730 research outputs found
A Deep Pulse Search in Eleven Low Mass X-Ray Binaries
We present a systematic coherent X-ray pulsation search in eleven low mass
X-ray binaries (LMXBs). We select a relatively broad variety of LMXBs,
including persistent and transient sources and spanning orbital periods between
0.3 and 17 hours. We use about 3.6 Ms of data collected by the Rossi X-Ray
Timing Explorer (RXTE) and XMM-Newton and apply a semi-coherent search strategy
to look for weak and persistent pulses in a wide spin frequency range. We find
no evidence for X-ray pulsations in these systems and consequently set upper
limits on the pulsed sinusoidal semi-amplitude between 0.14% and 0.78% for ten
outbursting/persistent LMXBs and 2.9% for a quiescent system. These results
suggest that weak pulsations might not form in (most) non-pulsating LMXBs.Comment: submitted to ApJ, 8 page
SWIGLAL: Python and Octave interfaces to the LALSuite gravitational-wave data analysis libraries
The LALSuite data analysis libraries, written in C, implement key routines critical to the successful detection of gravitational waves, such as the template waveforms describing the merger of two black holes or two neutron stars. SWIGLAL is a component of LALSuite which provides interfaces for Python and Octave, making LALSuite routines accessible directly from scripts written in those languages. It has enabled modern gravitational-wave data analysis software, used in the first detection of gravitational waves, to be written in Python, thereby benefiting from its ease of development and rich feature set, while still having access to the computational speed and scientific trustworthiness of the routines provided by LALSuite
Implementation and characterization of BinaryWeave: A new search pipeline for continuous gravitational waves from Scorpius X-1
Scorpius X-1 (Sco X-1) has long been considered one of the most promising targets for detecting continuous gravitational waves with ground-based detectors. Observational searches for Sco X-1 have achieved substantial sensitivity improvements in recent years, to the point of starting to rule out emission at the torque-balance limit in the low-frequency range \sim 40--180 Hz. In order to further enhance the detection probability, however, there is still much ground to cover for the full range of plausible signal frequencies \sim 20--1500 Hz, as well as a wider range of uncertainties in binary orbital parameters. Motivated by this challenge, we have developed BinaryWeave, a new search pipeline for continuous waves from a neutron star in a known binary system such as Sco X-1. This pipeline employs a semi-coherent StackSlide F-statistic using efficient lattice-based metric template banks, which can cover wide ranges in frequency and unknown orbital parameters. We present a detailed timing model and extensive injection-and-recovery simulations that illustrate that the pipeline can achieve high detection sensitivities over a significant portion of the parameter space when assuming sufficiently large (but realistic) computing budgets. Our studies further underline the need for stricter constraints on the Sco X-1 orbital parameters from electromagnetic observations, in order to be able to push sensitivity below the torque-balance limit over the entire range of possible source parameters
Deep searches for X-ray pulsations from Scorpius X-1 and Cygnus X-2 in support of continuous gravitational wave searches
Neutron stars in low mass X-ray binaries are hypothesised to emit continuous
gravitational waves that may be detectable by ground-based observatories. The
torque balance model predicts that a higher accretion rate produces
larger-amplitude gravitational waves, hence low mass X-ray binaries with high
X-ray flux are promising targets for gravitational wave searches. The detection
of X-ray pulsations would identify the spin frequency of these neutron stars,
and thereby improve the sensitivity of continuous gravitational-wave searches
by reducing the volume of the search parameter space. We perform a
semi-coherent search for pulsations in the two low mass X-ray binaries Scorpius
X-1 and Cygnus X-2 using X-ray data from the \textit{ Rossi X-ray Timing
Explorer} Proportional Counter Array. We find no clear evidence for pulsations,
and obtain upper limits (at confidence) on the fractional pulse
amplitude, with the most stringent being for Scorpius X-1 and
for Cygnus X-2. These upper limits improve upon those of Vaughan et
al. (1994) by factors of and respectively.Comment: 10 pages, 11 figure
Valency of rare earths in RIn3 and RSn3: Ab initio analysis of electric-field gradients
In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which
are divalent. This was experimentally determined in 1977 by perturbed angular
correlation measurements of the electric-field gradient on a 111Cd impurity. At
that time, the data were interpreted using a point charge model, which is now
known to be unphysical and unreliable. This makes the valency determination
potentially questionable. We revisit these data, and analyze them using ab
initio calculations of the electric-field gradient. From these calculations,
the physical mechanism that is responsible for the influence of the valency on
the electric-field gradient is derived. A generally applicable scheme to
interpret electric-field gradients is used, which in a transparent way
correlates the size of the field gradient with chemical properties of the
system.Comment: 10 page
N-dimensional electron in a spherical potential: the large-N limit
We show that the energy levels predicted by a 1/N-expansion method for an
N-dimensional Hydrogen atom in a spherical potential are always lower than the
exact energy levels but monotonically converge towards their exact eigenstates
for higher ordered corrections. The technique allows a systematic approach for
quantum many body problems in a confined potential and explains the remarkable
agreement of such approximate theories when compared to the exact numerical
spectrum.Comment: 8 pages, 1 figur
Searching for Galactic White Dwarf Binaries in Mock LISA Data using an F-Statistic Template Bank
We describe an F-statistic search for continuous gravitational waves from
galactic white-dwarf binaries in simulated LISA Data. Our search method employs
a hierarchical template-grid based exploration of the parameter space. In the
first stage, candidate sources are identified in searches using different
simulated laser signal combinations (known as TDI variables). Since each source
generates a primary maximum near its true "Doppler parameters" (intrinsic
frequency and sky position) as well as numerous secondary maxima of the
F-statistic in Doppler parameter space, a search for multiple sources needs to
distinguish between true signals and secondary maxima associated with other,
"louder" signals. Our method does this by applying a coincidence test to reject
candidates which are not found at nearby parameter space positions in searches
using each of the three TDI variables. For signals surviving the coincidence
test, we perform a fully coherent search over a refined parameter grid to
provide an accurate parameter estimation for the final candidates. Suitably
tuned, the pipeline is able to extract 1989 true signals with only 5 false
alarms. The use of the rigid adiabatic approximation allows recovery of signal
parameters with errors comparable to statistical expectations, although there
is still some systematic excess with respect to statistical errors expected
from Gaussian noise. An experimental iterative pipeline with seven rounds of
signal subtraction and re-analysis of the residuals allows us to increase the
number of signals recovered to a total of 3419 with 29 false alarms.Comment: 29 pages, 11 figures; submitted to Classical and Quantum Gravit
The very faint X-ray binary IGR J17062-6143: a truncated disc, no pulsations, and a possible outflow
We present a comprehensive X-ray study of the neutron star low-mass X-ray binary IGR J17062-6143, which has been accreting at low luminosities since its discovery in 2006. Analysing NuSTAR, XMM–Newton, and Swift observations, we investigate the very faint nature of this source through three approaches: modelling the relativistic reflection spectrum to constrain the accretion geometry, performing high-resolution X-ray spectroscopy to search for an outflow, and searching for the recently reported millisecond X-ray pulsations. We find a strongly truncated accretion disc at
77+22−18
gravitational radii (∼164 km) assuming a high inclination, although a low inclination and a disc extending to the neutron star cannot be excluded. The high-resolution spectroscopy reveals evidence for oxygen-rich circumbinary material, possibly resulting from a blueshifted, collisionally ionized outflow. Finally, we do not detect any pulsations. We discuss these results in the broader context of possible explanations for the persistent faint nature of weakly accreting neutron stars. The results are consistent with both an ultra-compact binary orbit and a magnetically truncated accretion flow, although both cannot be unambiguously inferred. We also discuss the nature of the donor star and conclude that it is likely a CO or O–Ne–Mg white dwarf, consistent with recent multiwavelength modelling
Reduced order modelling in searches for continuous gravitational waves - I. Barycentring time delays
The frequencies and phases of emission from extra-solar sources measured by Earth-bound observers are modulated by the motions of the observer with respect to the source, and through relativistic effects. These modulations depend critically on the source's sky-location. Precise knowledge of the modulations are required to coherently track the source's phase over long observations, for example, in pulsar timing, or searches for continuous gravitational waves. The modulations can be modelled as sky-location and time-dependent time delays that convert arrival times at the observer to the inertial frame of the source, which can often be the Solar system barycentre. We study the use of reduced order modelling for speeding up the calculation of this time delay for any sky-location. We find that the time delay model can be decomposed into just four basis vectors, and with these the delay for any sky-location can be reconstructed to sub-nanosecond accuracy. When compared to standard routines for time delay calculation in gravitational wave searches, using the reduced basis can lead to speed-ups of 30 times. We have also studied components of time delays for sources in binary systems. Assuming eccentricities <0.25, we can reconstruct the delays to within 100 s of nanoseconds, with best case speed-ups of a factor of 10, or factors of two when interpolating the basis for different orbital periods or time stamps. In long-duration phase-coherent searches for sources with sky-position uncertainties, or binary parameter uncertainties, these speed-ups could allow enhancements in their scopes without large additional computational burdens
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