154 research outputs found
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
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
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
Probing the outer edge of an accretion disk : a Her X-1 turn-on observed with RXTE
We present the analysis of Rossi X-ray Timing Explorer (RXTE) observations of the turn-on phase of a 35 day cycle of the X-ray binary Her X-1. During the early phases of the turn-on, the energy spectrum is composed of X-rays scattered into the line of sight plus heavily absorbed X-rays. The energy spectra in the 3â17 keV range can be described by a partial covering model, where one of the components is influenced by photoelectric absorption and Thomson scattering in cold material plus an iron emission line at 6.5 keV. In this paper we show the evolution of spectral parameters as well as the evolution of the pulse profile during the turn-on. We describe this evolution using Monte Carlo simulations which self-consistently describe the evolution of the X-ray pulse profile and of the energy spectrum
The 1999 Hercules X-1 Anomalous Low State
A failed main-on in the 35d cycle of Her X-1 was observed with the Rossi
X-Ray Timing Explorer (RXTE) on 1999 April 26. Exceptions to the normal 35d
cycle have been seen only twice before; in 1983 and again 1993. We present
timing and spectral results of this latest Anomalous Low State (ALS) along with
comparisons to the main-on and normal low states. Pulsations were observed in
the 3-18 keV band with a fractional RMS variation of (0.037+-0.003). Spectral
analysis indicates that the ALS spectrum has the same shape as the main-on but
is modified by heavy absorption and scattering. We find that 70% of the
observed emission has passed through a cold absorber (N_H=5.0x10^{23}cm^{-2}).
This partially absorbing spectral fit can be applied to the normal low state
with similar results. We find that the ALS observations may be interpreted as a
decrease in inclination of the accretion disk causing the central X-Ray source
to be obscured over the entire 35d cycle.Comment: revised text, 6 revised figures, accepted for publication in Ap
On the enigmatic X-ray Source V1408 Aql (=4U 1957+11)
Models for the characteristically soft X-ray spectrum of the compact X-ray
source V1498 Aql (=4U 1957+11) have ranged from optically thick Comptonization
to multicolor accretion disk models. We critically examine the X-ray spectrum
of V1408 Aql via archival Advanced Satellite for Cosmology and Astrophysics
(ASCA) data, archival Roentgensatellit (ROSAT) data, and recent Rossi X-Ray
Timing Explorer (RXTE) data. Although we are able to fit a variety of X-ray
spectral models to these data, we favor an interpretation of the X-ray spectrum
as being due to an accretion disk viewed at large inclination angles. Evidence
for this hypothesis includes long term (117 day, 235 day, 352 day)
periodicities seen by the RXTE All Sky Monitor (ASM), which we interpret as
being due to a warped precessing disk, and a 1 keV feature in the ASCA data,
which we interpret as being the blend of L fluorescence features from a disk
atmosphere or wind. We also present timing analysis of the RXTE data and find
upper limits of 4% for the root mean square (rms) variability between
f=0.001-16 Hz. The situation of whether the compact object is a black hole or
neutron star is still ambiguous; however, it now seems more likely that an
X-ray emitting, warped accretion disk is an important component of this system.Comment: High Frequency Power Spectrum corrected for unflagged `data dropouts'
(described in Appendix) and correct upper limits for variability presented.
All energy spectra and long term variability sections unchanged. Additional
references and acknowledgements added. 13 pages in emulateapj.st
Continuous monitoring of pulse period variations in Her X-1 using Swift/BAT
Context: Monitoring of pulse period variations in accreting binary pulsars is
an important tool to study the interaction between the magnetosphere of the
neutron star and the accretion disk. While the X-ray flux of the brightest
X-ray pulsars have been successfully monitored over many years (e.g. with
RXTE/ASM, CGRO/BATSE, Swift/BAT), the possibility to monitor their pulse timing
properties continuously has so far been very limited. Aims: In our work we show
that the Swift/BAT observations can be used to monitor coherent pulsations of
bright X-ray sources and use the Swift archival data to study one of the most
enigmatic X-ray pulsars, Hercules X-1. A quasi-continuous monitoring of the
pulse period and the pulse period derivative of an X-ray pulsar, here Her X-1,
is achieved over a long time (<~ 4 yrs). We compare our observational results
with predictions of accretion theory and use them to test different aspects of
the physical model of the system. Methods: In our analysis we use the data
accumulated with Swift/BAT starting from the beginning of 2005 (shortly after
launch) until the present time. To search for pulsations and for their
subsequent analysis we used the count rate measured by the BAT detector in the
entire field of view. Results: The slope of the correlation between the locally
determined spin-up rate and the X-ray luminosity is measured for Her X-1 and
found to be in agreement with predictions of basic accretion torque theory. The
observed behaviour of the pulse period together with the previously measured
secular decrease of the system's orbital period is discussed in the frame of a
model assuming ejection of matter close to the inner boundary of the accretion
disk.Comment: 7 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Six supersoft X-ray binaries: system parameters and twin-jet outflows
A comparison is made between the properties of CAL 83, CAL 87, RX
J0513.9-6951, 1E 0035.4-7230 (SMC 13), RX J0019.8+2156, and RX J0925.7-4758,
all supersoft X-ray binaries. Spectra with the same resolution and wavelength
coverage of these systems are compared and contrasted. Some new photometry is
also presented. The equivalent widths of the principal emission lines of H and
He II differ by more than an order of magnitude among these sources, although
those of the highest ionization lines (e.g. O VI) are very similar. In
individual systems, the velocity curves derived from various ions often differ
in phasing and amplitude, but those whose phasing is consistent with the light
curves (implying the lines are formed near the compact star) give masses of
and for the degenerate and mass-losing
stars, respectively. This finding is in conflict with currently prevailing
theoretical models for supersoft binaries. The three highest luminosity sources
show evidence of "jet" outflows, with velocities of .
In CAL 83 the shape of the He II 4686\AA profile continues to show evidence
that these jets may precess with a period of days.Comment: 27 pages including 5 tables, plus 6 figures. To appear in Ap
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