flat CDM, and
for the non-flat CDM, the results are ,
and . These results from
calibrated GRBs are consistent with that of SNe Ia. Meanwhile, the combined
data can improve cosmological constraints significantly, comparing to SNe Ia
alone. Our results show that the correlation is
promising to probe the high-redshift universe.Comment: 10 pages, 6 figures, 4 table, accepted by A&A. Table 4 contains
calibrated distance moduli of GRB
The Luminosity - E_p Relation within Gamma--Ray Bursts and Implications for Fireball Models
Using a sample of 2408 time-resolved spectra for 91 BATSE gamma-ray bursts
(GRBs) presented by Preece et al., we show that the relation between the
isotropic-equivalent luminosity (L_iso) and the spectral peak energy (E_p) in
the cosmological rest frame, L_iso \propto E_p^2, not only holds within these
bursts, but also holds among these GRBs, assuming that the burst rate as a
function of redshift is proportional to the star formation rate. The possible
implications of this relation for the emission models of GRBs are discussed. We
suggest that both the kinetic-energy-dominated internal shock model and the
magnetic-dissipation-dominated external shock model can well interpret this
relation. We constrain the parameters for these two models, and find that they
are in a good agreement with the parameters from the fittings to the afterglow
data (abridged).Comment: 3 pages plus 5 figures, emulateapj style, accepted for publication in
ApJ Letter
Internal Anisotropy of Collision Cascades
We investigate the internal anisotropy of collision cascades arising from the
branching structure. We show that the global fractal dimension cannot give an
adequate description of the geometrical structure of cascades because it is
insensitive to the internal anisotropy. In order to give a more elaborate
description we introduce an angular correlation function, which takes into
account the direction of the local growth of the branches of the cascades. It
is demonstrated that the angular correlation function gives a quantitative
description of the directionality and the interrelation of branches. The power
law decay of the angular correlation is evidenced and characterized by an
exponent and an angular correlation length different from the radius of
gyration. It is demonstrated that the overlapping of subcascades has a strong
effect on the angular correlation.Comment: RevteX, 8 pages, 6 .eps figures include
A rapid cosmic-ray increase in BC 3372-3371 from ancient buried tree rings in China
Cosmic rays interact with the Earth's atmosphere to produce C, which
can be absorbed by trees. Therefore, rapid increases of C in tree rings
can be used to probe previous cosmic-ray events. By this method, three C
rapidly increasing events have been found. Plausible causes of these events
include large solar proton events, supernovae or short gamma-ray bursts.
However, due to the lack of measurements of C by year, the occurrence
frequency of such C rapidly increasing events is poorly known. In
addition, rapid increases may be hidden in the IntCal13 data with five-year
resolution. Here we report the result of C measurements using an ancient
buried tree during the period between BC 3388 and 3358. We find a rapid
increase of about 9\textperthousand~ in the C content from BC 3372 to BC
3371. We suggest that this event could originate from a large solar proton
event.Comment: 23 pages, 3 figures, 2 tables, published in Nature Communication
Chirally symmetric but confining dense and cold matter
The folklore tradition about the QCD phase diagram is that at the chiral
restoration phase transition at finite density hadrons are deconfined and there
appears the quark matter. We address this question within the only known
exactly solvable confining and chirally symmetric model. It is postulated
within this model that there exists linear Coulomb-like confining interaction.
The chiral symmetry breaking and the quark Green function are obtained from the
Schwinger-Dyson (gap) equation while the color-singlet meson spectrum results
from the Bethe-Salpeter equation. We solve this model at T=0 and finite
chemical potential and obtain a clear chiral restoration phase transition
at the critical value \mu_{cr}. Below this value the spectrum is similar to the
previously obtained one at \mu = 0. At \mu > \mu_{cr} the quarks are still
confined and the physical spectrum consists of bound states which are arranged
into a complete set of exact chiral multiplets. This explicitly demonstrates
that a chirally symmetric matter consisting of confined but chirally symmetric
hadrons at finite chemical potential is also possible in QCD. If so, there must
be nontrivial implications for astrophysics.Comment: 7 pp; the paper has been expanded to make some technical details more
clear; 3 new figures have been added. To appear in PR
Comment on Decay
We calculate the rate for decay using Chiral
Perturbation Theory. This isospin violating process results from -
mixing, and its amplitude is proportional to . Experimental information on the branching
ratio for can provide insight into the pattern of
violation in radiative decays.Comment: 7 pages with 2 figures not included but available upon request,
CALT-68-191
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