219 research outputs found
A New Interpretation for the Second Peak of T Coronae Borealis Outbursts: A Tilting Disk around a Very Massive White Dwarf
A new interpretation for the second peak of T Coronae Borealis (T CrB)
outbursts is proposed based on a thermonuclear runaway (TNR) model. The system
consists of a very massive white dwarf (WD) with a tilting accretion disk and a
lobe-filling red-giant. The first peak of the visual light curve of T CrB
outbursts is well reproduced by the TNR model on a WD close to the
Chandrasekhar mass (), while the second peak
is reproduced by the combination of the irradiated M-giant and the irradiated
tilting disk. The derived fitting parameters are the WD mass , the M-giant companion mass
( is acceptable), the inclination angle of the orbit i \sim
70 \arcdeg, and the tilting angle of the disk i_{\rm prec} \sim 35 \arcdeg.
These parameters are consistent with the recently derived binary parameters of
T CrB.Comment: 6 pages including 2 figures, to be published in ApJ Letter
Long term flux variations in Cen X-3: clues from flux dependent orbital modulation and pulsed fraction
We have investigated the long term flux variation in Cen X-3 using orbital
modulation and pulsed fraction in different flux states using observations made
with the All Sky Monitor and the Proportional Counter Array on board the Rossi
X-ray Timing Explorer. In the high state, the eclipse ingress and egress are
found to be sharp whereas in the intermediate state the transitions are more
gradual. In the low state, instead of eclipse ingress and egress, the
lightcurve shows a smooth flux variation with orbital phase. The orbital
modulation of the X-ray lightcurve in the low state shows that the X-ray
emission observed in this state is from an extended object. The flux dependent
orbital modulations indicate that the different flux states of Cen X-3 are
primarily due to varying degree of obscuration. Measurement of the pulsed
fraction in different flux states is consistent with the X-ray emission of Cen
X-3 having one highly varying component with a constant pulsed fraction and an
unpulsed component and in the low state, the unpulsed component becomes
dominant. The observed X-ray emission in the low state is likely to be due to
scattering of X-rays from the stellar wind of the companion star. Though we can
not ascertain the origin and nature of the obscuring material that causes the
aperiodic long term flux variation, we point out that a precessing accretion
disk driven by radiative forces is a distinct possibility.Comment: 10 pages, 5 figures. Paper accepted for publication in MNRA
Interest-based RDF Update Propagation
Many LOD datasets, such as DBpedia and LinkedGeoData, are voluminous and
process large amounts of requests from diverse applications. Many data products
and services rely on full or partial local LOD replications to ensure faster
querying and processing. While such replicas enhance the flexibility of
information sharing and integration infrastructures, they also introduce data
duplication with all the associated undesirable consequences. Given the
evolving nature of the original and authoritative datasets, to ensure
consistent and up-to-date replicas frequent replacements are required at a
great cost. In this paper, we introduce an approach for interest-based RDF
update propagation, which propagates only interesting parts of updates from the
source to the target dataset. Effectively, this enables remote applications to
`subscribe' to relevant datasets and consistently reflect the necessary changes
locally without the need to frequently replace the entire dataset (or a
relevant subset). Our approach is based on a formal definition for
graph-pattern-based interest expressions that is used to filter interesting
parts of updates from the source. We implement the approach in the iRap
framework and perform a comprehensive evaluation based on DBpedia Live updates,
to confirm the validity and value of our approach.Comment: 16 pages, Keywords: Change Propagation, Dataset Dynamics, Linked
Data, Replicatio
A Theoretical Light-Curve Model for the Recurrent Nova V394 Coronae Austrinae
A theoretical light curve for the 1987 outburst of V394 Coronae Austrinae
(V394 CrA) is modeled to obtain various physical parameters of this recurrent
nova. We then apply the same set of parametersto a quiescent phase and confirm
that these parameters give a unified picture of the binary. The early visual
light curve (1-10 days after the optical maximum) is well reproduced by a
thermonuclear runaway model on a very massive WD close to the Chandrasekhar
limit (1.37 +- 0.01 M_sun). The ensuing plateau phase (10-30 days) is also
reproduced by the combination of a slightly irradiated MS and a fully
irradiated flaring-up disk with a radius ~1.4 times the Roche lobe size. The
best fit parameters are the WD mass 1.37 M_sun, the companion mass 1.5 M_sun
(0.8-2.0 M_sun is acceptable), the inclination angle of the orbit i~65-68
degree, and the flaring-up rim ~0.30 times the disk radius. The envelope mass
at the optical peak is estimated to be ~6 x 10^{-6} M_sun, which indicates an
average mass accretion rate of 1.5 x 10^{-7} M_sun yr^{-1} during the quiescent
phase between the 1949 and 1987 outbursts. In the quiescent phase, the observed
light curve can be reproduced with a disk size of 0.7 times the Roche lobe size
and a rather slim thickness of 0.05 times the accretion disk size at the rim.
About 0.5 mag sinusoidal variation of the light curve requires the mass
accretion rate higher than ~1.0 x 10^{-7} M_sun yr^{-1}, which is consistent
with the above estimation from the 1987 outburst. These newly obtained
quantities are exactly the same as those predicted in a new progenitor model of
Type Ia supernovae.Comment: 9 pages including 4 figures, to appear in the Astrophysical Journal,
Part
Changes in the long term intensity variations in Cyg X-2 and LMC X-3
We report the detection of changes in the long-term intensity variations in
two X-ray binaries, Cyg X-2 and LMC X-3. In this work, we have used the
long-term light curves obtained with the All-Sky Monitors (ASMs) of the Rossi
X-Ray Timing Explorer (RXTE), Ginga, Ariel 5, and Vela 5B and the scanning
modulation collimator of HEAO 1. It is found that in the light curves of both
the sources, obtained with these instruments at various times over the last 30
years, more than one periodic or quasi-periodic component is always present.
The multiple prominent peaks in the periodograms have frequencies unrelated to
each other. In Cyg X-2, RXTE-ASM data show strong peaks at 40.4 and 68.8 days,
and Ginga-ASM data show strong peaks at 53.7 and 61.3 days. Multiple peaks are
also observed in LMC X-3. The various strong peaks in the periodograms of LMC
X-3 appear at 104, 169, and 216 days (observed with RXTE-ASM) and 105, 214, and
328 days (observed with Ginga-ASM). The present results, when compared with the
earlier observations of periodicities in these two systems, demonstrate the
absence of any stable long period. The 78 day periodicity detected earlier in
Cyg X-2 was probably due to the short time base in the RXTE data that were
used, and the periodicity of 198 days in LMC X-3 was due to a relatively short
duration of observation with HEAO 1.Comment: 11 pages, 7 postscript figures include
A new clue to the transition mechanism between optical high and low states of the supersoft X-ray source RX J0513.9-6951, implied from the recurrent nova CI Aquilae 2000 outburst model
We have found a new clue to the transition mechanism between optical
high/X-ray off and optical low/X-ray on states of the LMC supersoft X-ray
source RX J0513.9-6951. A sharp ~1 mag drop is common to the CI Aql 2000
outburst. These drops are naturally attributed to cessation of optically thick
winds on white dwarfs. A detailed light-curve analysis of CI Aql indicates that
the size of a disk drastically shrinks when the wind stops. This causes ~1-2
mag drop in the optical light curve. In RX J0513.9-6951, the same mechanism
reproduces sharp ~1 mag drop from optical high to low states. We predict this
mechanism also works on the transition from low to high states. Interaction
between the wind and the companion star attenuates the mass transfer and drives
full cycles of low and high states.Comment: 9 pages including 5 figures, to appear in the Astrophysical Journa
Eclipsing Broad Emission Lines in Hercules X-1: Evidence for a Disk Wind?
We present disk wind model calculations for the broad emission lines seen in
the ultraviolet spectra of the X-ray binary Hercules X-1. Recent HST/STIS
observations of these lines suggest that they are kinematically linked to the
orbital motion of the neutron star and exhibit a red-shifted to blue-shifted
evolution of the line shape during the progression of the eclipse from ingress
to egress which is indicative of disk emission. Furthermore, these lines are
single-peaked which implies that they may be formed in a disk wind similar to
those we have proposed as producing the broad emission lines seen in the UV
spectra of active galactic nuclei. We compute line profiles as a function of
eclipse phase and compare them to the observed line profiles. Various effects
may modify the appearance of the lines including resonant scattering in the
wind itself, self-shadowing of the warped disk from the central continuum, and
self-obscuration of parts of the disk along the observer's line-of-sight. These
latter two effects can cause orbital and precessional phase dependent
variations in the emission lines. Hence, examination of the line profiles as a
function of these phases can, in principle, provide additional information on
the characteristics of the disk warp.Comment: 32 pages, 9 figures, LaTeX, Accepted for publication in Ap
Long-term developments in Her X-1: Correlation between the histories of the 35 day turn-on cycle and the 1.24 sec pulse period
We have studied the long-term (1971-2005) behaviour of the 1.24 sec pulse
period and the 35 day precession period of Her X-1 and show that both periods
vary in a highly correlated way (see also Staubert et al. 1997 and 2000). When
the spin-up rate decreases, the 35 day turn-on period shortens.This correlation
is most evident on long time scales (~2000 days),e.g.around four extended
spin-down episodes, but also on shorter time scales (a few 100 days) on which
quasi-periodic variations are apparent. We argue that the likely common cause
is variations of the mass accretion rate onto the neutron star.The data since
1991 allow a continuous sampling and indicate a lag between the turn-on
behaviour and the spin behaviour, in the sense that changes are first seen in
the spin, about one cycle later in the turn-on. Both the coronal wind model
(Schandl & Meyer 1994) as well as the stream-disk model (Shakura et al.999)
predict this kind of behaviour.Comment: 6 pages, 4 figures, In: The transient Milky Way: a perspective for
MIRAX, AIP Conf. Proc. 840 (2006) 65-7
On the tilting of protostellar disks by resonant tidal effects
We consider the dynamics of a protostellar disk surrounding a star in a
circular-orbit binary system. Our aim is to determine whether, if the disk is
initially tilted with respect to the plane of the binary orbit, the inclination
of the system will increase or decrease with time. The problem is formulated in
the binary frame in which the tidal potential of the companion star is static.
We consider a steady, flat disk that is aligned with the binary plane and
investigate its linear stability with respect to tilting or warping
perturbations. The dynamics is controlled by the competing effects of the m=0
and m=2 azimuthal Fourier components of the tidal potential. In the presence of
dissipation, the m=0 component causes alignment of the system, while the m=2
component has the opposite tendency. We find that disks that are sufficiently
large, in particular those that extend to their tidal truncation radii, are
generally stable and will therefore tend to alignment with the binary plane on
a time-scale comparable to that found in previous studies. However, the effect
of the m=2 component is enhanced in the vicinity of resonances where the outer
radius of the disk is such that the natural frequency of a global bending mode
of the disk is equal to twice the binary orbital frequency. Under such
circumstances, the disk can be unstable to tilting and acquire a warped shape,
even in the absence of dissipation. The outer radius corresponding to the
primary resonance is always smaller than the tidal truncation radius. For disks
smaller than the primary resonance, the m=2 component may be able to cause a
very slow growth of inclination through the effect of a near resonance that
occurs close to the disk center. We discuss these results in the light of
recent observations of protostellar disks in binary systems.Comment: 21 pages, 7 figures, to be published in the Astrophysical Journa
A Theoretical Light-Curve Model for the 1999 Outburst of U Scorpii
A theoretical light curve for the 1999 outburst of U Scorpii is presented in
order to obtain various physical parameters of the recurrent nova. Our U Sco
model consists of a very massive white dwarf (WD) with an accretion disk and a
lobe-filling, slightly evolved, main-sequence star (MS). The model includes a
reflection effect by the companion and the accretion disk together with a
shadowing effect on the companion by the accretion disk. The early visual light
curve (t ~ 1-15 days after maximum) is well reproduced by a thermonuclear
runaway model on a very massive WD close to the Chandrasekhar limit (M_{WD}=
1.37 \pm 0.01 M_\odot), in which optically thick winds blowing from the WD play
a key role in determining the nova duration. The duration of the strong wind
phase (t~0-17 days) is very consistent with the BeppoSAX supersoft X-ray
detection at t~19-20 days because supersoft X-rays are self-absorbed by the
massive wind. The envelope mass at the peak is estimated to be ~3x10^{-6}
M_\odot, which is indicating an average mass accretion rate ~2.5x10^{-7}
M_\odot yr^{-1} during the quiescent phase between 1987 and 1999. These
quantities are exactly the same as those predicted in a new progenitor model of
Type Ia supernovae.Comment: 7 pages, 3 figures, to appear in ApJL, vol. 52
- …