9,164 research outputs found
GRB Fireball Physics: Prompt and Early Emission
We review the fireball shock model of gamma-ray burst prompt and early
afterglow emission in light of rapid follow-up measurements made and enabled by
the multi-wavelength Swift satellite. These observations are leading to a
reappraisal and expansion of the previous standard view of the GRB and its
fireball. New information on the behavior of the burst and afterglow on minutes
to hour timescales has led, among other results, to the discovery and follow-up
of short GRB afterglows, the opening up of the z>6 redshift range, and the
first prompt multi-wavelength observations of a long GRB-supernova. We discuss
the salient observational results and some associated theoretical issues.Comment: 23 pages. Published in the New Journal of Physics Focus Issue, "Focus
on Gamma-Ray Bursts in the Swift Era" (Eds. D. H. Hartmann, C. D. Dermer & J.
Greiner). V2: Minor change
Observation of Scaling Violations in Scaled Momentum Distributions at HERA
Charged particle production has been measured in deep inelastic scattering
(DIS) events over a large range of and using the ZEUS detector. The
evolution of the scaled momentum, , with in the range 10 to 1280
, has been investigated in the current fragmentation region of the Breit
frame. The results show clear evidence, in a single experiment, for scaling
violations in scaled momenta as a function of .Comment: 21 pages including 4 figures, to be published in Physics Letters B.
Two references adde
Measurement of the open-charm contribution to the diffractive proton structure function
Production of D*+/-(2010) mesons in diffractive deep inelastic scattering has
been measured with the ZEUS detector at HERA using an integrated luminosity of
82 pb^{-1}. Diffractive events were identified by the presence of a large
rapidity gap in the final state. Differential cross sections have been measured
in the kinematic region 1.5 < Q^2 < 200 GeV^2, 0.02 < y < 0.7, x_{IP} < 0.035,
beta 1.5 GeV and |\eta(D*+/-)| < 1.5. The measured cross
sections are compared to theoretical predictions. The results are presented in
terms of the open-charm contribution to the diffractive proton structure
function. The data demonstrate a strong sensitivity to the diffractive parton
densities.Comment: 35 pages, 11 figures, 6 table
Magnetoresistance, specific heat and magnetocaloric effect of equiatomic rare-earth transition-metal magnesium compounds
We present a study of the magnetoresistance, the specific heat and the
magnetocaloric effect of equiatomic Mg intermetallics with , Eu, Gd, Yb and , Au and of GdAuIn. Depending on the
composition these compounds are paramagnetic (, Yb) or they
order either ferro- or antiferromagnetically with transition temperatures
ranging from about 13 to 81 K. All of them are metallic, but the resistivity
varies over 3 orders of magnitude. The magnetic order causes a strong decrease
of the resistivity and around the ordering temperature we find pronounced
magnetoresistance effects. The magnetic ordering also leads to well-defined
anomalies in the specific heat. An analysis of the entropy change leads to the
conclusions that generally the magnetic transition can be described by an
ordering of localized moments arising from the half-filled
shells of Eu or Gd. However, for GdAgMg we find clear evidence
for two phase transitions indicating that the magnetic ordering sets in
partially below about 125 K and is completed via an almost first-order
transition at 39 K. The magnetocaloric effect is weak for the antiferromagnets
and rather pronounced for the ferromagnets for low magnetic fields around the
zero-field Curie temperature.Comment: 12 pages, 7 figures include
Thermal expansion of the magnetically ordering intermetallics RTMg (R = Eu, Gd and T = Ag, Au)
We report measurements of the thermal expansion for two Eu- and two
Gd-based intermetallics which exhibit ferro- or antiferromagnetic phase
transitions. These materials show sharp positive (EuAgMg and GdAuMg) and
negative (EuAuMg and GdAgMg) peaks in the temperature dependence of the thermal
expansion coefficient which become smeared and/or displaced in an
external magnetic field. Together with specific heat data we determine the
initial pressure dependences of the transition temperatures at ambient pressure
using the Ehrenfest or Clausius-Clapeyron relation. We find large pressure
dependences indicating strong spin-phonon coupling, in particular for GdAgMg
and EuAuMg where a quantum phase transition might be reached at moderate
pressures of a few GPa.Comment: 6 pages, 3 figure
Isolated tau leptons in events with large missing transverse momentum at HERA
A search for events containing isolated tau leptons and large missing
transverse momentum, not originating from the tau decay, has been performed
with the ZEUS detector at the electron-proton collider HERA, using 130 pb^-1 of
integrated luminosity. A search was made for isolated tracks coming from
hadronic tau decays. Observables based on the internal jet structure were
exploited to discriminate between tau decays and quark- or gluon-induced jets.
Three tau candidates were found, while 0.40 +0.12 -0.13 were expected from
Standard Model processes, such as charged current deep inelastic scattering and
single W-boson production. To search for heavy-particle decays, a more
restrictive selection was applied to isolate tau leptons produced together with
a hadronic final state with high transverse momentum. Two candidate events
survive, while 0.20 +-0.05 events are expected from Standard Model processes.Comment: 28 pages, 4 figures, 3 tables, accepted by Phys. Lett. B. Updated
with minor changes to the text requested by the journal refere
Massive Stars and their Supernovae
Massive stars and their supernovae are prominent sources of radioactive
isotopes, the observations of which thus can help to improve our astrophysical
models of those. Our understanding of stellar evolution and the final explosive
endpoints such as supernovae or hypernovae or gamma-ray bursts relies on the
combination of magneto-hydrodynamics, energy generation due to nuclear
reactions accompanying composition changes, radiation transport, and
thermodynamic properties (such as the equation of state of stellar matter).
Nuclear energy production includes all nuclear reactions triggered during
stellar evolution and explosive end stages, also among unstable isotopes
produced on the way. Radiation transport covers atomic physics (e.g. opacities)
for photon transport, but also nuclear physics and neutrino nucleon/nucleus
interactions in late phases and core collapse. Here we want to focus on the
astrophysical aspects, i.e. a description of the evolution of massive stars and
their endpoints, with a special emphasis on the composition of their ejecta (in
form of stellar winds during the evolution or of explosive ejecta). Low and
intermediate mass stars end their evolution as a white dwarf with an unburned C
and O composition. Massive stars evolve beyond this point and experience all
stellar burning stages from H over He, C, Ne, O and Si-burning up to core
collapse and explosive endstages. In this chapter we discuss the
nucleosynthesis processes involved and the production of radioactive nuclei in
more detail.Comment: 79 pages; Chapter of "Astronomy with Radioactivities", a book in
Springer's 'lecture notes in physics series, Vol. 812, Eds. Roland Diehl,
Dieter H. Hartmann, and Nikos Prantzos, to appear in summer 201
A Systematic Analysis of Supernova Light in Gamma-Ray Burst Afterglows
We systematically reanalyzed all Gamma-Ray Burst (GRB) afterglow data
published through the end of 2002, in an attempt to detect the predicted
supernova light component and to gain statistical insight on its
phenomenological properties. We fit the observed photometric light curves as
the sum of an afterglow, an underlying host galaxy, and a supernova component.
The latter is modeled using published multi-color light curves of SN 1998bw as
a template. The total sample of afterglows with established redshifts contains
21 bursts (GRB 970228 - GRB 021211). For nine of these GRBs a weak supernova
excess (scaled to SN 1998bw) was found, what makes this to one of the first
samples of high-z core collapse supernovae. Among this sample are all bursts
with redshifts less than ~0.7. These results strongly support the notion that
in fact all afterglows of long-duration GRBs contain light from an associated
supernova. A statistics of the physical parameters of these GRB-supernovae
shows that SN 1998bw was at the bright end of its class, while it was not
special with respect to its light curve shape. Finally, we have searched for a
potential correlation of the supernova luminosities with the properties of the
corresponding bursts and optical afterglows, but we have not found such a
relation.Comment: 25 pages, 7 figures, accepted by ApJ; revised, shortened and updated
compared to version 1; Title slightly changed; all figures showing individual
afterglow light curves removed, as advised by the referee; conclusions
unchange
Nucleosynthesis and Gamma-Ray Line Spectroscopy with INTEGRAL
Cosmic nucleosynthesis co-produces unstable isotopes, which emit
characteristic gamma-ray emission lines upon their radioactive decay that can
be measured with SPI on INTEGRAL. High spectral resolution allows to derive
velocity constraints on nucleosynthesis ejecta down to ~100 km/s. Core-collapse
supernovae apparently do not always produce significant amounts of 44Ti, as in
the Galaxy fewer sources than expected from the supernova rate have been found.
INTEGRAL's 44Ti data on the well-observed Cas A and SN1987A events are evidence
that non-spherical explosions and 44Ti production may be correlated.
Characteristic gamma-ray lines from radioactive decays of long-lived 26Al and
60Fe isotopes have been exploited to obtain information on the structure and
dynamics of massive stars in their late evolution and supernovae, as their
yields are sensitive to those details. The extended INTEGRAL mission
establishes a database of sufficiently-deep observations of several specific
regions of massive star groups, such as Cygnus, Carina, and Sco-Cen. In the
inner Galaxy, 26Al nucleosynthesis gamma-rays help to unravel the Galaxy's
structure and the role of a central bar, as the kinematically-shifted 26Al
gamma-ray line energy records the longitude-velocity behavior of hot
interstellar gas. Thus, INTEGRAL has consolidated the feasibility of
constraining cosmic nucleosynthesis through gamma-ray line observations. Due to
its extended mission INTEGRAL maintains its chance to also see rare
sufficiently-nearby events, such as a nova to provide first nova
nucleosynthesis measurements of 7Be and 22Na production.Comment: Conference "The extreme and variable high-energy sky", Italy Sep
2011. 10 pages, 4 figure
Using longitudinal survival probabilities to test field vigour estimates in sugar maple (Acer saccharum Marsh.)
Tree mortality is a major force driving forest dynamics. To foresters, however, tree mortality is often considered a loss in productivity. To reduce tree mortality, silvicultural systems, such as selection cuts, aim at removing trees that are more likely to die. In order to identify trees with higher risks of mortality, field classifications are employed that assess vigour based on external characteristics of trees. We used a novel longitudinal approach for estimating survival probabilities based on ring-width measurements, initially developed by Bigler and Bugmann [Bigler, C., Bugmann, H., 2004. Predicting the time of tree death using dendrochronological data. Ecol. Appl. 14 (3), 902-914], to parameterize a survival probability model for sugar maple (Acer saccharum Marsh.) and to test whether field-assessed tree vigour classes are corroborated by survival probabilities determined from radial growth history. Data from 56 dead and 321 live sugar maples were collected in stands in western Quebec (Canada) that had undergone a selection cut ≈10 years prior to sampling. Our results showed that tree vigour established from external defects and pathological symptoms, using the classification of Boulet [Boulet, B., 2005. D
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