1,098 research outputs found
Evidence of magnetic accretion in an SW Sex star: discovery of variable circular polarization in LS Pegasi
We report on the discovery of variable circular polarization in the SW Sex
star LS Pegasi. The observed modulation has an amplitude of ~0.3 % and a period
of 29.6 minutes, which we assume as the spin period of the magnetic white
dwarf. We also detected periodic flaring in the blue wing of Hbeta, with a
period of 33.5 minutes. The difference between both frequencies is just the
orbital frequency, so we relate the 33.5-min modulation to the beat between the
orbital and spin period. We propose a new accretion scenario in SW Sex stars,
based on the shock of the disk-overflown gas stream against the white dwarf's
magnetosphere, which extends to the corotation radius. From this geometry, we
estimate a magnetic field strength of B(1) ~ 5-15 MG. Our results indicate that
magnetic accretion plays an important role in SW Sex stars and we suggest that
these systems are probably Intermediate Polars with the highest mass accretion
rates.Comment: Accepted by ApJ Letters. LaTeX, 14 pages, 3 PostScript figure
Precision Ephemerides For Gravitational Wave Searches: II. Cyg X-2
Accreting neutron stars in low-mass X-ray binaries (LMXBs) are candidate
high-frequency persistent gravitational wave sources. These may be detectable
with next generation interferometers such as Advanced LIGO/VIRGO within this
decade. However, the search sensitivity is expected to be limited principally
by the uncertainty in the binary system parameters. We combine new optical
spectroscopy of Cyg X-2 obtained with the Liverpool Telescope (LT) with
available historical radial velocity data, which gives us improved orbital
parameter uncertainties based on a 44-year baseline. We obtained an improvement
of a factor of 2.6 in the orbital period precision and a factor of 2 in the
epoch of inferior conjunction T_0. The updated orbital parameters imply a mass
function of 0.65 +/- 0.01 M_sun, leading to a primary mass (M_1) of 1.67 +/-
0.22 M_sun (for i=62.5 +/- 4 deg). In addition, we estimate the likely orbital
parameter precision through to the expected Advanced LIGO and VIRGO detector
observing period and quantify the corresponding improvement in sensitivity via
the required number of templates.Comment: 7 pages, 3 figures, accepted for publication by Ap
Staff experiences of formulating within a team setting
This study evaluates psychology-led formulation sessions within an assessment and treatment service. Five staff members completed interviews exploring their experiences of formulation and their perception of its usefulness to clients. Results suggested that they perceived formulation to be beneficial on a number of levels for themselves and their practice but were uncertain about the tangible benefits for clients
On the Origin of the Absorption Features in SS433
We present high-resolution optical spectroscopy of the X-ray binary system
SS433, obtained over a wide range of orbital phases. The spectra display
numerous weak absorption features, and include the clearest example seen to
date of those features, resembling a mid-A type supergiant spectrum, that have
previously been associated with the mass donor star. However, the new data
preclude the hypothesis that these features originate solely within the
photosphere of the putative mass donor, indicating that there may be more than
one region within the system producing an A supergiant-like spectrum, probably
an accretion disc wind. Indeed, whilst we cannot confirm the possibility that
the companion star is visible at certain phase combinations, it is possible
that all supergiant-like features observed thus far are produced solely in a
wind. We conclude that great care must be taken when interpreting the behaviour
of these weak features.Comment: Accepted for publication in MNRAS, 8 pages, 6 figure
Doppler tomography of the transient X-ray binary Centaurus X-4 in quiescence
We present ESO-NTT low resolution spectroscopy of the neutron star X-ray
transient Cen X-4 in quiescence over a complete orbital cycle. Our data reveal
the presence of a K3-7 V companion which contributes 63% to the 5600-6900A flux
and orbits the neutron star with a velocity semi-amplitude of K_2=145.8 +/- 1.0
km s^{-1}. This, combined with a previous determination of the inclination
angle and mass ratio, yields a neutron star and companion mass of M_1=1.5 +/-
1.0 M_Sun and M_2=0.31 +/- 0.27 M_Sun, respectively. The mass donor is thus
undermassive for the inferred spectral type indicating it is probably evolved,
in agreement with previous studies. Doppler tomography of the H_alpha line
shows prominent emission located on the companion and a slightly asymmetric
accretion disc distribution similar to that seen in systems with precessing
eccentric discs. Strong H_alpha emission from the companion can be explained by
X-ray irradiation from the primary. No evidence is found for a hot spot in
H_alpha, whereas one is revealed via Doppler tomography of the HeI lines. This
can be interpreted as the hot spot and outer regions of the disc being at a
higher temperature than in other systems.Comment: 9 pages, 5 figures, accepted for publication in A&
Aspherical supernova explosions and formation of compact black hole low-mass X-ray binaries
It has been suggested that black-hole low-mass X-ray binaries (BHLMXBs) with
short orbital periods may have evolved from BH binaries with an
intermediate-mass secondary, but the donor star seems to always have higher
effective temperatures than measured in BHLMXBs (Justham, Rappaport &
Podsiadlowski 2006). Here we suggest that the secondary star is originally an
intermediate-mass (\sim 2-5 M_{\sun}) star, which loses a large fraction of
its mass due to the ejecta impact during the aspherical SN explosion that
produced the BH. The resulted secondary star could be of low-mass (\la 1
M_{\sun}). Magnetic braking would shrink the binary orbit, drive mass transfer
between the donor and the BH, producing a compact BHLMXB.Comment: 4 pages, accepted for publication in MNRAS Letter
ULTRACAM observations of the black hole X-ray transient XTE J1118+480 in quiescence
We present high time-resolution multicolour observations of the quiescent
soft X-ray transient XTE J1118+480 obtained with ULTRACAM. Superimposed on the
double-humped continuum g' and i'-band lightcurves are rapid flare events which
typically last a few minutes. The power density spectrum of the lightcurves can
be described by a broken power-law model with a break frequency at ~2 mHz or a
power-law model plus a broad quasi-periodic oscillation (QPO) at ~2 mHz. In the
context of the cellular-automaton we estimate the size of the quiescent
advection-dominated flow (ADAF) region to be ~10^4 Schwarzschild radii, similar
to that observed in other quiescent black hole X-ray transients, suggesting the
same underlying physics. The similarites between the low/hard and quiescent
state PDS suggest a similar origin for the optical and X-ray variability, most
likely from regions at/near the ADAF.Comment: 9 pages, 6 figures, accepted by MNRA
Optical studies of the X-ray transient XTE J2123-058 -I. Photometry
We present optical photometry of the X-ray transient XTE J2123-058, obtained
in July-October 1998. The light curves are strongly modulated on the 5.95hrs
orbital period, and exhibit dramatic changes in amplitude and form during the
decline. We used synthetic models which include the effect of partial eclipses
and X-ray heating effects, to estimate the system parameters, and we constrain
the binary inclination to be i=73+-4 degrees. The model is successful in
reproducing the light curves at different stages of the decay by requiring the
accretion disc to become smaller and thinner by 30% as the system fades by 1.7
mags in the optical. From Aug 26 the system reaches quiescence with a mean
magnitude of R=21.7+-0.1 and our data are consistent with the optical
variability being dominated by the companion's ellipsoidal modulation.Comment: 6 pages, 6 figure
Near-Infrared Spectroscopy of Low Mass X-ray Binaries : Accretion Disk Contamination and Compact Object Mass Determination in V404 Cyg and Cen X-4
We present near-infrared (NIR) broadband (0.80--2.42 m) spectroscopy of
two low mass X-ray binaries: V404 Cyg and Cen X-4. One important parameter
required in the determination of the mass of the compact objects in these
systems is the binary inclination. We can determine the inclination by modeling
the ellipsoidal modulations of the Roche-lobe filling donor star, but the
contamination of the donor star light from other components of the binary,
particularly the accretion disk, must be taken into account. To this end, we
determined the donor star contribution to the infrared flux by comparing the
spectra of V404 Cyg and Cen X-4 to those of various field K-stars of known
spectral type. For V404 Cyg, we determined that the donor star has a spectral
type of K3 III. We determined the fractional donor contribution to the NIR flux
in the H- and K-bands as and , respectively. We
remodeled the H-band light curve from \citet{sanwal1996} after correcting for
the donor star contribution to obtain a new value for the binary inclination.
From this, we determined the mass of the black hole in V404 Cyg to be . We performed the same spectral analysis for Cen X-4
and found the spectral type of the donor star to be in the range K5 -- M1V. The
donor star contribution in Cen X-4 is in the H-band while in the
K-band, the accretion disk can contribute up to 10% of the infrared flux. We
remodeled the H-band light curve from \citet{shahbaz1993}, again correcting for
the fractional contribution of the donor star to obtain the inclination. From
this, we determined the mass of the neutron star as . However, the masses obtained for both systems should
be viewed with some caution since contemporaneous light curve and spectral data
are required to obtain definitive masses
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