310 research outputs found
Radio Frequency Models of Novae in eruption. I. The Free-Free Process in Bipolar Morphologies
Observations of novae at radio frequencies provide us with a measure of the
total ejected mass, density profile and kinetic energy of a nova eruption. The
radio emission is typically well characterized by the free-free emission
process. Most models to date have assumed spherical symmetry for the eruption,
although it has been known for as long as there have been radio observations of
these systems, that spherical eruptions are to simplistic a geometry. In this
paper, we build bipolar models of the nova eruption, assuming the free-free
process, and show the effects of varying different parameters on the radio
light curves. The parameters considered include the ratio of the minor- to
major-axis, the inclination angle and shell thickness (further parameters are
provided in the appendix). We also show the uncertainty introduced when fitting
spherical model synthetic light curves to bipolar model synthetic light curves.
We find that the optically thick phase rises with the same power law () for both the spherical and bipolar models. In the bipolar case
there is a "plateau" phase -- depending on the thickness of the shell as well
as the ratio of the minor- to major-axis -- before the final decline, that
follows the same power law () as in the spherical case.
Finally, fitting spherical models to the bipolar model synthetic light curves
requires, in the worst case scenario, doubling the ejected mass, more than
halving the electron temperature and reducing the shell thickness by nearly a
factor of 10. This implies that in some systems we have been over predicting
the ejected masses and under predicting the electron temperature of the ejecta.Comment: 9 pages, 6 figures, accepted for publication in ApJ, accompanying
movie to figure 3 available at
http://www.ast.uct.ac.za/~valerio/papers/radioI
The X-Ray Evolution of the Symbiotic Star V 407 Cygni During Its 2010 Outburst
We present a summary of Swift and Suzaku X-ray observations of the 2010 nova outburst of the symbiotic star, V 407 Cyg. The Suzaku spectrum obtained on day 30 indicates the presence of the supersoft component from the white dwarf surface, as well as optically thin component from the shock between the nova ejecta and the Mira wind. The Swift observations then allow us to track the evolution of both components from day 4 to day 150. Most notable is the sudden brightening of the optically think component around day 20. We identify this as the time when the blast wave reached the immediate vicinity of the photosphere of the Mira. We have developed a simplified model of the blast wave-wind interaction that can reproduce the gross features of the X-ray evolution of V407 Cyg. If the model is correct, the binary separation is likely to be large and the mass loss rate of the Mira is likely to be relatively low
The Distance to Nova V959 Mon from VLA Imaging
Determining reliable distances to classical novae is a challenging but
crucial step in deriving their ejected masses and explosion energetics. Here we
combine radio expansion measurements from the Karl G. Jansky Very Large Array
with velocities derived from optical spectra to estimate an expansion parallax
for nova V959 Mon, the first nova discovered through its gamma-ray emission. We
spatially resolve the nova at frequencies of 4.5-36.5 GHz in nine different
imaging epochs. The first five epochs cover the expansion of the ejecta from
2012 October to 2013 January, while the final four epochs span 2014 February to
2014 May. These observations correspond to days 126 through 199 and days 615
through 703 after the first detection of the nova. The images clearly show a
non-spherical ejecta geometry. Utilizing ejecta velocities derived from 3D
modelling of optical spectroscopy, the radio expansion implies a distance
between 0.9 +/- 0.2 and 2.2 +/- 0.4 kpc, with a most probable distance of 1.4
+/- 0.4 kpc. This distance implies a gamma-ray luminosity much less than the
prototype gamma-ray-detected nova, V407 Cyg, possibly due to the lack of a red
giant companion in the V959 Mon system. V959 Mon also has a much lower
gamma-ray luminosity than other classical novae detected in gamma-rays to date,
indicating a range of at least a factor of 10 in the gamma-ray luminosities for
these explosions.Comment: 11 pages, 8 figures, 3 tables, submitted to ApJ 2015-01-21, under
revie
The Peculiar Multi-Wavelength Evolution Of V1535 Sco
We present multi-wavelength observations of the unusual nova V1535 Sco
throughout its outburst in 2015. Early radio observations were consistent with
synchrotron emission, and early X-ray observations revealed the presence of
high-energy (>1 keV) photons. These indicated that strong shocks were present
during the first ~2 weeks of the nova's evolution. The radio spectral energy
distribution was consistent with thermal emission from week 2 to week 6.
Starting in week 7, the radio emission again showed evidence of synchrotron
emission and there was an increase in X-ray emission, indicating a second shock
event. The optical spectra show evidence for at least two separate outflows,
with the faster outflow possibly having a bipolar morphology. The optical and
near infrared light curves and the X-ray measurements of the hydrogen column
density indicated that the companion star is likely a K giant.Comment: 20 pages, 13 figures, under review at ApJ, updated to match the most
recent version submitted to the refere
Interface Mixing in Fe/Si Multilayers Observed by the In Situ Conductance Measurements
In this contribution the in situ conductance vs. deposition time dependences of Fe/Si multilayers are analysed. The plot of resistance multiplied by the square of the thickness as a function of iron thickness shows that during the iron deposition initially amorphous-like Fe-Si mixture is formed, next the mixture crystallises, and finally bcc-Fe phase appears. The interface mixing is also manifested by the reduction of the total multilayer thickness measured by small angle X-ray diffraction
Tracking the Enigmatic Globular Cluster Ultracompact X-ray Binary X1850--087: Extreme Radio Variability in the Hard State
The conditions under which accreting neutron stars launch radio-emitting jets
and/or outflows are still poorly understood. The ultracompact X-ray binary
X1850--087, located in the globular cluster NGC 6712, is a persistent
atoll-type X-ray source that has previously shown unusual radio continuum
variability. Here we present the results of a pilot radio monitoring program of
X1850--087 undertaken with the Karl G. Jansky Very Large Array, with
simultaneous or quasi-simultaneous Swift/XRT data obtained at each epoch. The
binary is clearly detected in the radio in two of the six new epochs. When
combined with previous data, these results suggest that X1850--087 shows radio
emission at a slightly elevated hard state X-ray luminosity of L_X > 2x10^36
erg/s, but no radio emission in its baseline hard state L_X ~10^36 erg/s. No
clear X-ray spectral changes are associated with this factor of > 10 radio
variability. At all detected epochs X1850--087 has a flat-to-inverted radio
spectral index, more consistent with the partially absorbed optically thick
synchrotron of a compact jet rather than the evolving optically thick to thin
emission associated with transient expanding synchrotron-emitting ejecta. If
the radio emission in X1850--087 is indeed due to a compact jet, then it is
plausibly being launched and quenched in the hard state on timescales as short
as a few days. Future radio monitoring of X1850--087 could help elucidate the
conditions under which compact jets are produced around hard state accreting
neutron stars.Comment: 9 pages, 2 figures, 2 tables, accepted for publication in Ap
Obscuration of Supersoft X-ray Sources by Circumbinary Material - A Way to Hide Type Ia Supernova Progenitors?
The progenitors of supernovae (SNe) type Ia are usually assumed to be either
a single white dwarf (WD) accreting from a non-degenerate companion (the SD
channel) or the result of two merging WDs (DD channel). However, no consensus
currently exists as to which progenitor scenario is the correct one, or whether
the observed SN Ia rate is produced by a combination of both channels. Unlike a
DD progenitor a SD progenitor is expected to emit supersoft X-rays for a
prolonged period of time (~1 Myr) as a result of the burning of accreted matter
on the surface of the WD. An argument against the SD channel as a significant
producer of SNe type Ia has been the lack of observed supersoft X-ray sources
(SSS) and the lower-than-expected integrated soft X-ray flux from elliptical
galaxies.
We wish to determine if it is possible to obscure the supersoft X-ray
emission from a nuclear burning white dwarf in an accreting single degenerate
binary system. In case of obscured systems we wish to determine their general
observational characteristics.
We examine the emergent X-ray emission from a canonical SSS system surrounded
by a spherically symmetric configuration of material, assuming a black body
spectrum with T_BB=50 eV and L=10^38 erg/s. The circumbinary material is
assumed to be of solar chemical abundances, and we leave the mechanism behind
the mass loss into the circumbinary region unspecified.
If steadily accreting, nuclear burning WDs are canonical SSS our analysis
suggests that they can be obscured by relatively modest circumbinary mass loss
rates. This may explain the discrepancy of SSS compared to the SN Ia rate
inferred from observations if the SD progenitor scenario contributes
significantly to the SN Ia rate. Recycled emissions from obscured systems may
be visible in other wavebands than X-rays. It may also explain the lack of
observed SSS in symbiotic binary systems.Comment: 10 pages, 4 figures, accepted A&
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