288 research outputs found
Illumination in binaries
We give a simple, but accurate method that can be used to account for
illumination in compact binary systems which have a low-mass companion, even if
spherically symmetric illumination of the secondary star (not necessarily on
the main sequence) is not assumed. This is done by introducing a multiplicative
factor Phi in the Stefan-Boltzmann surface boundary condition, which accounts
for the blocking of the intrinsic secondary flux by X-ray heating of the
photospheric layers. Numerical fits and tables for Phi are given for
unperturbed effective temperatures in the range 2500 - 5600 K and log g in the
range 1.0 - 5.0Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Supplement Serie
Outbursts in ultracompact X-ray binaries
Very faint X-ray binaries appear to be transient in many cases with peak
luminosities much fainter than that of usual soft X-ray transients, but their
nature still remains elusive. We investigate the possibility that this
transient behaviour is due to the same thermal/viscous instability which is
responsible for outbursts of bright soft X-ray transients, occurring in
ultracompact binaries for adequately low mass-transfer rates. More generally,
we investigate the observational consequences of this instability when it
occurs in ultracompact binaries. We use our code for modelling the
thermal-viscous instability of the accretion disc, assumed here to be hydrogen
poor. We also take into account the effects of disc X-ray irradiation, and
consider the impact of the mass-transfer rate on the outburst brightness. We
find that one can reproduce the observed properties of both the very faint and
the brighter short transients (peak luminosity, duration, recurrence times),
provided that the viscosity parameter in quiescence is slightly smaller
(typically a factor of between two and four) than in bright soft X-ray
transients and normal dwarf nova outbursts, the viscosity in outburst being
unchanged. This possibly reflects the impact of chemical composition on
non-ideal MHD effects affecting magnetically driven turbulence in poorly
ionized discs.Comment: 9 pages, 7 figures, Astronomy and Astrophysics, in pres
Hydrodynamic simulations of irradiated secondaries in dwarf novae
We investigate numerically the surface flow on the secondary star during
outbursts. We use a simple model for the irradiation and the geometry of the
secondary star: the irradiation temperature is treated as a free parameter and
the secondary is replaced by a spherical star with a space-dependent Coriolis
force that mimics the effect of the Roche geometry. The Euler equations are
solved in spherical coordinates with the TVD-MacCormack scheme. We show that
the Coriolis force leads to the formation of a circulation flow from high
latitude region to the close vicinity of the point. However no heat can
be efficiently transported to the region due to the rapid radiative
cooling of the hot material as it enters the equatorial belt shadowed from
irradiation. Under the assumption of hydrostatic equilibrium, the Coriolis
force could lead to a moderate increase of the mass transfer rate by pushing
the gas in the vertical direction at the point, but only during the
initial phases of the outburst (about 15 -- 20 orbital periods). We conclude
that the Coriolis force does not prevent a flow from the heated regions of the
secondary towards the region, at least during the initial phase of an
outburst, but the resulting increase of the mass transfer rate is moderate, and
it is unlikely to be able to account for the duration of long outbursts.Comment: 11 pages, 11 figures, accepted for publication in A&
Dwarf nova outbursts in intermediate polars
The disc instability model (DIM) has been very successful in explaining the
dwarf nova outbursts observed in cataclysmic variables. When, as in
intermediate polars (IP), the accreting white dwarf is magnetized, the disc is
truncated at the magnetospheric radius, but for mass-transfer rates
corresponding to the thermal-viscous instability such systems should still
exhibit dwarf-nova outbursts. Yet, the majority of intermediate polars in which
the magnetic field is not large enough to completely disrupt the accretion
disc, seem to be stable, and the rare observed outbursts, in particular in
systems with long orbital periods, are much shorter than normal dwarf-nova
outbursts. We investigate the predictions of the disc instability model for
intermediate polars in order to determine which of the observed properties of
these systems can be explained by the DIM. We use our numerical code for the
time evolution of accretion discs, modified to include the effects of the
magnetic field, with constant or variable mass transfer from the secondary
star. We show that intermediate polars have mass transfer low enough and
magnetic fields large enough to keep the accretion disc stable on the cold
equilibrium branch. We show that the infrequent and short outbursts observed in
long period systems, such as e.g., TV Col, cannot be attributed to the
thermal-viscous instability of the accretion disc, but instead have to be
triggered by an enhanced mass-transfer from the secondary, or, more likely, by
some instability coupling the white dwarf magnetic field with that generated by
the magnetorotational instability operating in the accretion disc. Longer
outbursts (a few days) could result from the disc instability.Comment: 7 pages, 5 figures; submitted to Astronomy & Astrophysic
Hot white dwarfs and the UV delay in dwarf novae
We calculate the effect of illumination of dwarf nova accretion discs by
radiation from a hot, central, white dwarf. We show that only for very hot
white dwarfs (Teff ~ 40 000$ K) the inner region of quiescent dwarf nova discs
are partially depleted so that the delay between the rise to outburst of the
optical and UV fluxes would be increased as suggested recently by King (1997).
This depletion, however, must create several small outbursts between main
outbursts, contrary to observations. Lower white dwarf temperatures may cause
the outburts to be of the `inside-out' type removing the UV delay. We conclude
that white dwarf irradiation of dwarf nova discs is not very efficient for
example because the UV radiation from the hot white dwarf does not penetrate
deep enough in the disc atmosphere. The total ablation of the inner disc by
e.g. evaporation (possibly related to illumination) appears to be a very
promising possibility, accounting for both the EUV delay and the general
lightcurves properties.Comment: 6 pages, 8 figures; accepted for publication in MNRA
Evidence for Antipodal Hot Spots During X-ray Bursts From 4U 1636-536
The discovery of high-frequency brightness oscillations in thermonuclear
X-ray bursts from several neutron-star low-mass X-ray binaries has important
implications for the beat frequency model of kilohertz quasi-periodic
brightness oscillations, the propagation of nuclear burning, the structure of
the subsurface magnetic fields in neutron stars, and the equation of state of
high-density matter. These implications depend crucially on whether the
observed frequency is the stellar spin frequency or its first overtone. Here we
report an analysis of five bursts from 4U 1636-536 which exhibit strong
oscillations at approximately 580 Hz. We show that combining the data from the
first 0.75 seconds of each of the five bursts yields a signal at 290 Hz that is
significant at the level when the number of trials is taken
into account. This strongly indicates that 290 Hz is the spin frequency of this
neutron star and that 580 Hz is its first overtone, in agreement with other
arguments about this source but in contrast to suggestions in the literature
that 580 Hz is the true spin frequency. The method used here, which is an
algorithm for combining time series data from the five bursts so that the
phases of the 580 Hz oscillations are aligned, may be used in any source to
search for weak oscillations that have frequencies related in a definite way to
the frequency of a strong oscillation.Comment: 9 pages including one figure, uses aaspp4.sty, submitted to The
Astrophysical Journal Letters on September 1
X-ray irradiation in low mass binary systems
We calculate self-consistent models of X-ray irradiated accretion discs in
close binary systems. We show that a point X-ray source powered by accretion
and located in the disc plane cannot modify the disc structure, mainly because
of the self-screening by the disc of its outer regions. Since observations show
that the emission of the outer disc regions in low mass X-ray binaries is
dominated by the reprocessed X-ray flux, accretion discs in these systems must
be either warped or irradiated by a source above the disc plane, or both. We
analyse the thermal-viscous stability of irradiated accretion discs and derive
the stability criteria of such systems. We find that, contrary to the usual
assumptions, the critical accretion rate below which a disc is unstable is
rather uncertain since the correct formula describing irradiation is not well
known.Comment: to be published in MNRAS, uses epsfig.st
The nature of dwarf nova outbursts
We show that if the dwarf-nova disc instability model includes the effects of
heating by stream impact and tidal torque dissipation in the outer disc, the
calculated properties of dwarf-nova outbursts change considerably, and several
notorious deficiencies of this model are repaired. In particular: (1)
outside-in outbursts occur for mass transfer rates lower than in the standard
model as required by observations; (2) the presence of long (wide) and short
(narrow) outbursts with similar peak luminosities is a natural property of the
model. Mass-transfer fluctuations by factors ~ 2 can explain the occurrence of
both long and short outbursts above the cataclysmic variable period gap,
whereas below 2 hr only short normal outbursts are expected (in addition to
superoutbursts which are not dealt with in this article). With additional
heating by the stream and tidal torques, such fluctuations can also explain the
occurrence of both outside-in and inside-out outbursts in SS Cyg and similar
systems. The occurrence of outside-in outbursts in short orbital-period, low
mass-transfer-rate systems requires the disc to be much smaller than the
tidal-truncation radius. In this case the recurrence time of both inside-out
and outside-in outbursts have a similar dependence on the mass-transfer rate.Comment: 11 pages, 8 figures Submitted to Astronomy and Astrophysic
The thermal-viscous disk instability model in the AGN context
Accretion disks in AGN should be subject to the same type of instability as
in cataclysmic variables (CVs) or in low-mass X-ray binaries (LMXBs), which
leads to dwarf nova and soft X-ray transient outbursts. It has been suggested
that this thermal/viscous instability can account for the long term variability
of AGNs. We test this assertion by presenting a systematic study of the
application of the disk instability model (DIM) to AGNs. We are using the
adaptative grid numerical code we have developed in the context of CVs,
enabling us to fully resolve the radial structure of the disk. We show that,
because in AGN disks the Mach numbers are very large, the heating and cooling
fronts are so narrow that they cannot be resolved by the numerical codes that
have been used until now. In addition, these fronts propagate on time scales
much shorter than the viscous time. As a result, a sequence of heating and
cooling fronts propagate back and forth in the disk, leading only to small
variations of the accretion rate onto the black hole, with short quiescent
states occurring for very low mass transfer rates only. Truncation of the inner
part of the disk by e.g. an ADAF does not alter this result, but enables longer
quiescent states. Finally we discuss the effects of irradiation by the central
X-ray source, and show that, even for extremely high irradiation efficiencies,
outbursts are not a natural outcome of the model.Comment: Astronomy & Astrophysics - in pres
Z Cam stars: a particular response to a general phenomenon
We show that the disc instability model can reproduce all the observed
properties of Z Cam stars if the energy equation includes heating of the outer
disc by the mass-transfer stream impact and by tidal torques and if the
mass-transfer rate from the secondary varies by about 30% around the value
critical for stability. In particular the magnitude difference between outburst
maxima and standstills corresponds to observations, all outbursts are of the
inside-out type and can be divided into two classes: long (wide) and short
(narrow) outbursts, as observed. Mass transfer rate fluctuations should occur
in other dwarf novae but one can exclude variations similar to those observed
in magnetic systems (AM Her's and some DQ Her's) and some nova-like systems (VY
Scl's), in which \dot{M} become very small during low states; these would
produce mini-outburst which, although detectable, have never been observed.Comment: submitted to Astronomy & Astrophysics, 8 pages, 4 figure
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