51 research outputs found
Superhumps in V348 Pup
The eclipsing novalike cataclysmic variable star V348 Pup exhibits a
persistent luminosity modulation with a period 6 per cent longer than its 2.44
hour orbital-period (Porb). This has been interpreted as a `positive superhump'
resulting from a slowly precessing non-axisymmetric accretion disc
gravitationally interacting with the secondary. We find a clear modulation of
mid-eclipse times on the superhump period, which agrees well with the
predictions of a simple precessing eccentric disc model. Our modelling shows
that the disc light centre is on the far side of the disc from the donor star
when the superhump reaches maximum light. This phasing suggests a link between
superhumps in V348 Pup and late superhumps in SU UMa systems. Modelling of the
full lightcurve and maximum entropy eclipse mapping both show that the disc
emission is concentrated closer to the white dwarf at superhump maximum than at
superhump minimum. We detect additional signals consistent with the beat
periods between the implied disc precession period and both (1/2)Porb and
(1/3)Porb.Comment: 13 pages, 13 figures, accepted for publication in MNRA
SPH simulations of irradiation-driven warped accretion discs and the long periods in X-ray binaries
We present three dimensional smoothed particle hydrodynamics (SPH)
calculations of irradiation-driven warping of accretion discs. Initially
unwarped planar discs are unstable to the radiation reaction when the disc is
illuminated by a central radiation source. The disc warps and tilts and
precesses slowly in a retrograde direction; its shape continuously flexes in
response to the changing orientation of the Roche potential. We simulate ten
systems: eight X-ray binaries, one cataclysmic variable (CV), and a `generic'
low mass X-ray binary (LMXB). We adopt system parameters from observations and
tune a single parameter: our model X-ray luminosity () to reproduce the
observed or inferred super-orbital periods. Without exception, across a wide
range of parameter space, we find an astonishingly good match between the
observed and the model . We conclude irradiation-driven warping
is the mechanism underlying the long periods in X-ray binaries. Our Her X-1
simulation simultaneously reproduces the observed , the "main-" and
"short-high" X-ray states and the orbital inclination. Our simulations of SS
433 give a maximum warp angle of , a good match to the cone
traced by the jets, but this angle is reached only in the outer disc. In all
cases, the overall disc tilt is less than \degrees{13} and the maximum disc
warp is less than and or equal to \degrees{21}.Comment: 17 pages, 14 figures, shorter abstract (24 lines limit
Doppler Tomography of Dwarf Nova IY UMa during Quiescence
Quiescent Doppler tomography of the newly discovered deeply-eclipsing SU UMa
system IY UMa reveals properties of the region where the accretion stream from
the donor impacts the edge of the disc. A very strong bright spot is produced
and the Keplerian disc emission in the impact region is disrupted or obscured.
The differing properties of Halpha, Hbeta and He I emission will allow physical
parameters of the converging flow region to be studied.Comment: 6 pages, 5 figures. To appear in Proceedings of Astro-Tomography
Workshop, Brussels, July 2000, Eds. H. Boffin, D. Steeghs, Springer-Verlag
Lecture Notes in Physic
Superhumps in Low-Mass X-Ray Binaries
We propose a mechanism for the superhump modulations observed in optical
photometry of at least two black hole X-ray transients (SXTs). As in extreme
mass-ratio cataclysmic variables (CVs), superhumps are assumed to result from
the presence of the 3:1 orbital resonance in the accretion disc. This causes
the disc to become non-axisymmetric and precess. However the mechanism for
superhump luminosity variations in low mass X-ray binaries (LMXBs) must differ
from that in CVs, where it is attributed to a tidally-driven modulation of the
disc's viscous dissipation, varying on the beat between the orbital and disc
precession period. By contrast in LMXBs, tidal dissipation in the outer
accretion disc is negligible: the optical emission is overwhelming dominated by
reprocessing of intercepted central X-rays. Thus a different origin for the
superhump modulation is required. Recent observations and numerical simulations
indicate that in an extreme mass-ratio system the disc area changes on the
superhump period. We deduce that the superhumps observed in SXTs arise from a
modulation of the reprocessed flux by the changing area. Therefore, unlike the
situation in CVs, where the superhump amplitude is inclination-independent,
superhumps should be best seen in low-inclination LMXBs, whereas an orbital
modulation from the heated face of the secondary star should be more prominent
at high inclinations. Modulation at the disc precession period (10s of days)
may indicate disc asymmetries such as warping. We comment on the orbital period
determinations of LMXBs, and the possibility and significance of possible
permanent superhump LMXBs.Comment: 6 pages, 1 encapsulated figure. MNRAS in press; replaced to correct
typographical error
Comprehensive simulations of superhumps
(Abridged) We use 3D SPH calculations with higher resolution, as well as with
more realistic viscosity and sound-speed prescriptions than previous work to
examine the eccentric instability which underlies the superhump phenomenon in
semi-detached binaries. We illustrate the importance of the two-armed spiral
mode in the generation of superhumps. Differential motions in the fluid disc
cause converging flows which lead to strong spiral shocks once each superhump
cycle. The dissipation associated with these shocks powers the superhump. We
compare 2D and 3D results, and conclude that 3D simulations are necessary to
faithfully simulate the disc dynamics. We ran our simulations for unprecedented
durations, so that an eccentric equilibrium is established except at high mass
ratios where the growth rate of the instability is very low. Our improved
simulations give a closer match to the observed relationship between superhump
period excess and binary mass ratio than previous numerical work. The observed
black hole X-ray transient superhumpers appear to have systematically lower
disc precession rates than the cataclysmic variables. This could be due to
higher disc temperatures and thicknesses. The modulation in total viscous
dissipation on the superhump period is overwhelmingly from the region of the
disc within the 3:1 resonance radius. As the eccentric instability develops,
the viscous torques are enhanced, and the disc consequently adjusts to a new
equilibrium state, as suggested in the thermal-tidal instability model. We
quantify this enhancement in the viscosity, which is ~10 per cent for q=0.08.
We characterise the eccentricity distributions in our accretion discs, and show
that the entire body of the disc partakes in the eccentricity.Comment: 18 pages (mn2e LaTeX), 14 figures, 5 tables, Accepted for publication
in MNRA
Late superhumps and the stream-disc impact in IY UMa
We use the hot spot eclipse times of the newly discovered deeply-eclipsing
dwarf nova IY UMa to trace out the shape of its disc during the late superhump
era. We find an eccentric disc. We show that the brightness of the stream-disc
impact region varies as expected with |deltaV|^2, where deltaV is the
differential velocity of the stream with respect to the velocity of the disc at
the impact point. We conclude that the hot spot is the source of late superhump
light.Comment: 7 pages, 9 figures, accepted for publication in MNRAS. Minor changes
to original content, additional modelling and figure
Mass transfer during low mass X-ray transient decays
The outbursts of low mass X-ray binaries are prolonged relative to those of
dwarf nova cataclysmic variables as a consequence of X-ray irradiation of the
disc. We show that the time-scale of the decay light curve and its luminosity
at a characteristic time are linked to the radius of the accretion disc. Hence
a good X-ray light curve permits two independent estimates of the disc radius.
In the case of the milli-second pulsars SAX J1808.4-3658 and XTE J0929-314 the
agreement between these estimates is very strong. Our analysis allows new
determinations of distances and accretion disc radii. Our analysis will allow
determination of accretion disc radii for sources in external galaxies, and
hence constrain system parameters where other observational techniques are not
possible. We also use the X-ray light curves to estimate the mass transfer
rate. The broken exponential decay observed in the 2002 outburst of SAX
J1808.4-3658 may be caused by the changing self-shadowing of the disc.Comment: 11 pages, 15 figures, 3 tables. Accepted MNRAS Oct 200
Simulations of spectral lines from an eccentric precessing accretion disc
Two dimensional SPH simulations of a precessing accretion disc in a q=0.1
binary system (such as XTE J1118+480) reveal complex and continuously varying
shape, kinematics, and dissipation. The stream-disc impact region and disc
spiral density waves are prominent sources of energy dissipation.The dissipated
energy is modulated on the period P_{sh} = ({P_{orb}}^{-1}-{P_{prec}}^{-1}^{-1}
with which the orientation of the disc relative to the mass donor repeats. This
superhump modulation in dissipation energy has a variation in amplitude of ~10%
relative to the total dissipation energy and evolves, repeating exactly only
after a full disc precession cycle. A sharp component in the light curve is
associated with centrifugally expelled material falling back and impacting the
disc. Synthetic trailed spectrograms reveal two distinct "S-wave" features,
produced respectively by the stream gas and the disc gas at the stream-disc
impact shock. These S-waves are non-sinusoidal, and evolve with disc precession
phase. We identify the spiral density wave emission in the trailed spectrogram.
Instantaneous Doppler maps show how the stream impact moves in velocity space
during an orbit. In our maximum entropy Doppler tomogram the stream impact
region emission is distorted, and the spiral density wave emission is
uppressed. A significant radial velocity modulation of the whole line profile
occurs on the disc precession period. We compare our SPH simulation with a
simple 3D model: the former is appropriate for comparison with emission lines
while the latter is preferable for skewed absorption lines from precessing
discs.Comment: See http://physics.open.ac.uk/FHMR/ for associated movie (avi) files.
The full paper is in MNRAS press. Limited disk space limit of 650k, hence low
resolution figure file
Three dimensional SPH simulations of radiation-driven warped accretion discs
We present three dimensional smoothed particle hydrodynamics (SPH)
calculations of warped accretion discs in X-ray binary systems. Geometrically
thin, optically thick accretion discs are illuminated by a central radiation
source. This illumination exerts a non-axisymmetric radiation pressure on the
surface of the disc resulting in a torque that acts on the disc to induce a
twist or warp. Initially planar discs are unstable to warping driven by the
radiation torque and in general the warps also precess in a retrograde
direction relative to the orbital flow. We simulate a number of X-ray binary
systems which have different mass ratios using a number of different
luminosities for each. Radiation-driven warping occurs for all systems
simulated. For mass ratios q ~ 0.1 a moderate warp occurs in the inner disc
while the outer disc remains in the orbital plane (c.f. X 1916-053). For less
extreme mass ratios the entire disc tilts out of the orbital plane (c.f. Her
X-1). For discs that are tilted out of the orbital plane in which the outer
edge material of the disc is precessing in a prograde direction we obtain both
positive and negative superhumps simultaneously in the dissipation light curve
(c.f. V603 Aql).Comment: 12 pages, 12 figures, paper accepted for publication by MNRA
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