1,130,611 research outputs found
Star Formation History up to z = 7.4: Implications for Gamma-Ray Bursts and the Cosmic Metallicity Evolution
The current Swift sample of gamma-ray bursts (GRBs) with measured redshifts
allows to test the assumption that GRBs trace the star formation in the
Universe. Some authors have claimed that the rate of GRBs increases with cosmic
redshift faster than the star formation rate, whose cause is not known yet. In
this paper, I investigate the possibility for interpreting the observed
discrepancy between the GRB rate history and the star formation rate history by
the cosmic metallicity evolution, motivated by the observation that the cosmic
metallicity evolves with redshift and GRBs prefer to occur in low metallicity
galaxies. First, I derive a star formation history up to redshift z=7.4 from an
updated sample of star formation rate densities obtained by adding the new UV
measurements of Bouwens et al. and the new UV and infrared measurements of
Reddy et al. to the existing sample compiled by Hopkins & Beacom. Then,
adopting a simple model for the relation between the GRB production and the
cosmic metallicity history as proposed by Langer & Norman, I show that the
observed redshift distribution of the Swift GRBs can be reproduced with a
fairly good accuracy. Although the results are limited by the small size of the
GRB sample and the poorly understood selection biases in detection and
localization of GRBs and in redshift determination, they suggest that GRBs
trace both the star formation and the metallicity evolution. If the star
formation history can be accurately measured with other approaches, which is
presumably achievable in the near future, it will be possible to determine the
cosmic metallicity evolution with the study on the redshift distribution of
GRBs.Comment: 16 pages, 11 figures and 2 tables, accepted for publication in MNRAS.
New version contains an updated star formation history and a review on the
cosmic metallicity measuremen
Constraining GRB Initial Lorentz Factor with the Afterglow Onset Feature and Discovery of a Tight Gamma_0-E_iso Correlation
The onset of GRB afterglow is characterized by a smooth bump in the early
afterglow lightcurve. We make an extensive search for such a feature. Twenty
optically selected GRBs and 12 X-ray selected GRBs are found, among which 17
optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements.
We fit the lightcurves with a smooth broken power-law and measure the temporal
characteristic timescales of the bumps at FWHM. Strong mutual correlations
among these timescales are found, and a dimmer and broader bump tends to peak
at a later peak time. The ratio of rising to decaying timescales is almost
universal among bursts, but the ratio of the rising time to the peak time
varies from 0.3~1. The E_iso is tightly correlated with the peak luminosity and
the peak time of the bump in the burst frame. Assuming that the bumps signal
the deceleration of the GRB fireballs in a constant density medium, we
calculate the initial Lorentz factor (Gamma_0) and the deceleration radius
(R_dec) of the GRBs in the optical-selected sample. It is found that Gamma_0
are typically a few hundreds, and the typical deceleration radius is
R_dec~10^{17} cm. More intriguingly, a tight correlation between the Gamma_0
and E_iso is found, namely Gamma_0 ~ 195 E_iso, 52}^{0.27} (satisfied for both
the optical and X-ray z-known samples). It is helpful to understand GRB
physics, and may serve as an indicator of Gamma_0. We find that the early
bright X-rays are usually dominated by a different component from the external
shock emission, but occasionally (for one case) an achromatic deceleration
feature is observed. Components in X-rays would contribute to the diversity of
the observed X-ray lightcurves (abridge).Comment: 15 pages, including 4 tables and 7 figures, Submitted to Ap
Low-Luminosity Gamma-Ray Bursts as a Distinct GRB Population:A Firmer Case from Multiple Criteria Constraints
The intriguing observations of Swift/BAT X-ray flash XRF 060218 and the
BATSE-BeppoSAX gamma-ray burst GRB 980425, both with much lower luminosity and
redshift compared to other observed bursts, naturally lead to the question of
how these low-luminosity (LL) bursts are related to high-luminosity (HL)
bursts. Incorporating the constraints from both the flux-limited samples
observed with CGRO/BATSE and Swift/BAT and the redshift-known GRB sample, we
investigate the luminosity function for both LL- and HL-GRBs through
simulations. Our multiple criteria, including the log N - log P distributions
from the flux-limited GRB sample, the redshift and luminosity distributions of
the redshift-known sample, and the detection ratio of HL- and LL- GRBs with
Swift/BAT, provide a set of stringent constraints to the luminosity function.
Assuming that the GRB rate follows the star formation rate, our simulations
show that a simple power law or a broken power law model of luminosity function
fail to reproduce the observations, and a new component is required. This
component can be modeled with a broken power, which is characterized by a sharp
increase of the burst number at around L < 10^47 erg s^-1}. The lack of
detection of moderate-luminosity GRBs at redshift ~0.3 indicates that this
feature is not due to observational biases. The inferred local rate, rho_0, of
LL-GRBs from our model is ~ 200 Gpc^-3 yr^-1 at ~ 10^47 erg s^-1, much larger
than that of HL-GRBs. These results imply that LL-GRBs could be a separate GRB
population from HL-GRBs. The recent discovery of a local X-ray transient
080109/SN 2008D would strengthen our conclusion, if the observed non-thermal
emission has a similar origin as the prompt emission of most GRBs and XRFs.Comment: 22 pages, 9 figures, 3 tables; MNRAS, in press; Updated analysis and
figure
Testing the Epeak - Eiso relation for GRBs detected by Swift and Suzaku-WAM
One of the most prominent, yet controversial associations derived from the
ensemble of prompt-phase observations of gamma-ray bursts (GRBs) is the
apparent correlation in the source frame between the peak energy Epeak) of the
nu-F(nu) spectrum and the isotropic radiated energy, Eiso. Since most gamma-ray
bursts (GRBs) have Epeak above the energy range (15-150 keV) of the Burst Alert
Telescope (BAT) on Swift, determining accurate Epeak values for large numbers
of Swift bursts has been difficult. However, by combining data from Swift/BAT
and the Suzaku Wide-band All-Sky Monitor (WAM), which covers the energy range
from 50-5000 keV, for bursts which are simultaneously detected, one can
accurately fit Epeak and Eiso and test the relationship between them for the
Swift sample. Between the launch of Suzaku in July 2005 and the end of April
2009, there were 48 gamma-ray bursts (GRBs) which triggered both Swift/BAT and
WAM and an additional 48 bursts which triggered Swift and were detected by WAM,
but did not trigger. A BAT-WAM team has cross-calibrated the two instruments
using GRBs, and we are now able to perform joint fits on these bursts to
determine their spectral parameters. For those bursts with spectroscopic
redshifts, we can also calculate the isotropic energy. Here we present the
results of joint Swift/BAT-Suzaku/WAM spectral fits for 91 of the bursts
detected by the two instruments. We show that the distribution of spectral fit
parameters is consistent with distributions from earlier missions and confirm
that Swift bursts are consistent with earlier reported relationships between
Epeak and isotropic energy. We show through time-resolved spectroscopy that
individual burst pulses are also consistent with this relationship.Comment: Accepted for publication in the Astrophysical Journa
Detection of Gamma-Ray Polarization in Prompt Emission of GRB 100826A
We report the polarization measurement in prompt -ray emission of GRB
100826A with the Gamma-Ray Burst Polarimeter (GAP) aboard the small solar power
sail demonstrator IKAROS. We detected the firm change of polarization angle
(PA) during the prompt emission with 99.9% () confidence level, and
the average polarization degree () of % with 99.4% () confidence level. Here the quoted errors are given at 1
confidence level for two parameters of interest. The systematic errors have
been carefully included in this analysis, unlike any previous reports. Such a
high can be obtained in several emission models of gamma-ray bursts
(GRBs), including synchrotron and photospheric models. However, it is difficult
to explain the observed significant change of PA within the framework of
axisymmetric jet as considered in many theoretical works. The non-axisymmetric
(e.g., patchy) structures of the magnetic fields and/or brightness inside the
relativistic jet are therefore required within the observable angular scale of
. Our observation strongly indicates that the polarization
measurement is a powerful tool to constrain the GRB production mechanism, and
more theoretical works are needed to discuss the data in more details.Comment: 6 pages, 3 figures, accepted for publication in ApJ
GRB 090417B and its Host Galaxy: A Step Towards an Understanding of Optically-Dark Gamma-Ray Bursts
GRB 090417B was an unusually long burst with a T_90 duration of at least 2130
s and a multi-peaked light curve at energies of 15-150 keV. It was optically
dark and has been associated with a bright star-forming galaxy at a redshift of
0.345 that is broadly similar to the Milky Way. This is one of the few cases
where a host galaxy has been clearly identified for a dark gamma-ray burst and
thus an ideal candidate for studying the origin of dark bursts. We find that
the dark nature of GRB 090417B cannot be explained by high redshift, incomplete
observations, or unusual physics in the production of the afterglow. Assuming
the standard relativistic fireball model for the afterglow we find that the
optical flux is at least 2.5 mag fainter than predicted by the X-ray flux. The
Swift/XRT X -ray data are consistent with the afterglow being obscured by a
dense, localized sheet of dust approximately 30-80 pc from the burst along the
line of sight. Our results suggest that this dust sheet imparts an extinction
of A_V >~ 12 mag, which is sufficient to explain the missing optical flux. GRB
090417B is an example of a gamma-ray burst that is dark due to the localized
dust structure in its host galaxy.Comment: Accepted for publication in Ap
Gamma-Ray Bursts: Jets and Energetics
The relativistic outflows from gamma-ray bursts are now thought to be
narrowly collimated into jets. After correcting for this jet geometry there is
a remarkable constancy of both the energy radiated by the burst and the kinetic
energy carried by the outflow. Gamma-ray bursts are still the most luminous
explosions in the Universe, but they release energies that are comparable to
supernovae. The diversity of cosmic explosions appears to be governed by the
fraction of energy that is coupled to ultra-relativistic ejecta.Comment: Paper presented at "The Restless High-Energy Universe", May 5-8 2003
Royal Tropical Institute, Amsterda
The polarisation of afterglow emission reveals GRB jet structure
We numerically compute light and polarisation curves of gamma-ray burst
afterglows for various configurations of the jet luminosity structure and for
different dynamical evolutions. We especially consider the standard homogeneous
``top hat'' jet and the ``universal structured jet'' with power-law wings. We
also investigate a possible more physical variation of the ``top hat'' model:
the ``Gaussian jet''. The polarisation curves for the last two jet types are
shown here for the first time together with the computation of X-ray and radio
polarised fluxes. We show that the lightcurves of the total flux from these
configurations are very similar to each other, and therefore only very high
quality data could allow us to pin down the underlying jet structure. We
demonstrate instead that polarisation curves are a powerful means to solve the
jet structure, since the predicted behaviour of polarisation and its position
angle at times around the jet break are very different if not opposite. We
conclude that the afterglow polarisation measurements provide clear footprints
of any outflow energy distribution (unlike the lightcurves of the total flux)
and the joint analysis of the total and polarised flux should reveal GRBs jet
structure.Comment: 16 pages, 18 figures, MNRAS, temp, 321. Light curves and polarisation
curves for a Gaussian jet added. Cartoon of the three jet structures adde
GRB 050505: A high redshift burst discovered by Swift
We report the discovery and subsequent multi-wavelength afterglow behaviour
of the high redshift (z = 4.27) Gamma Ray Burst GRB 050505. This burst is the
third most distant burst, measured by spectroscopic redshift, discovered after
GRB 000131 (z = 4.50) and GRB 050904 (z = 6.29). GRB 050505 is a long GRB with
a multipeaked gamma-ray light curve, with a duration of T_90 = 63+/-2 s and an
inferred isotropic release in gamma-rays of ~4.44 x 10^53 ergs in the 1-10^4
keV rest frame energy range. The Swift X-Ray Telescope followed the afterglow
for 14 days, detecting two breaks in the light curve at 7.4(+/-1.5) ks and 58.0
(+9.9/-15.4) ks after the burst trigger. The power law decay slopes before,
between and after these breaks were 0.25 (+0.16/-0.17), 1.17 (+0.08/-0.09) and
1.97 (+0.27/-0.28) respectively. The light curve can also be fit with a
`smoothly broken' power law model with a break observed at ~ T+18.5 ks, with
decay slopes of ~0.4 and ~1.8 before and after the break respectively. The
X-ray afterglow shows no spectral variation over the course of the Swift
observations, being well fit with a single power law of photon index ~1.90.
This behaviour is expected for the cessation of continued energisation of the
ISM shock followed by a break caused by a jet, either uniform or structured.
Neither break is consistent with a cooling break. The spectral energy
distribution indeed shows the cooling frequency to be below the X-ray but above
optical frequencies. The optical -- X-ray spectrum also shows that there is
significant X-ray absorption in excess of that due to our Galaxy but very
little optical/UV extinction, with E(B-V) ~0.10 for a SMC-like extinction
curve.Comment: 9 pages, 6 figures. Accepted by MNRA
The dark GRB080207 in an extremely red host and the implications for GRBs in highly obscured environments
[Abridged] We present comprehensive X-ray, optical, near- and mid-infrared,
and sub-mm observations of GRB 080207 and its host galaxy. The afterglow was
undetected in the optical and near-IR, implying an optical to X-ray index <0.3,
identifying GRB 080207 as a dark burst. Swift X-ray observations show extreme
absorption in the host, which is confirmed by the unusually large optical
extinction found by modelling the X-ray to nIR afterglow spectral energy
distribution. Our Chandra observations obtained 8 days post-burst allow us to
place the afterglow on the sky to sub-arcsec accuracy, enabling us to pinpoint
an extremely red galaxy (ERO). Follow-up host observations with HST, Spitzer,
Gemini, Keck and the James Clerk Maxwell Telescope (JCMT) provide a photometric
redshift solution of z ~1.74 (+0.05,-0.06) (1 sigma), 1.56 < z < 2.08 at 2
sigma) for the ERO host, and suggest that it is a massive and morphologically
disturbed ultra-luminous infrared galaxy (ULIRG) system, with L_FIR ~ 2.4 x
10^12 L_solar. These results add to the growing evidence that GRBs originating
in very red hosts always show some evidence of dust extinction in their
afterglows (though the converse is not true -- some extinguished afterglows are
found in blue hosts). This indicates that a poorly constrained fraction of GRBs
occur in very dusty environments. By comparing the inferred stellar masses, and
estimates of the gas phase metallicity in both GRB hosts and sub-mm galaxies we
suggest that many GRB hosts, even at z>2 are at lower metallicity than the
sub-mm galaxy population, offering a likely explanation for the dearth of
sub-mm detected GRB hosts. However, we also show that the dark GRB hosts are
systematically more massive than those hosting optically bright events, perhaps
implying that previous host samples are severely biased by the exclusion of
dark events.Comment: 13 pages, 6 figures, accepted for publication in MNRA
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