570 research outputs found
The mass of the neutron star in the low-mass X-ray binary 2A 1822-371
Using phase resolved spectroscopic observations obtained with the Ultraviolet
and Visual Echelle Spectrograph on ESO's Kueyen Very Large Telescope
supplemented by spectroscopic observations obtained with the Boller and Chivens
Spectrograph on the Walter Baade Magellan telescope, we found sinusoidal
radial-velocity variations with a semi-amplitude 327+-17 km/s. From previous
observations and from the fact that the epoch of minimum velocity arrived early
with respect to the epoch calculated from pulse timing we know that the
companion star is suffering from irradiation. Since we most likely observed
primarily the side of the companion star facing the observer at phase ~0.75 the
velocity quoted above is not the true radial velocity semi-amplitude of the
companion star. Assuming a uniform contribution to the line profile from this
hemisphere yields a radial velocity semi-amplitude of 280+-26 km/s for a
systemic velocity of 54+-24 km/s; if the contribution is instead weighted
somewhat more towards the side of the companion facing the X-ray source then
the true semi-amplitude is larger than this value. Together with the well
constrained inclination (81<i<84 degrees) and the mass-function determined from
pulse-timing analysis (2.03+-0.03 x 10^-2 Msun), we derive a lower limit to the
mass of the neutron star and to that of the companion star of 0.97+-0.24 Msun
and 0.33+-0.05 Msun, respectively (1 sigma; including uncertainties in the
inclination). We briefly discuss other aspects of the spectrum and the
implications of our findings.Comment: 6 pages, 2 figures, accepted for publication by MNRA
Optical spectroscopy of (candidate) ultra-compact X-ray binaries
We present (preliminary) results of our systematic spectroscopic study of
(candidate) ultra-compact X-ray binaries. Most candidates are confirmed and we
found the first optical spectra of (pure) carbon-oxygen accretion discs.Comment: To appear in the proceedings of the conference Interacting Binaries
in Cefalu, 200
Hypercompact stellar clusters: morphological renditions and spectro-photometric models
Numerical relativity predicts that the coalescence of a black hole-binary
causes the newly formed black hole to recoil, and evidence for such recoils has
been found in the gravitational waves observed during the merger of
stellar-mass black holes. Recoiling (super)massive black holes are expected to
reside in hypercompact stellar clusters (HCSCs). Simulations of galaxy assembly
predict that hundreds of HCSCs should be present in the halo of a Milky
Way-type galaxy, and a fraction of those around the Milky Way should have
magnitudes within the sensitivity limit of existing surveys. However, recoiling
black holes and their HCSCs are still waiting to be securely identified. With
the goal of enabling searches through recent and forthcoming databases, we
improve over existing literature to produce realistic renditions of HCSCs bound
to black holes with a mass of 10 M. Including the effects of a
population of blue stragglers, we simulate their appearance in Pan-STARRS and
in forthcoming images. We also derive broad-band spectra and the
corresponding multi-wavelength colours, finding that the great majority of the
simulated HCSCs fall on the colour-colour loci defined by stars and galaxies,
with their spectra resembling those of giant K-type stars. We discuss the
clusters properties, search strategies, and possible interlopers.Comment: Accepted for publication on MNRAS, 17 pages, 7 figure
A flexible rotor on flexible supports: modeling & experiments
In this study, a flexible rotor with variable support stiffness has been analyzed. Simple support models consisting of mass, spring systems are extracted from modal analysis of the isolated support and by applying static loads to the finite element model of the supports. The derived equivalent models of the supports are then implemented in the finite element based structural model which predicts the dynamic behavior of the rotor. Finally experimental modal analysis of the rotor is performed with different support stiffnesses. The experimental and theoretical results have been compared and different support modeling approaches have been examined
Thermal modeling of a mini rotor-stator system
In this study the temperature increase and heat dissipation in the air gap of a cylindrical mini rotor stator system has been analyzed. A simple thermal model based on lumped parameter thermal networks has been developed. With this model the temperature dependent air properties for the fluid-rotor interaction models have been calculated. Next the complete system has also been modeled by using computational fluid dynamics (CFD) with Ansys-CFX and Ansys. The results have been compared and the capability of the thermal networks method to calculate the temperature of the air between the rotor and stator of a high speed micro rotor has been discussed
The infrared counterpart of the Z source GX5-1
We have obtained UKIRT infrared observations of the field of the bright
Galactic Z source GX5-1. From an astrometric plate solution tied to Tycho-ACT
standards we have obtained accurate positions for the stars in our field which,
combined with an accurate radio position, have allowed us to identify the
probable infrared counterpart of GX5-1. Narrow-band photometry marginally
suggests excess Br-gamma emission in the counterpart, supporting its
association with an accretion-disc source. No significant variability is
observed in a limited number of observations. We compare the H and K magnitudes
with those of other Z sources, and briefly discuss possible sources of infrared
emission in these systems.Comment: 5 pages, 2 figures, accepted by MNRA
On the stratification of a class of specially structured matrices
We consider specially structured matrices representing optimization problems with quadratic objective functions and (finitely many) affine linear equality constraints in an n-dimensional Euclidean space. The class of all such matrices will be subdivided into subsets ['strata'], reflecting the features of the underlying optimization problems. From a differential-topological point of view, this subdivision turns out to be very satisfactory: Our strata are smooth manifolds, constituting a so-called Whitney Regular Stratification, and their dimensions can be explicitly determined. We indicate how, due to Thom's Transversality Theory, this setting leads to some fundamental results on smooth one-parameter families of linear-quadratic optimization problems with ( finitely many) equality and inequality constraints
Discovery of a Quasar with Double-Peaked Broad Balmer Emission Lines
Most massive galaxies contain a supermassive black hole (SMBH) at their
center. When galaxies merge, their SMBHs sink to the center of the new galaxy
where they are thought to eventually merge. During this process an SMBH binary
is formed. The presence of two sets of broad emission lines in the optical
spectrum of an active galactic nucleus (AGN) has been interpreted as evidence
for two broad line regions (BLR), one surrounding each SMBH in a binary. We
modeled the broad Balmer emission lines in SDSS spectra of 373 extreme
variability AGNs using one broad and several narrow Gaussian components. We
report on the discovery of SDSS J021647.53011341.5 (hereafter J0216) as a
double-peaked broad emission line source. Among the 373 AGNs there were five
sources that are known double-peaked emission line sources. Three of these have
been reported as candidate SMBH binaries in previous studies. We present all
six objects and their double-peaked broad Balmer emission lines, and discuss
the implications for a tidal disruption event (TDE) interpretation of the
extreme variability assuming the double-peaked sources are SMBH binaries.Comment: 6 pages, 1 figure, 1 table. Accepted for publication in MNRA
Dependence of kHz QPO Properties on Normal-Branch Oscillation Phase in Scorpius X-1
We analysed RXTE data of Sco X-1 which show kHz quasi-periodic oscillations
(QPOs) and the 6--8 Hz normal-branch oscillation (NBO) simultaneously.
Using power spectra of 0.03--0.5 s data segments, we find that both the upper
kHz QPO frequency and the ratio of lower to upper kHz QPO amplitude are
anticorrelated to variations in the X-ray count-rate taking place on the NBO
time scale. The frequency dependence is similar to (but probably weaker than)
that found on longer time scales, but the power ratio dependence is opposite to
it. A model where radiative stresses on the disk material, modulated at the NBO
frequency, lead to changes in can explain the data; this implies some
of the NBO flux changes originate from inside the inner disk radius. We discuss
how these findings affect our understanding of kHz QPOs and of the
low-frequency variability of low-mass X-ray binaries.Comment: 13 pages including 3 figures, accepted for publlication in ApJ
Letter
Mass models of NGC 6624 without an intermediate-mass black hole
An intermediate-mass black hole (IMBH) was recently reported to reside in the
centre of the Galactic globular cluster (GC) NGC 6624, based on timing
observations of a millisecond pulsar (MSP) located near the cluster centre in
projection. We present dynamical models with multiple mass components of NGC
6624 - without an IMBH - which successfully describe the surface brightness
profile and proper motion kinematics from the Hubble Space Telescope (HST) and
the stellar mass function at different distances from the cluster centre. The
maximum line-of-sight acceleration at the position of the MSP accommodates the
inferred acceleration of the MSP, as derived from its first period derivative.
With discrete realizations of the models we show that the higher-order period
derivatives - which were previously used to derive the IMBH mass - are due to
passing stars and stellar remnants, as previously shown analytically in
literature. We conclude that there is no need for an IMBH to explain the timing
observations of this MSP.Comment: 8 pages, 7 figures, MNRAS. Updated to match final journal styl
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