131 research outputs found
Models for the soft X-ray emission of post-outburst classical novae
A hydrostatic and stationary white dwarf envelope model has been developed
for the study of the post-outburst phases of classical novae and their soft
X-ray emission. Several white dwarf masses and chemical compositions typical
for classical novae have been considered. The results show that the luminosity,
maximum effective temperature and envelope masses depend on the white dwarf
mass and on the chemical composition. Envelope masses for which equilibrium
solutions exist are pretty small ~10^{-7}-10^{-6} Msun, thus leading to a short
duration of the soft X-ray emitting phase of classical novae, in agreement with
most of the observations. The models presented provide a useful tool for the
determination of the white dwarf properties from observable parameters in the
X-ray range.Comment: 15 pages, 11 figures, 2 tables, accepted for publication in Astronomy
& Astrophysic
V5116 Sgr, an Eclipsing Supersoft Post-Outburst Nova?
V5116 Sgr (Nova Sgr 2005 No. 2), discovered on 2005 July 4, was observed with
XMM-Newton in March 2007, 20 months after the optical outburst. The X-ray
spectrum shows that the nova had evolved to a pure supersoft X-ray source, with
no significant emission at energies above 1 keV. The X-ray light-curve shows
abrupt decreases and increases of the flux by a factor ~8. It is consistent
with a periodicity of 2.97 h, the orbital period suggested by Dobrotka et al.
(2007), although the observation lasted just a little more than a whole period.
We estimate the distance to V5116 Sgr to be 11+/-3 kpc. A simple blackbody
model does not fit correctly the EPIC spectra, with reduced chi^2>4. In
contrast, ONe rich white dwarf atmosphere models provide a good fit, with
nH=1.3(+/-0.1)e21 cm^-2, T=6.1(+/-0.1)e5 K, and L=3.9(+/-0.8)e37(D/10kpc)^2
erg/s (during the high-flux periods). This is consistent with residual hydrogen
burning in the white dwarf envelope. The white dwarf atmosphere temperature is
the same both in the low and the high flux periods, ruling out an intrinsic
variation of the X-ray source as the origin of the flux changes. We speculate
that the X-ray light-curve may result from a partial coverage by an asymmetric
accretion disk in a high inclination system.Comment: 2 figures, emulateapj, to appear in ApJ
Looking for black-holes in X-ray binaries with XMM-Newton: XTE J1817-330 and XTE J1856+053
The X-ray binary XTE J1817-330 was discovered in outburst on 26 January 2006
with RXTE/ASM. One year later, another X-ray transient discovered in 1996, XTE
J1856+053, was detected by RXTE during a new outburst on 28 February 2007. We
triggered XMM-Newton target of opportunity observations on these two objects to
constrain their parameters and search for a stellar black holes. We summarize
the properties of these two X-ray transients and show that the soft X-ray
spectra indicate indeed the presence of an accreting stellar black hole in each
of the two systems.Comment: to appear in the proceedings of the Second Kolkata Conference on
Observational Evidence for Black Holes in the Universe, Feb. 2008, Editor
Sandip Chakrabarti, AI
Simulations of stellar winds from X-ray bursts. Characterization of solutions and observable variables
Photospheric radius expansion during X-ray bursts can be used to measure
neutron star radii and help constrain the equation of state of neutron star
matter. Understanding the stellar wind dynamics is important for interpreting
observations. Stellar wind models, though studied in past decades, have thus
regained interest and need to be revisited with updated data and methods. In
this work we study the radiative wind model in the context of XRBs, with modern
techniques and physics input. We focus on characterization of the solutions and
study of observable magnitudes as a function of free model parameters. We
implement a spherically-symmetric non-relativistic wind model in a stationary
regime, with updated opacity tables and modern numerical techniques. Total mass
and energy outflows are treated as free parameters. A high
resolution parameter space exploration was performed to allow better
characterization of observable magnitudes. High correlation was found between
different photospheric magnitudes and free parameters. For instance, the
photospheric ratio of gravitational energy outflow to radiative luminosity is
in direct proportion to the photospheric wind velocity. The correlations found
could help determine the physical conditions of the inner layers, where nuclear
reactions take place, by means of observable photospheric values. Further
studies are needed to determine the range of physical conditions in which the
correlations are valid.Comment: Accepted in Astronomy and Astrophysics. New version with corrected
typo in eq. (36) and some minor changes after language editio
GRO J1655-40: from ASCA and XMM-Newton Observations
We have analysed four ASCA observations (1994--1995, 1996--1997) and three
XMM-Newton observations (2005) of this source, in all of which the source is in
high/soft state. We modeled the continuum spectra with relativistic disk model
kerrbb, estimated the spin of the central black hole, and constrained the
spectral hardening factor f_col and the distance. If kerrbb model applies, for
normally used value of f_col, the distance cannot be very small, and f_col
changes with observations.Comment: 2 pages, 1 figure, Conference proceedings to appear in "The Central
Engine of Active Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San
Francisco: ASP
Constraints on the mass and radius of the accreting neutron star in the Rapid Burster
The Rapid Burster (MXB 1730-335) is a unique object, showing both type I and
type II X-ray bursts. A type I burst of the Rapid Burster was observed with
Swift/XRT on 2009 March 5, showing photospheric radius expansion for the first
time in this source. We report here on the mass and radius determination from
this photospheric radius expansion burst using a Bayesian approach. After
marginalization over the likely distance of the system (5.8-10 kpc) we obtain
M=1.1+/-0.3 M_sun and R=9.6+/-1.5 km (1-sigma uncertainties) for the compact
object, ruling out the stiffest equations of state for the neutron star. We
study the sensitivity of the results to the distance, the color correction
factor, and the hydrogen mass fraction in the envelope. We find that only the
distance plays a crucial role.Comment: 7 pages, 8 figures, Accepted for publication in The Astrophysical
Journa
XMM-Newton observations of XTE J1817-330 and XTE J1856+053
The black hole candidate XTE J1817-330 was discovered in outburst on 26
January 2006 with RXTE/ASM. One year later, on 28 February 2007, another X-ray
transient discovered in 1996, XTE J1856+053, was detected by RXTE during a new
outburst. We report on the spectra obtained by XMM-Newton of these two black
hole candidates.Comment: Replaced with corrected versio
Canine Hepacivirus NS3 Serine Protease Can Cleave the Human Adaptor Proteins MAVS and TRIF
Canine hepacivirus (CHV) was recently identified in domestic dogs and horses. The finding that CHV is genetically the virus most closely related to hepatitis C virus (HCV) has raised the question of whether HCV might have evolved as the result of close contact between dogs and/or horses and humans. The aim of this study was to investigate whether the NS3/4A serine protease of CHV specifically cleaves human mitochondrial antiviral signaling protein (MAVS) and Toll-IL-1 receptor domain-containing adaptor inducing interferon-beta (TRIF). The proteolytic activity of CHV NS3/4A was evaluated using a bacteriophage lambda genetic screen. Human MAVS- and TRIF-specific cleavage sites were engineered into the lambda cI repressor. Upon infection of Escherichia coli cells coexpressing these repressors and a CHV NS3/4A construct, lambda phage replicated up to 2000-fold more efficiently than in cells expressing a CHV protease variant carrying the inactivating substitution S139A. Comparable results were obtained when several HCV NS3/4A constructs of genotype 1b were assayed. This indicates that CHV can disrupt the human innate antiviral defense signaling pathway and suggests a possible evolutionary relationship between CHV and HCV
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