97 research outputs found
On the toughness of thermoplastic polymer nanocomposites as assessed by the essential work of fracture (EWF) approach
The essential work of fracture (EWF) approach is widely used to determine the plane stress fracture toughness of highly ductile polymers and related systems. To shed light on how the toughness is affected by nanofillers EWF-suited model polymers, viz. amorphous copolyester and polypropylene block copolymer were modified by multiwall carbon nanotube (MWCNT), graphene (GR), boehmite alumina (BA), and organoclay (MMT) in 1 wt% each. EWF tests were performed on deeply double-edge notched tensile-loaded specimens under quasistatic loading conditions. Data reduction occurred by energy partitioning between yielding and necking/tearing. The EWF prerequisites were not met with the nanocomposites containing MWCNT and GR by contrast to those with MMT and BA. Accordingly, the toughness of nanocomposites with homogeneously dispersed and low aspect ratio fillers may be properly determined using the EWF. Results indicated that incorporation of nanofillers may result in an adverse effect between the specific essential and non-essential EWF parameters
A complete sample of bright Swift Gamma-Ray Bursts: X-ray afterglow luminosity and its correlation with the prompt emission
We investigate wheter there is any correlation between the X-ray afterglow
luminosity and the prompt emission properties of a carefully selected
sub-sample of bright Swift long Gamma-Ray Bursts (GRBs) nearly complete in
redshift (~90%). Being free of selection effects (except flux limit), this
sample provides the possibility to compare the rest frame physical properties
of GRB prompt and afterglow emission in an unbiased way. The afterglow X-ray
luminosities are computed at four different rest frame times (5 min, 1 hr, 11
hr and 24 hr after trigger) and compared with the prompt emission isotropic
energy E_iso, the isotropic peak luminosity L_iso and the rest frame peak
energy E_peak. We find that the rest frame afterglow X-ray luminosity do
correlate with these prompt emission quantities, but the significance of each
correlation decreases over time. This result is in agreement with the idea that
the GRB X-ray light curve can be described as the result of a combination of
different components whose relative contribution and weight change with time,
with the prompt and afterglow emission dominating at early and late time,
respectively. In particular, we found evidence that the plateau and the shallow
decay phase often observed in GRB X-ray light curves are powered by activity
from the central engine. The existence of the L_X-E_iso correlation at late
times (t_rf > 11 hr) suggests a similar radiative efficiency among different
bursts with on average about 6% of the total kinetic energy powering the prompt
emission.Comment: 8 pages, 4 figures, 3 tables. Accepted for publication in MNRA
A model for cyclotron resonance scattering features
(abbreviated version of the abstract) We study the physics of cyclotron line
formation in the high-energy spectra of accreting X-ray pulsars using Monte
Carlo methods, assuming that the line-forming region is a low-density electron
plasma in a sub-critical magnetic field. We investigate the dependence of the
shape of the fundamental line on angle, geometry, optical depth and
temperature. We also discuss variations of the line ratios for non-uniform
magnetic fields. These numerical predictions for the line profiles are linked
to results from observational data analysis using an XSPEC model based on the
Monte Carlo simulations. We apply this model to observational data from RXTE
and INTEGRAL. The predicted strong emission wings of the fundamental cyclotron
feature are not found in observational data, hinting at a bottom illuminated
slab geometry for line formation.Comment: 16 pages, 15 figures, Astron. Astrophys. (in press
Lithium abundances along the RGB: FLAMES-GIRAFFE spectra of a large sample of low-mass Bulge stars
Context: A small number of K-type giants on the red giant branch (RGB) is
known to be very rich in lithium (Li). This fact is not accounted for by
standard stellar evolution theory. The exact phase and mechanism of Li
enrichment is still a matter of debate. Aims: Our goal is to probe the
abundance of Li along the RGB, from its base to the tip, to confine Li-rich
phases that are supposed to occur on the RGB. Methods: For this end, we
obtained medium-resolution spectra with the FLAMES spectrograph at the VLT in
GIRAFFE mode for a large sample of 401 low-mass RGB stars located in the
Galactic bulge. The Li abundance was measured in the stars with a detectable Li
670.8 nm line by means of spectral synthesis with COMARCS model atmospheres. A
new 2MASS (J-K) - Teff calibration from COMARCS models is presented in the
Appendix. Results: Thirty-one stars with a detectable Li line were identified,
three of which are Li-rich according to the usual criterion (). The stars are distributed all along the RGB, not concentrated in
any particular phase of the red giant evolution (e.g. the luminosity bump or
the red clump). The three Li-rich stars are clearly brighter than the
luminosity bump and red clump, and do not show any signs of enhanced mass loss.
Conclusions: We conclude that the Li enrichment mechanism cannot be restricted
to a clearly defined phase of the RGB evolution of low-mass stars
(M\sim1M_{\sun}), contrary to earlier suggestions from disk field stars.Comment: 6 pages (14 with appendix), 5 figures (1 in appendix), accepted for
publication in A&
Swift observations of the prompt X-ray emission and afterglow from GRB050126 and GRB050219A
We report on the temporal and spectral characteristics of the early X-ray
emission from the Gamma Ray Bursts GRB050126 and GRB050219A as observed by
Swift. The X-ray light-curves of these 2 bursts both show remarkably steep
early decays (F(t)\propto t^{-3}), breaking to flatter slopes on timescales of
a few hundred seconds. For GRB050126 the burst shows no evidence of spectral
evolution in the 20-150 keV band, and the spectral index of the gamma-ray and
X-ray afterglows are significantly different suggesting a separate origin. By
contrast the BAT spectrum of GRB050219A displays significant spectral
evolution, becoming softer at later times, with Gamma evolving toward the XRT
photon index seen in the early X-ray afterglow phase. For both bursts, the
0.2-10 keV spectral index pre- and post-break in the X-ray decay light-curve
are consistent with no spectral evolution. We suggest that the steep early
decline in the X-ray decay light-curve is either the curvature tail of the
prompt emission; X-ray flaring activity; or external forward shock emission
from a jet with high density regions of small angular size (> Gamma^{-1}). The
late slope we associate with the forward external shock.Comment: 13 pages, 12 figures, accepted for publication in Astronomy &
Astrophysic
Possible Effects of Pair Echoes on Gamma-Ray Burst Afterglow Emission
High-energy emission from gamma-ray bursts (GRBs) is widely expected but had
been sparsely observed until recently when the Fermi satellite was launched. If
>TeV gamma rays are produced in GRBs and can escape from the emission region,
they are attenuated by the cosmic infrared background photons, leading to
regeneration of GeV-TeV secondary photons via inverse-Compton scattering. This
secondary emission can last for a longer time than the duration of GRBs, and it
is called a pair echo. We investigate how this pair echo emission affects
spectra and light curves of high energy afterglows, considering not only prompt
emission but also afterglow as the primary emission. Detection of pair echoes
is possible as long as the intergalactic magnetic field (IGMF) in voids is
weak. We find (1) that the pair echo from the primary afterglow emission can
affect the observed high-energy emission in the afterglow phase after the jet
break, and (2) that the pair echo from the primary prompt emission can also be
relevant, but only when significant energy is emitted in the TeV range,
typically E_{gamma, >0.1 TeV} > (Y/(1+Y)) epsilon_e E_k. Even non-detections of
the pair echoes could place interesting constraints on the strength of IGMF.
The more favorable targets to detect pair echoes may be the "naked" GRBs
without conventional afterglow emission, although energetic naked GRBs would be
rare. If the IGMF is weak enough, it is predicted that the GeV emission extends
to >30-300 s.Comment: 9 pages, 10 figures, accepted for publication in MNRAS, with extended
description
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
An unexpectedly rapid decline in the X-ray afterglow emission of long gamma-ray bursts
Long gamma-ray bursts (GRBs) are commonly accepted to originate in the
explosion of particularly massive stars, which gives rise to a highly
relativistic jet. Internal inhomogeneities in the expanding flow give rise to
internal shock waves that are believed to produce the gamma-rays we see. As the
jet travels further outward into the surrounding circumstellar medium
`external' shocks give rise to the afterglow emission seen in the X-ray,
optical and radio bands. Here we report on the early phases of the X-ray
emission of five GRBs. Their X-ray light curves are characterised by a rapid
fall-off for the first few hundred seconds, followed by a less rapid decline
lasting several hours. This steep decline, together with detailed spectral
properties of two particular bursts, shows that violent shock interactions take
place in the early jet outflows.Comment: 10 pages, 2 tables, 3 figures. Note: This paper has been accepted for
publication in Nature, but is embargoed for discussion in the popular press
until formal publication in Natur
Panchromatic study of GRB 060124: from precursor to afterglow
We present observations of GRB 060124, the first event for which both the
prompt and the afterglow emission could be observed simultaneously and in their
entirety by the three Swift instruments. Indeed, Swift-BAT triggered on a
precursor ~570s before the main burst peak, and this allowed Swift to repoint
the narrow field instruments to the burst position ~350s before the main burst
occurred. GRB 060124 also triggered Konus-Wind, which observed the prompt
emission in a harder gamma-ray band (up to 2MeV). Thanks to these exceptional
circumstances, the temporal and spectral properties of the prompt emission can
be studied in the optical, X-ray and gamma-ray ranges. While the X-ray emission
(0.2-10keV) clearly tracks the gamma-ray burst, the optical component follows a
different pattern, likely indicating a different origin, possibly the onset of
external shocks. The prompt GRB spectrum shows significant spectral evolution,
with both the peak energy and the spectral index varying. As observed in
several long GRBs, significant lags are measured between the hard- and
low-energy components, showing that this behaviour extends over 3 decades in
energy. The GRB peaks are also much broader at soft energies. This is related
to the temporal evolution of the spectrum, and can be accounted for by assuming
that the electron spectral index softened with time. The burst energy
(E_iso~5x10^{53} erg) and average peak energy (E_p~300keV) make GRB 060124
consistent with the Amati relation. The X-ray afterglow is characterized by a
decay which presents a break at t_b~10^5s.Comment: 15 pages, 14 figures, 6 tables. Astronomy and Astrophysics, accepted.
Revised UVOT photometr
Implementation and testing of the first prompt search for gravitational wave transients with electromagnetic counterparts
Aims. A transient astrophysical event observed in both gravitational wave
(GW) and electromagnetic (EM) channels would yield rich scientific rewards. A
first program initiating EM follow-ups to possible transient GW events has been
developed and exercised by the LIGO and Virgo community in association with
several partners. In this paper, we describe and evaluate the methods used to
promptly identify and localize GW event candidates and to request images of
targeted sky locations.
Methods. During two observing periods (Dec 17 2009 to Jan 8 2010 and Sep 2 to
Oct 20 2010), a low-latency analysis pipeline was used to identify GW event
candidates and to reconstruct maps of possible sky locations. A catalog of
nearby galaxies and Milky Way globular clusters was used to select the most
promising sky positions to be imaged, and this directional information was
delivered to EM observatories with time lags of about thirty minutes. A Monte
Carlo simulation has been used to evaluate the low-latency GW pipeline's
ability to reconstruct source positions correctly.
Results. For signals near the detection threshold, our low-latency algorithms
often localized simulated GW burst signals to tens of square degrees, while
neutron star/neutron star inspirals and neutron star/black hole inspirals were
localized to a few hundred square degrees. Localization precision improves for
moderately stronger signals. The correct sky location of signals well above
threshold and originating from nearby galaxies may be observed with ~50% or
better probability with a few pointings of wide-field telescopes.Comment: 17 pages. This version (v2) includes two tables and 1 section not
included in v1. Accepted for publication in Astronomy & Astrophysic
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