2,074 research outputs found
Outburst Morphology in the Soft X-ray Transient Aquila X-1
We present optical and near-IR (OIR) observations of the major outbursts of
the neutron star soft X-ray transient binary system Aquila X-1, from summer
1998 -- fall 2007. The major outbursts of the source over the observed timespan
seem to exhibit two main types of light curve morphologies, (a) the classical
Fast-Rise and Exponential-Decay (FRED) type outburst seen in many soft X-ray
transients and (b) the Low-Intensity State (LIS) where the
optical-to-soft-X-ray flux ratio is much higher than that seen during a FRED.
Thus there is no single correlation between the optical (R-band) and soft X-ray
(1.5-12 keV, as seen by the ASM onboard RXTE) fluxes even within the hard state
for Aquila X-1, suggesting that LISs and FREDs have fundamentally different
accretion flow properties. Time evolution of the OIR fluxes during the major
LIS and FRED outbursts is compatible with thermal heating of the irradiated
outer accretion disk. No signature of X-ray spectral state changes or any
compact jet are seen in the OIR, showing that the OIR color-magnitude diagram
(CMD) can be used as a diagnostic tool to separate thermal and non-thermal
radiation from X-ray binaries where orbital and physical parameters of the
system are reasonably well known. We suggest that the LIS may be caused by
truncation of the inner disk in a relatively high mass accretion state,
possibly due to matter being diverted into a weak outflow.Comment: Accepted for publication in ApJ. Uses aastex. 27 pages and 10 figure
Merging -means with hierarchical clustering for identifying general-shaped groups
Clustering partitions a dataset such that observations placed together in a
group are similar but different from those in other groups. Hierarchical and
-means clustering are two approaches but have different strengths and
weaknesses. For instance, hierarchical clustering identifies groups in a
tree-like structure but suffers from computational complexity in large datasets
while -means clustering is efficient but designed to identify homogeneous
spherically-shaped clusters. We present a hybrid non-parametric clustering
approach that amalgamates the two methods to identify general-shaped clusters
and that can be applied to larger datasets. Specifically, we first partition
the dataset into spherical groups using -means. We next merge these groups
using hierarchical methods with a data-driven distance measure as a stopping
criterion. Our proposal has the potential to reveal groups with general shapes
and structure in a dataset. We demonstrate good performance on several
simulated and real datasets.Comment: 16 pages, 1 table, 9 figures; accepted for publication in Sta
Magnetic and orbital order in overdoped bilayer manganites
The magnetic and orbital orders for the bilayer manganites in the doping
region have been investigated from a model that incorporates the
two orbitals at each Mn site, the inter-orbital Coulomb interaction and
lattice distortions. The usual double exchange operates via the orbitals.
It is shown that such a model reproduces much of the phase diagram recently
obtained for the bilayer systems in this range of doping. The C-type phase with
() spin order seen by Ling et al. appears as a natural consequence
of the layered geometry and is stabilised by the static distortions of the
system. The orbital order is shown to drive the magnetic order while the
anisotropic hopping across the orbitals, layered nature of the underlying
structure and associated static distortions largely determine the orbital
arrangements.Comment: 8 pages, 5 figure
Identification of two new HMXBs in the LMC: a 2013 s pulsar and a probable SFXT
We report on the X-ray and optical properties of two high-mass X-ray binary
systems located in the Large Magellanic Cloud (LMC). Based on the obtained
optical spectra, we classify the massive companion as a supergiant star in both
systems. Timing analysis of the X-ray events collected by XMM-Newton revealed
the presence of coherent pulsations (spin period 2013 s) for XMMU
J053108.3-690923 and fast flaring behaviour for XMMU J053320.8-684122. The
X-ray spectra of both systems can be modelled sufficiently well by an absorbed
power-law, yielding hard spectra and high intrinsic absorption from the
environment of the systems. Due to their combined X-ray and optical properties
we classify both systems as SgXRBs: the 19 confirmed X-ray pulsar
and a probable supergiant fast X-ray transient in the LMC, the second such
candidate outside our Galaxy.Comment: 12 pages, 10 figures, accepted for publication in MNRA
The appearance of a compact jet in the soft-intermediate state of 4U 1543-47
Recent advancements in the understanding of jet-disc coupling in black hole
candidate X-ray binaries (BHXBs) have provided close links between radio jet
emission and X-ray spectral and variability behaviour. In 'soft' X-ray states
the jets are suppressed, but the current picture lacks an understanding of the
X-ray features associated with the quenching or recovering of these jets. Here
we show that a brief, ~4 day infrared (IR) brightening during a predominantly
soft X-ray state of the BHXB 4U 1543-47 is contemporaneous with a strong X-ray
Type B quasi-periodic oscillation (QPO), a slight spectral hardening and an
increase in the rms variability, indicating an excursion to the
soft-intermediate state (SIMS). This IR 'flare' has a spectral index consistent
with optically thin synchrotron emission and most likely originates from the
steady, compact jet. This core jet emitting in the IR is usually only
associated with the hard state, and its appearance during the SIMS places the
'jet line' between the SIMS and the soft state in the hardness-intensity
diagram for this source. IR emission is produced in a small region of the jets
close to where they are launched (~ 0.1 light-seconds), and the timescale of
the IR flare in 4U 1543-47 is far too long to be caused by a single, discrete
ejection. We also present a summary of the evolution of the jet and X-ray
spectral/variability properties throughout the whole outburst, constraining the
jet contribution to the X-ray flux during the decay.Comment: Accepted to MNRAS. 11 pages, 6 figure
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