3,182 research outputs found
Astrophysical Fluids of Novae: High Resolution Pre-decay X-ray spectrum of V4743 Sagittarii
Eight X-ray observations of V4743 Sgr (2002), observed with Chandra and
XMM-Newton are presented. The nova turned off some time between days 301.9 and
371, and the X-ray flux subsequently decreased from day 301.9 to 526 following
an exponential decline time scale of days. We use the absorption
lines present in the SSS spectrum for diagnostic purposes, and characterize the
physics and the dynamics of the expanding atmosphere during the explosion of
the nova. The information extracted from this first stage is then used as input
for computing full photoionization models of the ejecta in V4743 Sgr. The SSS
spectrum is modeled with a simple black-body and multiplicative Gaussian lines,
which provides us of a general kinematical picture of the system, before it
decays to its faint phase (Ness et al. 2003). In the grating spectra taken
between days 180.4 and 370, we can resolve the line profiles of absorption
lines arising from H-like and He-like C, N, and O, including transitions
involving higher principal quantum numbers. Except for a few interstellar
lines, all lines are significantly blue-shifted, yielding velocities between
1000 and 6000 km/s which implies an ongoing mass loss. It is shown that
significant expansion and mass loss occur during this phase of the explosion,
at a rate . Our measurements show that the efficiency of the amount of
energy used for the motion of the ejecta, defined as the ratio between the
kinetic luminosity and the radiated luminosity , is
of the order of one.Comment: 25 pages, 9 figures. Accepted in book: Recent Advances in Fluid
Dynamics with Environmental Applications, pp.365-39
The SSS phase of RS Ophiuchi observed with Chandra and XMM-Newton I.: Data and preliminary Modeling
The phase of Super-Soft-Source (SSS) emission of the sixth recorded outburst
of the recurrent nova RS Oph was observed twice with Chandra and once with
XMM-Newton. The observations were taken on days 39.7, 54.0, and 66.9 after
outburst. We confirm a 35-sec period on day 54.0 and found that it originates
from the SSS emission and not from the shock. We discus the bound-free
absorption by neutral elements in the line of sight, resonance absorption lines
plus self-absorbed emission line components, collisionally excited emission
lines from the shock, He-like intersystem lines, and spectral changes during an
episode of high-amplitude variability. We find a decrease of the oxygen K-shell
absorption edge that can be explained by photoionization of oxygen. The
absorption component has average velocities of -1286+-267 km/s on day 39.7 and
of -771+-65 km/s on day 66.9. The wavelengths of the emission line components
are consistent with their rest wavelengths as confirmed by measurements of
non-self absorbed He-like intersystem lines. We have evidence that these lines
originate from the shock rather than the outer layers of the outflow and may be
photoexcited in addition to collisional excitations. We found collisionally
excited emission lines that are fading at wavelengths shorter than 15A that
originate from the radiatively cooling shock. On day 39.5 we find a systematic
blue shift of -526+-114 km/s from these lines. We found anomalous He-like f/i
ratios which indicates either high densities or significant UV radiation near
the plasma where the emission lines are formed. During the phase of strong
variability the spectral hardness light curve overlies the total light curve
when shifted by 1000sec. This can be explained by photoionization of neutral
oxygen in the line of sight if the densities of order 10^{10}-10^{11} cm^{-3}.Comment: 16 pages, 10 figures, 4 tables. Accepted by ApJ; v2: Co-author
Woodward adde
Swift X-ray Observations of Classical Novae
The new gamma-ray burst mission Swift has obtained pointed observations of
several classical novae in outburst. We analyzed all the observations of
classical novae from the Swift archive up to 30 June, 2006. We analyzed usable
observations of 12 classical novae and found 4 non-detections, 3 weak sources
and 5 strong sources. This includes detections of 2 novae exhibiting spectra
resembling those of Super Soft X-ray binary Source spectra (SSS) implying
ongoing nuclear burning on the white dwarf surface. With these new Swift data,
we add to the growing statistics of the X-ray duration and characteristics of
classical novae.Comment: Accepted for ApJ; this version contains additional material: 18
pages, 16 figure
YOUNG STARS IN AN OLD BULGE: A NATURAL OUTCOME OF INTERNAL EVOLUTION IN THE MILKY WAY
The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have reported that they span a range of ages, emphasizing that the bulge may not be a monolithic structure. In this Letter we demonstrate that the presence of young stars that are located predominantly nearer to the plane is expected for a bulge that has formed from the disk via dynamical instabilities. Using an N-body+ smoothed particle hydrodynamics simulation of a disk galaxy forming out of gas cooling inside a dark matter halo and forming stars, we find a qualitative agreement between our model and the observations of younger metal-rich stars in the bulge. We are also able to partially resolve the apparent contradiction in the literature between results that argue for a purely old bulge population and those that show a population comprised of a range in ages; the key is where to look
Quantum inelastic conductance through molecular wires
We calculate non-perturbatively the inelastic effects on the conductance
through a conjugated molecular wire-metal heterojunction, including realistic
electron-phonon coupling. We show that at sub-band-gap energies the current is
dominated by quantum coherent transport of virtual polarons through the
molecule. In this regime, the tunneling current is strongly increased relative
to the case of elastic scattering. It is essential to describe the full quantum
coherence of the polaron formation and transport in order to obtain correct
physics. Our results are generally applicable to one-dimensional atomic or
molecular wires.Comment: 4 pages, 4 figures, accepted for publication in Physical Review
Letter
Swift observations of the 2006 outburst of the recurrent nova RS Ophiuchi: I. Early X-ray emission from the shocked ejecta and red giant wind
RS Ophiuchi began its latest outburst on 2006 February 12. Previous outbursts
have indicated that high velocity ejecta interact with a pre-existing red giant
wind, setting up shock systems analogous to those seen in Supernova Remnants.
However, in the previous outburst in 1985, X-ray observations did not commence
until 55 days after the initial explosion. Here we report on Swift observations
covering the first month of the 2006 outburst with the Burst Alert (BAT) and
X-ray Telescope (XRT) instruments. RS Oph was clearly detected in the BAT 14-25
keV band from t=0 to days. XRT observationsfrom 0.3-10 keV, started at
3.17 days after outburst. The rapidly evolving XRT spectra clearly show the
presence of both line and continuum emission which can be fitted by thermal
emission from hot gas whose characteristic temperature, overlying absorbing
column, , and resulting unabsorbed total flux decline monotonically
after the first few days. Derived shock velocities are in good agreement with
those found from observations at other wavelengths. Similarly, is in
accord with that expected from the red giant wind ahead of the forward shock.
We confirm the basic models of the 1985 outburst and conclude that standard
Phase I remnant evolution terminated by days and the remnant then
rapidly evolved to display behaviour characteristic of Phase III. Around t=26
days however, a new, luminous and highly variable soft X-ray source began to
appear whose origin will be explored in a subsequent paper.Comment: 20 pages, 4 figures (2 updated), accepted by Ap
Soliton effects in dangling-bond wires on Si(001)
Dangling bond wires on Si(001) are prototypical one dimensional wires, which
are expected to show polaronic and solitonic effects. We present electronic
structure calculations, using the tight binding model, of solitons in
dangling-bond wires, and demonstrate that these defects are stable in
even-length wires, although approximately 0.1 eV higher in energy than a
perfect wire. We also note that in contrast to conjugated polymer systems,
there are two types of soliton and that the type of soliton has strong effects
on the energetics of the bandgap edges, with formation of intra-gap states
between 0.1 eV and 0.2 eV from the band edges. These intra-gap states are
localised on the atoms comprising the soliton.Comment: 6 pages, 3 figures, 3 tables, submitted to Phys. Rev.
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