299 research outputs found
Interpretation of UV Absorption Lines in SN1006
We present a theoretical interpretation of the broad silicon and iron UV
absorption features observed with the Hubble Space Telescope in the spectrum of
the Schweizer-Middleditch star behind the remnant of Supernova 1006. These
features are caused by supernova ejecta in SN1006. We propose that the
redshifted SiII2 1260 A feature consists of both unshocked and shocked SiII.
The sharp red edge of the line at 7070 km/s indicates the position of the
reverse shock, while its Gaussian blue edge reveals shocked Si with a mean
velocity of 5050 km/s and a dispersion of 1240 km/s, implying a reverse shock
velocity of 2860 km/s. The measured velocities satisfy the energy jump
condition for a strong shock, provided that all the shock energy goes into
ions, with little or no collisionless heating of electrons. The line profiles
of the SiIII and SiIV absorption features indicate that they arise mostly from
shocked Si. The total mass of shocked and unshocked Si inferred from the SiII,
SiIII and SiIV profiles is M_Si = 0.25 \pm 0.01 Msun on the assumption of
spherical symmetry. Unshocked Si extends upwards from 5600 km/s. Although there
appears to be some Fe mixed with the Si at lower velocities < 7070 km/s, the
absence of FeII absorption with the same profile as the shocked SiII suggests
little Fe mixed with Si at higher (before being shocked) velocities. The column
density of shocked SiII is close to that expected for SiII undergoing steady
state collisional ionization behind the reverse shock, provided that the
electron to SiII ratio is low, from which we infer that most of the shocked Si
is likely to be of a fairly high degree of purity, unmixed with other elements.
We propose that the ambient interstellar density on the far side of SN1006 is
anomalously low compared to the density around the rest of the remnant. ThisComment: 24 pages, with 8 figures included. Accepted for publication in the
Astrophysical Journa
1SXPS: A deep Swift X-ray Telescope point source catalog with light curves and spectra
We present the 1SXPS (Swift-XRT Point Source) catalog of 151,524 X-ray
point-sources detected by the Swift-XRT in 8 years of operation. The catalog
covers 1905 square degrees distributed approximately uniformly on the sky. We
analyze the data in two ways. First we consider all observations individually,
for which we have a typical sensitivity of ~3e-13 erg/cm2/s (0.3--10 keV). Then
we co-add all data covering the same location on the sky: these images have a
typical sensitivity of ~9e-14 erg/cm2/s (0.3--10 keV). Our sky coverage is
nearly 2.5 times that of 3XMM-DR4, although the catalog is a factor of ~1.5
less sensitive. The median position error is 5.5" (90% confidence), including
systematics. Our source detection method improves on that used in previous XRT
catalogs and we report >68,000 new X-ray sources. The goals and observing
strategy of the Swift satellite allow us to probe source variability on
multiple timescales, and we find ~30,000 variable objects in our catalog. For
every source we give positions, fluxes, time series (in four energy bands and
two hardness ratios), estimates of the spectral properties, spectra and
spectral fits for the brightest sources, and variability probabilities in
multiple energy bands and timescales.Comment: 27 pages, 19 figures; accepted for publication in ApJS. The
accompanying website, http://www.swift.ac.uk/1SXPS is live; the Vizier entry
should be available shortl
The Unexpected Role of Evolving Longitudinal Electric Fields in Generating Energetic Electrons in Relativistically Transparent Plasmas
Superponderomotive-energy electrons are observed experimentally from the
interaction of an intense laser pulse with a relativistically transparent
target. For a relativistically transparent target, kinetic modeling shows that
the generation of energetic electrons is dominated by energy transfer within
the main, classically overdense, plasma volume. The laser pulse produces a
narrowing, funnel-like channel inside the plasma volume that generates a field
structure responsible for the electron heating. The field structure combines a
slowly evolving azimuthal magnetic field, generated by a strong laser-driven
longitudinal electron current, and, unexpectedly, a strong propagating
longitudinal electric field, generated by reflections off the walls of the
funnel-like channel. The magnetic field assists electron heating by the
transverse electric field of the laser pulse through deflections, whereas the
longitudinal electric field directly accelerates the electrons in the forward
direction. The longitudinal electric field produced by reflections is 30 times
stronger than that in the incoming laser beam and the resulting direct laser
acceleration contributes roughly one third of the energy transferred by the
transverse electric field of the laser pulse to electrons of the
super-ponderomotive tail
Longitudinal Ion Acceleration from High-Intensity Laser Interactions with Underdense Plasma
Longitudinal ion acceleration from high-intensity (I ~ 10^20 Wcm^-2) laser
interactions with helium gas jet targets (n_e ~ 0.04 n_c) have been observed.
The ion beam has a maximum energy for He^2+ of approximately 40 MeV and was
directional along the laser propagation path, with the highest energy ions
being collimated to a cone of less than 10 degrees. 2D particle-in-cell
simulations have been used to investigate the acceleration mechanism. The time
varying magnetic field associated with the fast electron current provides a
contribution to the accelerating electric field as well as providing a
collimating field for the ions. A strong correlation between the plasma density
and the ion acceleration was found. A short plasma scale-length at the vacuum
interface was observed to be beneficial for the maximum ion energies, but the
collimation appears to be improved with longer scale-lengths due to enhanced
magnetic fields in the ramp acceleration region.Comment: 18 pages, 6 figure
Dynamic Control of Laser Produced Proton Beams
The emission characteristics of intense laser driven protons are controlled
using ultra-strong (of the order of 10^9 V/m) electrostatic fields varying on a
few ps timescale. The field structures are achieved by exploiting the high
potential of the target (reaching multi-MV during the laser interaction).
Suitably shaped targets result in a reduction in the proton beam divergence,
and hence an increase in proton flux while preserving the high beam quality.
The peak focusing power and its temporal variation are shown to depend on the
target characteristics, allowing for the collimation of the inherently highly
divergent beam and the design of achromatic electrostatic lenses.Comment: 9 Pages, 5 figure
Complicated variations of early optical afterglow of GRB 090726
We report on a detection of an early rising phase of optical afterglow (OA)
of a long GRB 090726. We resolve a complicated profile of the optical light
curve. We also investigate the relation of the optical and X-ray emission of
this event. We make use of the optical photometry of this OA obtained by the
0.5 m telescope of AI AS CR, supplemented by the data obtained by other
observers, and the X-ray Swift/XRT data.
The optical emission peaked at ~ 17.5 mag (R) at t-T0 ~ 500 s. We find a
complex profile of the light curve during the early phase of this OA: an
approximately power-law rise, a rapid transition to a plateau, a weak flare
superimposed on the center of this plateau, and a slowly steepening early
decline followed by a power-law decay. We discuss several possibilities to
explain the short flare on the flat top of the optical light curve at t-T0 ~
500 s; activity of the central engine is favored although reverse shock cannot
be ruled out. We show that power-law outflow with Theta_obs/Theta_c > 2.5 is
the best case for OA of GRB 090726. The initial Lorentz factor is Gamma_0 ~
230-530 in case of propagation of the blast wave in a homogeneous medium, while
propagation of this wave in a wind environment gives Gamma_0 ~ 80-300. The
value of Gamma_0 in GRB 090726 thus falls into the lower half of the range
observed in GRBs and it may even lie on the lower end. We also show that both
the optical and X-ray emission decayed simultaneously and that the spectral
profile from X-ray to the optical band did not vary. This OA belongs to the
least luminous ones in the phase of its power-law decay corresponding to that
observed for the ensemble of OAs of long GRBs.Comment: 5 pages, 5 figures, accepted to A&
The puzzling temporally variable optical and X-ray afterglow of GRB 101024A
Aim: To present the optical observations of the afterglow of GRB 101024A and
to try to reconcile these observations with the X-ray afterglow data of GRB
101024A using current afterglow models
Method: We employ early optical observations using the Zadko Telescope
combined with X-ray data and compare with the reverse shock/forward shock
model.
Results: The early optical light curve reveals a very unusual steep decay
index of alpha~5. This is followed by a flattening and possibly a plateau phase
coincident with a similar feature in the X-ray. We discuss these observations
in the framework of the standard reverse shock/forward shock model and energy
injection.We note that the plateau phase might also be the signature of the
formation of a new magnetar.Comment: 5 pages, 2 figures. Accepted for publication in Astronomy and
Astrophysic
Early multi-wavelength emission from Gamma-ray Bursts: from Gamma-ray to X-ray
The study of the early high-energy emission from both long and short
Gamma-ray bursts has been revolutionized by the Swift mission. The rapid
response of Swift shows that the non-thermal X-ray emission transitions
smoothly from the prompt phase into a decaying phase whatever the details of
the light curve. The decay is often categorized by a steep-to-shallow
transition suggesting that the prompt emission and the afterglow are two
distinct emission components. In those GRBs with an initially steeply-decaying
X-ray light curve we are probably seeing off-axis emission due to termination
of intense central engine activity. This phase is usually followed, within the
first hour, by a shallow decay, giving the appearance of a late emission hump.
The late emission hump can last for up to a day, and hence, although faint, is
energetically very significant. The energy emitted during the late emission
hump is very likely due to the forward shock being constantly refreshed by
either late central engine activity or less relativistic material emitted
during the prompt phase. In other GRBs the early X-ray emission decays
gradually following the prompt emission with no evidence for early temporal
breaks, and in these bursts the emission may be dominated by classical
afterglow emission from the external shock as the relativistic jet is slowed by
interaction with the surrounding circum-burst medium. At least half of the GRBs
observed by Swift also show erratic X-ray flaring behaviour, usually within the
first few hours. The properties of the X-ray flares suggest that they are due
to central engine activity. Overall, the observed wide variety of early
high-energy phenomena pose a major challenge to GRB models.Comment: Accepted for publication in the New Journal of Physics focus issue on
Gamma Ray Burst
The First Survey of X-ray Flares from Gamma Ray Bursts Observed by Swift: Spectral Properties and Energetics
Observations of gamma ray bursts (GRBs) with Swift produced the initially
surprising result that many bursts have large X-ray flares superimposed on the
underlying afterglow. The flares were sometimes intense, had rapid rise and
decay phases, and occurred late relative to the ``prompt'' phase. Some
remarkable flares are observed with fluence comparable to the prompt GRB
fluence. Many GRBs have several flares, which are sometimes overlapping. Short,
intense, repetitive, and late flaring can be most easily understood within the
context of the standard fireball model with the internal engine that powers the
prompt GRB emission in an active state at late times. However, other models for
flares have been proposed. Flare origin can be investigated by comparing the
flare spectra to that of the afterglow and the initial prompt emission. In this
work, we have analyzed all significant X-ray flares from the first 110 GRBs
observed by Swift. From this sample 33 GRBs were found to have significant
X-ray flares, with 77 flares that were detected above the 3 level. In
addition to temporal analysis presented in a companion paper, a variety of
spectral models have been fit to each flare. In some cases, we find that the
spectral fits favor a Band function model, which is more akin to the prompt
emission than to that of an afterglow. We find that the average fluence of the
flares is 2.4e-7 erg/cm^2/s in the 0.2-10 keV energy band, which is
approximately a factor of ten below the average prompt GRB fluence. These
results, when combined with those presented in the companion paper on temporal
properties of flares, supports the hypothesis that most X-ray flares are
late-time activity of the internal engine that spawned the initial GRB; not an
afterglow related effect.Comment: accepted by ApJ; 39 pages with 14 figures and 7 table
High-power, kilojoule laser interactions with near-critical density plasma
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98754/1/PhysPlasmas_18_056706.pd
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