2,116 research outputs found
Long-term Properties of Accretion Disks in X-ray Binaries: II. Stability of Radiation-Driven Warping
A significant number of X-ray binaries are now known to exhibit long-term
``superorbital'' periodicities on timescales of 10 - 100 days. Several
physical mechanisms have been proposed that give rise to such periodicities, in
particular warping and/or precession of the accretion disk. Recent theoretical
work predicts the stability to disk warping of X-ray binaries as a function of
the mass ratio, binary radius, viscosity and accretion efficiency, and here we
examine the constraints that can be placed on such models by current
observations.
In paper I we used a dynamic power spectrum (DPS) analysis of long-term X-ray
datasets (CGRO, RXTE), focusing on the remarkable, smooth variations in the
superorbital period exhibited by SMC X-1. Here we use a similar DPS analysis to
investigate the stability of the superorbital periodicities in the neutron star
X-ray binaries Cyg X-2, LMC X-4 and Her X-1, and thereby confront stability
predictions with observation. We find that the period and nature of
superorbital variations in these sources is consistent with the predictions of
warping theory.
We also use a dynamic lightcurve analysis to examine the behaviour of Her X-1
as it enters and leaves the 1999 Anomalous Low State (ALS). This reveals a
significant phase shift some 15 cycles before the ALS, which indicates a change
in the disk structure or profile leading into the ALS.Comment: 12 pages, 14 figures, Re-submitted to MNRAS after referee's comment
3 W of single-frequency output at 532 nm by intracavity frequency doubling of a diode-bar-pumped Nd:YAG ring laser 3 W of single-frequency output at 532 nm by intracavity frequency doubling of a diode-bar-pumped Nd:YAG ring laser
A beam-shaped 20W diode-bar has longitudinally pumped a Nd:YAG laser in a ring configuration. Unidirectional single-frequency operation is enforced by a Faraday rotator. Intracavity frequency doubling, using a KTP crystal has produced 3W of stable, single-frequency TEMoo output at 532nm
A Chemical Composition Survey of the Iron-Complex Globular Cluster NGC 6273 (M 19)
Recent observations have shown that a growing number of the most massive
Galactic globular clusters contain multiple populations of stars with different
[Fe/H] and neutron-capture element abundances. NGC 6273 has only recently been
recognized as a member of this "iron-complex" cluster class, and we provide
here a chemical and kinematic analysis of > 300 red giant branch (RGB) and
asymptotic giant branch (AGB) member stars using high resolution spectra
obtained with the Magellan-M2FS and VLT-FLAMES instruments. Multiple lines of
evidence indicate that NGC 6273 possesses an intrinsic metallicity spread that
ranges from about [Fe/H] = -2 to -1 dex, and may include at least three
populations with different [Fe/H] values. The three populations identified here
contain separate first (Na/Al-poor) and second (Na/Al-rich) generation stars,
but a Mg-Al anti-correlation may only be present in stars with [Fe/H] > -1.65.
The strong correlation between [La/Eu] and [Fe/H] suggests that the s-process
must have dominated the heavy element enrichment at higher metallicities. A
small group of stars with low [alpha/Fe] is identified and may have been
accreted from a former surrounding field star population. The cluster's large
abundance variations are coupled with a complex, extended, and multimodal blue
horizontal branch (HB). The HB morphology and chemical abundances suggest that
NGC 6273 may have an origin that is similar to omega Cen and M 54.Comment: Accepted for Publication in The Astrophysical Journal; 50 pages; 18
figures; 8 tables; higher resolution figures are available upon request or in
the published journal articl
X-ray behaviour of Circinus X-1 - I: X-ray Dips as a diagnostic of periodic behaviour
We examine the periodic nature of detailed structure (particularly dips) in
the RXTE/ASM lightcurve of Circinus X-1. The significant phase wandering of the
X-ray maxima suggests their identification with the response on a viscous
timescale of the accretion disk to perturbation. We find that the X-ray dips
provide a more accurate system clock than the maxima, and thus use these as
indicators of the times of periastron passage. We fit a quadratic ephemeris to
these dips, and find its predictive power for the X-ray lightcurve to be
superior to ephemerides based on the radio flares and the full archival X-ray
lightcurve. Under the hypothesis that the dips are tracers of the mass transfer
rate from the donor, we use their occurrence rate as a function of orbital
phase to explore the (as yet unconstrained) nature of the donor. The high
term in the ephemeris provides another piece of evidence that Cir X-1
is in a state of dynamical evolution, and thus is a very young post-supernova
system. We further suggest that the radio ``synchrotron nebula'' immediately
surrounding Cir X-1 is in fact the remnant of the event that created the
compact object, and discuss briefly the evidence for and against such an
interpretation.Comment: 11 pages, 11 figures, accepted for publication in MNRA
The Arches Cluster: Extended Structure and Tidal Radius
At a projected distance of ~26 pc from Sgr A*, the Arches cluster provides
insight to star formation in the extreme Galactic Center (GC) environment.
Despite its importance, many key properties such as the cluster's internal
structure and orbital history are not well known. We present an astrometric and
photometric study of the outer region of the Arches cluster (R > 6.25") using
HST WFC3IR. Using proper motions we calculate membership probabilities for
stars down to F153M = 20 mag (~2.5 M_sun) over a 120" x 120" field of view, an
area 144 times larger than previous astrometric studies of the cluster. We
construct the radial profile of the Arches to a radius of 75" (~3 pc at 8 kpc),
which can be well described by a single power law. From this profile we place a
3-sigma lower limit of 2.8 pc on the observed tidal radius, which is larger
than the predicted tidal radius (1 - 2.5 pc). Evidence of mass segregation is
observed throughout the cluster and no tidal tail structures are apparent along
the orbital path. The absence of breaks in the profile suggests that the Arches
has not likely experienced its closest approach to the GC between ~0.2 - 1 Myr
ago. If accurate, this constraint indicates that the cluster is on a prograde
orbit and is located front of the sky plane that intersects Sgr A*. However,
further simulations of clusters in the GC potential are required to interpret
the observed profile with more confidence.Comment: 24 pages (17-page main text, 7-page appendix), 24 figures, accepted
to Ap
A tree of linearisable second-order evolution equations by generalised hodograph transformations
We present a list of (1+1)-dimensional second-order evolution equations all
connected via a proposed generalised hodograph transformation, resulting in a
tree of equations transformable to the linear second-order autonomous evolution
equation. The list includes autonomous and nonautonomous equations.Comment: arXiv version is already officia
The Quintuplet Cluster: Extended Structure and Tidal Radius
The Quintuplet star cluster is one of only three known young ( Myr)
massive (M M) clusters within pc of the Galactic
Center. In order to explore star cluster formation and evolution in this
extreme environment, we analyze the Quintuplet's dynamical structure. Using the
HST WFC3-IR instrument, we take astrometric and photometric observations of the
Quintuplet covering a field-of-view, which is times
larger than those of previous proper motion studies of the Quintuplet. We
generate a catalog of the Quintuplet region with multi-band, near-infrared
photometry, proper motions, and cluster membership probabilities for
stars. We present the radial density profile of candidate Quintuplet
cluster members with M out to pc from the cluster
center. A lower limit of pc is placed on the tidal radius,
indicating the lack of a tidal truncation within this radius range. Only weak
evidence for mass segregation is found, in contrast to the strong mass
segregation found in the Arches cluster, a second and slightly younger massive
cluster near the Galactic Center. It is possible that tidal stripping hampers a
mass segregation signature, though we find no evidence of spatial asymmetry.
Assuming that the Arches and Quintuplet formed with comparable extent, our
measurement of the Quintuplet's comparatively large core radius of
pc provides strong empirical evidence that young massive
clusters in the Galactic Center dissolve on a several Myr timescale.Comment: 25 pages (21-page main text, 4-page appendix), 18 figures, submitted
to Ap
SS433:the microquasar link with ULXs?
SS433 is the prototype microquasar in the Galaxy and may even be analogous to
the ULX sources if the jets' kinetic energy is taken into account. However, in
spite of 20 years of study, our constraints on the nature of the binary system
are extremely limited as a result of the difficulty of locating spectral
features that can reveal the nature and motion of the mass donor. Newly
acquired, high resolution blue spectra taken when the (precessing) disc is
edge-on suggest that the binary is close to a common-envelope phase, and hence
providing kinematic constraints is extremely difficult. Nevertheless, we do
find evidence for a massive donor, as expected for the inferred very high mass
transfer rate, and we compare SS433's properties with those of Cyg X-3.Comment: 4 pages, 3 figures, to appear in "Compact binaries in the Galaxy and
beyond
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