661 research outputs found
Angular momentum losses and the orbital period distribution of cataclysmic variables below the period gap: effects of circumbinary disks
The population synthesis of cataclysmic variables below the period is
investigated. A grid of detailed binary evolutionary sequences has been
calculated and included in the simulations to take account of additional
angular momentum losses beyond that associated with gravitational radiation and
mass loss, due to nova outbursts, from the system. As a specific example, we
consider the effect of a circumbinary disk to gain insight into the ingredients
necessary to reproduce the observed orbital period distribution. The resulting
distributions show that the period minimum lies at about 80 minutes with the
number of systems monotonically increasing with increasing orbital period to a
maximum near 90 minutes. There is no evidence for an accumulation of systems at
the period minimum which is a common feature of simulations in which only
gravitational radiation losses are considered. The period distribution is found
to be fairly flat for orbital periods ranging from about 85 to 120 minutes. The
steepness of the lower edge of the period gap can be reproduced, for example,
by an input of systems at periods near 2.25 hrs due to a flow of cataclysmic
variable binary systems from orbital periods longer than 2.75 hrs. The good
agreement with the cumulated distribution function of observed systems within
the framework of our model indicates that the angular momentum loss by a
circumbinary disk or a mechanism which mimics its features coupled with a
weighting factor to account for selection effects in the discovery of such
systems and a flow of systems from above the period gap to below the period gap
are important ingredients for understanding the overall period distribution of
cataclysmic variable binary systems.Comment: Accepted for publication in Ap
The eclipsing X-ray pulsar X-7 in M33
Using our extensive ROSAT X-ray observations of M33, we confirm a 3.45 day
eclipse period for the Einstein source X-7 (Larson & Schulman, 1997) and
discover evidence for a 0.31-s pulse period. The orbital period, pulse period
and observed X-ray luminosity are remarkably similar to SMC X-1. We therefore
suggest M33 X-7 is a neutron star high mass X-ray binary with a 15-40 Msol O/B
companion and a binary separation of 25-33 Rsol if the companion is almost
filling its Roche lobe.Comment: accepted for publication in MNRA
INTEGRAL observation of the high-mass X-ray transient V 0332+53 during the 2005 outburst decline
The decline of the high mass X-ray transient V 0332+53 during the Dec. 2004 to Feb. 2005 outburst is analysed from the data recorded by INTEGRAL. The flux is shown to decrease exponentially until 2005 Feb. 10, with a decay time scale of ∼30 days above 20 keV and ∼20 days at lower energies, and to decrease linearly thereafter.
The energy spectrum is well modelled throughout the decay by a power law with a folding energy of ∼7.5 keV, and with two cyclotron absorption features. The folding energy does not vary significantly over the decay, but the spectrum becomes harder with time. Most importantly, we show that the parameters describing the fundamental cyclotron line around 27 keV do vary with time: its energy and depth increase (by about 17% for the energy in ∼6 weeks), while its width decreases. These changes of the cyclotron line parameters are interpreted as resulting from a change in the extent of the cyclotron scattering region.
Two quasi-periodic oscillations are also observed at various times during the observations, one at 0.05 Hz and another one near the pulsation frequency around 0.23 Hz
Spitzer Space Telescope Observations of the Magnetic Cataclysmic Variable AE Aqr
The magnetic cataclysmic variable AE Aquarii hosts a rapidly rotating white
dwarf which is thought to expel most of the material streaming onto it.
Observations of AE Aqr have been obtained in the wavelength range of 5 - 70
microns with the IRS, IRAC, and MIPS instruments on board the Spitzer Space
Telescope. The spectral energy distribution reveals a significant excess above
the K4V spectrum of the donor star with the flux increasing with wavelength
above 12.5 microns. Superposed on the energy distribution are several hydrogen
emission lines, identified as Pf alpha and Hu alpha, beta, gamma. The infrared
spectrum above 12.5 microns can be interpreted as synchrotron emission from
electrons accelerated to a power-law distribution dN=E^{-2.4}dE in expanding
clouds with an initial evolution timescale in seconds. However, too many
components must then be superposed to explain satisfactorily both the
mid-infrared continuum and the observed radio variability. Thermal emission
from cold circumbinary material can contribute, but it requires a disk
temperature profile intermediate between that produced by local viscous
dissipation in the disk and that characteristic of a passively irradiated disk.
Future high-time resolution observations spanning the optical to radio regime
could shed light on the acceleration process and the subsequent particle
evolution.Comment: 15 pages, 3 figures, accepted for publication in Ap
Optical and Infrared Light Curves of the Eclipsing X-ray Binary V395 Car = 2S 0921-630
We present results of optical and infrared photometric monitoring of the
eclipsing low-mass X-ray binary V395 Car (2S 0921-630). Our observations reveal
a clear, repeating orbital modulation with an amplitude of about one magnitude
in B, and V and a little less in J. Combining our data with archival
observations spanning about 20 years, we derive an updated ephemeris with
orbital period 9.0026+/-0.0001d. We attribute the modulation to a combination
of the changing aspect of the irradiated face of the companion star and
eclipses of the accretion disk around the neutron star. Both appear to be
necessary as a secondary eclipse of the companion star is clearly seen. We
model the B, V, and J lightcurves using a simple model of an accretion disk and
companion star and find a good fit is possible for binary inclinations of
82.2+/-1.0 degrees. We estimate the irradiating luminosity to be about 8x10^35
erg/s, in good agreement with X-ray constraints.Comment: 6 pages, accepted for publication in MNRA
What Role for Angiogenesis in Childhood Acute Lymphoblastic Leukaemia?
The role of angiogenesis in acute leukaemia has been discussed since the cloning of the gene of vascular endothelial growth factor (VEGF) from the acute myelogenous leukemia cell line (HL60) and, thereafter, when the first studies reported increased bone marrow vascularity and elevation of angiogenic cytokines in acute lymphoblastic leukaemia (ALL). VEGF and basic fibroblast growth factor (bFGF) are the major proangiogenic cytokines that have been studied, and evaluation of their prognostic impact in childhood ALL has been reported in several studies, though with controversial results. The antiangiogenic response, contributing to the angiogenic balance, has scarcely been reported. The origin of the factors, their prognostic value, and their relevance as good markers of what really happens in the bone marrow are discussed in this paper. The place of antiangiogenic drugs in ALL has to be defined in the global treatment strategy
High-energy gamma-ray emission from the inner jet of LS I+61 303: the hadronic contribution revisited
LS I+61 303 has been detected by the Cherenkov telescope MAGIC at very high
energies, presenting a variable flux along the orbital motion with a maximum
clearly separated from the periastron passage. In the light of the new
observational constraints, we revisit the discussion of the production of
high-energy gamma rays from particle interactions in the inner jet of this
system. The hadronic contribution could represent a major fraction of the TeV
emission detected from this source. The spectral energy distribution resulting
from p-p interactions is recalculated. Opacity effects introduced by the photon
fields of the primary star and the stellar decretion disk are shown to be
essential in shaping the high-energy gamma-ray light curve at energies close to
200 GeV. We also present results of Monte Carlo simulations of the
electromagnetic cascades developed very close to the periastron passage. We
conclude that a hadronic microquasar model for the gamma-ray emission in LS I
+61 303 can reproduce the main features of its observed high-energy gamma-ray
flux.Comment: 6 pages. Sligth improvements made. Accepted version by Astrophysics
and Space Scienc
Dependent patients discharged home from PRM departments: relevant indicators
OBJECTIVE: To evaluate simple, measurable indicators of optimal organizational procedures for the hospital-to-home discharge of dependent patients.MATERIAL AND METHOD: All the general practitioners (GPs) in the Maine-et-Loire county of France were sent a questionnaire asking them to rank the three main criteria (from the most important to the least significant) from a list of 14. We analyzed the median ranking for each item and identified the most important items in terms of their relative frequency. RESULTS: The response rate was 10.77% (104 out of 966). Four criteria had a median score over 9: contact with the GP prior to discharge, informing the GP of the discharge date, training for the patient and his/her family in activities of daily living and providing a list of people to be contacted in the event of a problem at home. Respite hospitalization (in the event of difficulties at home) was cited as one of the three most relevant criteria. DISCUSSION-CONCLUSION: The criteria highlighted by the GPs were not highly specific for the discharge of a dependent patient. However, it would be interesting to extend this study by interviewing other stakeholders and determining whether these criteria indeed improve the organization of hospital-to-home discharge
The origin and fate of short-period low-mass black-hole binaries
We present results of a population synthesis study for semidetached short
orbital period binaries which contain low-mass(<1.5 Msun) donors and black hole
(>4 Msun) accretors. Evolution of these binaries is determined by nuclear
evolution of the donors and/or orbital angular momentum loss due to magnetic
braking by the stellar wind of the donors and gravitational wave radiation.
According to our model, the estimated total number of this type of black-hole
binaries in the Galaxy is about 10000. If the magnetic braking is described by
the Verbunt & Zwaan formula, the model predicts around 3000 transient systems
with periods >2 hours and around 300 luminous stable systems with periods
between 3 and 8 hours. Several dozens of these bright systems should be above
the RXTE ASM sensitivity limit. The absence of such systems implies that
angular momentum losses are reduced by a factor more than 2 with respect to the
Verbunt & Zwaan prescription. We show that it is unlikely that the transient
behaviour of black-hole short-period X-ray binaries is explained by the evolved
nature of the stellar companion. A substantial fraction of black-hole binaries
with periods >3 hours could be faint with truncated, stable cold accretion
discs as proposed by Menou et al. Most of the semidetached black-hole binaries
are expected to have periods shorter than ~2 hours. Properties of such, still
to be observed, very small mass-ratio (q<0.02) binaries are different from
those of their longer period cousins.Comment: 13 pages, 6 figures, accepted for publication in A&
- …