124 research outputs found
Developmental programming of congenital heart diseases and cardiovascular risk factors : Periconception epidemiologic and epigenetic studies
Radial Distribution of X-ray Point Sources near the Galactic Center
(Abridged) We present the LogN-LogS and spatial distributions of X-ray point
sources in seven Galactic Bulge (GB) fields within 4 deg from the Galactic
Center (GC). We compare the properties of 1159 X-ray point sources discovered
in our deep (100 ks) Chandra observations of three low extinction Window fields
near the GC with the X-ray sources in the other GB fields centered around Sgr
B2, Sgr C, the Arches Cluster and Sgr A* using Chandra archival data. To reduce
the systematic errors induced by the uncertain X-ray spectra of the sources
coupled with field-and-distance dependent extinction, we classify the X-ray
sources using quantile analysis and estimate their fluxes accordingly. The
result indicates the GB X-ray population is highly concentrated at the center,
more heavily than the stellar distribution models. We also compare the total
X-ray and infrared surface brightness using the Chandra and Spitzer
observations of the regions. The radial distribution of the total infrared
surface brightness from the 3.6 band m images appears to resemble the
radial distribution of the X-ray point sources better than predicted by the
stellar distribution models. Assuming a simple power law model for the X-ray
spectra, the closer to the GC the intrinsically harder the X-ray spectra
appear, but adding an iron emission line at 6.7 keV in the model allows the
spectra of the GB X-ray sources to be largely consistent across the region.
This implies that the majority of these GB X-ray sources can be of the same or
similar type. Their X-ray luminosity and spectral properties support the idea
that the most likely candidate is magnetic cataclysmic variables (CVs),
primarily intermediate polars (IPs). Their observed number density is also
consistent with the majority being IPs.Comment: 17 pages, 7 figures, 6 tables, accepted for publication in
Astrophysical Journa
Sample of LMXBs in the Galactic bulge. I. Optical and near-infrared constraints from the Virtual Observatory
We report on the archival optical and near-infrared observations of 6 low
mass X-ray binaries situated in the Galactic bulge. We processed several recent
Chandra and XMM-Newton as well as Einstein datasets of a binary systems
suspected to be ultracompact, which gave us arcsec-scale positional uncertainty
estimates. We then undertook comprehensive search in existing archives and
other Virtual Observatory resources in order to discover unpublished
optical/NIR data on these objects. We found and analysed data from ESO Archive
and UKIRT Infrared Deep Sky Survey (UKIDSS) on SLX 1735-269, 3A 1742-294, SLX
1744-299, SLX 1744-300, GX 3+1, IGR J17505-2644 systems and publish their
finding charts and optical flux constraints in this paper, as well as simple
estimates of the physical parameters of these objects.Comment: 8 pages, 6 figures, 1 table; accepted for publication in MNRA
Large amplitude variability from the persistent ultracompact X-ray binary in NGC 1851
Using archival RXTE data, we show that the ultracompact X-ray binary in NGC
1851 exhibits large amplitude X-ray flux varations of more than a factor of 10
on timescales of days to weeks and undergoes sustained periods of months where
the time-averaged luminosty varies by factors of two. Variations of this
magnitude and timescale have not been reported previously in other ultracompact
X-ray binaries. Mass transfer in ultracompact binaries is thought to be driven
by gravitational radiation and the predicted transfer rates are so high that
the disks of ultracompact binaries with orbits as short as that of this object
should not be susceptible to ionization instabilities. Therefore the
variability characteristics we observe were unexpected, and need to be
understood. We briefly discuss a few alternatives for producing the observed
variations in light of the fact that the viscous timescale of the disk is of
order a week, comparable to the shorter time scale variation that is observed
but much less than the longer term variation. We also discuss the implications
for interpretation of observations of extragalactic binaries if the type of
variability seen in the source in NGC 1851 is typical.Comment: 7 pages, 5 figures, accepted for publication in MNRA
Formation of ultra-compact X-ray binaries through circum-binary disk-driven mass transfer
The formation of ultra-compact X-ray binaries (UCXBs) has not been well
understood. Previous works show that ultra-short orbital periods ( hr) may
be reached through mass transfer driven by magnetic braking in normal
low/intermediate-mass X-ray binaries (L/IMXBs) only for an extremely small
range of initial binary parameters, which makes it difficult to account for the
rather large population of UCXBs. In this paper we report the calculated
results on mass transfer processes in L/IMXBs with a circum-binary disk. We
show that when the orbital angular momentum loss due to a circum-binary disk is
included, ultra-short orbital periods could be reached for a relatively wide
range of initial binary parameters. The results of our binary models suggest an
alternative formation channel for UCXBs.Comment: 25 pages, 8 figures, accepted for publication in Ap
On the progenitors of millisecond pulsars by the recycling evolutionary channel
The recycling model suggested that low-mass X-ray binaries (LMXBs) could
evolve into binary millisecond pulsars (BMSPs). In this work, we attempt to
investigate the progenitor properties of BMSPs formed by the recycling
evolutionary channel, and if sub-millisecond pulsars can be produced by this
channel. Using Eggleton's stellar evolution code, considering that the dead
pulsars can be spun up to a short spin period by the accreting material and
angular momentum from the donor star, we have calculated the evolution of close
binaries consisting of a neutron star and a low-mass main-sequence donor star,
and the spin evolution of NSs. In calculation, some physical process such as
the thermal and viscous instability of a accretion disk, propeller effect, and
magnetic braking are included. Our calculated results indicate that, all LMXBs
with a low-mass donor star of 1.0 - 2.0 and a short orbital period
(\la 3-4 \rm d) can form millisecond pulsars with a spin period less than 10
ms. However, it is difficult to produce sub-millisecond pulsars by this
evolutionary channel. In addition, our evolutionary scenario cannot account for
the existence of BMSPs with a long orbital period (P_{\rm orb}\ga 70-80\rm
d).Comment: 7 pages,5 figures, MNRAS in pres
Efficient interface conditions for the semi-vectorial finite-difference beam propagation method
Setting upper limits on the strength of periodic gravitational waves from PSR J1939+2134 using the first science data from the GEO 600 and LIGO detectors
Data collected by the GEO 600 and LIGO interferometric gravitational wave detectors during their first observational science run were searched for continuous gravitational waves from the pulsar J1939+2134 at twice its rotation frequency. Two independent analysis methods were used and are demonstrated in this paper: a frequency domain method and a time domain method. Both achieve consistent null results, placing new upper limits on the strength of the pulsar's gravitational wave emission. A model emission mechanism is used to interpret the limits as a constraint on the pulsar's equatorial ellipticity
Cucurbit[8]uril templated supramolecular ring structure formation and protein assembly modulation
Binary and Millisecond Pulsars
We review the main properties, demographics and applications of binary and
millisecond radio pulsars. Our knowledge of these exciting objects has greatly
increased in recent years, mainly due to successful surveys which have brought
the known pulsar population to over 1700. There are now 80 binary and
millisecond pulsars associated with the disk of our Galaxy, and a further 103
pulsars in 24 of the Galactic globular clusters. Recent highlights have been
the discovery of the first ever double pulsar system and a recent flurry of
discoveries in globular clusters, in particular Terzan 5.Comment: 77 pages, 30 figures, available on-line at
http://www.livingreviews.org/lrr-2005-
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