708 research outputs found
Gravitational Wave Hotspots: Ranking Potential Locations of Single-Source Gravitational Wave Emission
The steadily improving sensitivity of pulsar timing arrays (PTAs) suggests
that gravitational waves (GWs) from supermassive black hole binary (SMBHB)
systems in the nearby universe will be de- tectable sometime during the next
decade. Currently, PTAs assume an equal probability of detection from every sky
position, but as evidence grows for a non-isotropic distribution of sources, is
there a most likely sky position for a detectable single source of GWs? In this
paper, a collection of galactic catalogs is used to calculate various metrics
related to the detectability of a single GW source resolv- able above a GW
background, assuming that every galaxy has the same probability of containing a
SMBHB. Our analyses of these data reveal small probabilities that one of these
sources is currently in the PTA band, but as sensitivity is improved regions of
consistent probability density are found in predictable locations, specifically
around local galaxy clusters.Comment: 9 pages, 9 figures, accepted for submission in Ap
Model-based asymptotically optimal dispersion measure correction for pulsar timing
In order to reach the sensitivity required to detect gravitational waves,
pulsar timing array experiments need to mitigate as much noise as possible in
timing data. A dominant amount of noise is likely due to variations in the
dispersion measure. To correct for such variations, we develop a statistical
method inspired by the maximum likelihood estimator and optimal filtering. Our
method consists of two major steps. First, the spectral index and amplitude of
dispersion measure variations are measured via a time-domain spectral analysis.
Second, the linear optimal filter is constructed based on the model parameters
found in the first step, and is used to extract the dispersion measure
variation waveforms. Compared to current existing methods, this method has
better time resolution for the study of short timescale dispersion variations,
and generally produces smaller errors in waveform estimations. This method can
process irregularly sampled data without any interpolation because of its
time-domain nature. Furthermore, it offers the possibility to interpolate or
extrapolate the waveform estimation to regions where no data is available.
Examples using simulated data sets are included for demonstration.Comment: 15 pages, 15 figures, submitted 15th Sept. 2013, accepted 2nd April
2014 by MNRAS. MNRAS, 201
Hard X-ray timing and spectral characteristics of the energetic pulsar PSR J0205+6449 in supernova remnant 3C58
PSR J0205+6449 is a young rotation-powered pulsar in SNR 3C 58. It is one of
only three young (<10,000 year old) pulsars which are so far detected in the
radio and the classical X-ray bands, as well as at hard X-rays above 20 keV and
at high-energy (>100 MeV) -rays. The other two young pulsars are the
Crab and PSR B1509-58. Our aim is to derive the timing and spectral
characteristics of PSR J0205+6449 over the broad X-ray band from ~0.5 to ~270
keV. We used all publicly available RXTE observations of PSR J0205+6449 to
first generate accurate ephemerides over the period September 30, 2000 - March
18, 2006. Next, phase-folding procedures yielded pulse profiles using data from
RXTE PCA and HEXTE, and XMM-Newton EPIC PN. While our timing solutions are
consistent with earlier results, our work shows sharper structures in the PCA
X-ray profile. The X-ray pulse profile consists of two sharp pulses, separated
in phase by 0.488(2), which can be described with 2 asymmetric Lorentzians,
each with the rising wing steeper than the trailing wing, and
full-width-half-maximum 1.41(5) ms and 2.35(22) ms, respectively. We find an
indication for a flux increase by a factor ~2, about 3.5 sigma above the
time-averaged value, for the second, weaker pulse during a two-week interval,
while its pulse shape did not change. The spectrum of the pulsed X-ray emission
is of non-thermal origin, exhibiting a power-law shape with photon index Gamma
= 1.03(2) over the energy band ~0.5 to ~270 keV. In the energy band covered
with the PCA (~3-30 keV) the spectra of the two pulses have the same photon
index, namely, 1.04(3) and 1.10(8), respectively.Comment: 10 pages; 7 figures (2 in color), resubmitted to A&A, including
referee comment
Very long baseline astrometry of PSR J1012+5307 and its implications on alternative theories of gravity
PSR J1012+5307, a millisecond pulsar in orbit with a helium white dwarf (WD),
has been timed with high precision for about 25 years. One of the main
objectives of this long-term timing is to use the large asymmetry in
gravitational binding energy between the neutron star and the WD to test
gravitational theories. Such tests, however, will be eventually limited by the
accuracy of the distance to the pulsar. Here, we present VLBI (very long
baseline interferometry) astrometry results spanning approximately 2.5 years
for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the
MSPSRPI project. These provide the first proper motion and absolute position
for PSR J1012+5307 measured in a quasi-inertial reference frame. From the VLBI
results, we measure a distance of kpc (all the estimates
presented in the abstract are at 68% confidence) for PSR J1012+5307, which is
the most precise obtained to date. Using the new distance, we improve the
uncertainty of measurements of the unmodeled contributions to orbital period
decay, which, combined with three other pulsars, places new constraints on the
coupling constant for dipole gravitational radiation
and the fractional time derivative of
Newton's gravitational constant in the local universe. As the uncertainties of the
observed decays of orbital period for the four leading pulsar-WD systems become
negligible in years, the uncertainties for and
will be improved to and
, respectively, predominantly limited by the distance
uncertainties.Comment: published in ApJ (2020ApJ...896...85D
Starch-based microspheres produced by emulsion crosslinking with a potential media dependent responsive behavior to be used as drug delivery carriers
This paper describes the development and characterization
of starch microspheres for being used as drug
delivery carriers in tissue engineering applications. The developed
starch microspheres can be further loaded with specific
growth factors and immobilized in scaffolds, or administrated
separately with scaffolds. Furthermore and due to
the processing conditions used, it is expected that these microspheres
can be also used to encapsulate living cells. The
aim of this study was to evaluate the efficacy of this methodology
for further studies with biologically active agents or
living cells. The starch microspheres were prepared using
an emulsion crosslinking technique at room temperature to
allow for the loading of biologically active agents. A preliminary
study was performed to evaluate the incorporation of
a model drug (nonsteroidal anti-inflammatory drug-NSAID)
and investigate its release profile as function of changes in
the medium parameters, such as ionic strength and pH. The
developed starch-based drug delivery system has shown to be
dependent on the ionic strength of the release medium. From
preliminary results, the release seems to be pH-dependent
due to the drug solubility. It was found that the developed
microspheres and the respective processing route are appropriate
for further studies. In fact, and based in the processing
conditions and characterization, the developed system
present a potential for the loading of different growth factors or even living cells on future studies with these systems
for improving bone regeneration in tissue engineering, especially
because the crosslinking reaction of the microspheres
takes place at room temperature
Reconciling optical and radio observations of the binary millisecond pulsar PSR J1640+2224
Previous optical and radio observations of the binary millisecond pulsar PSR
J1640+2224 have come to inconsistent conclusions about the identity of its
companion, with some observations suggesting the companion is a low-mass
helium-core (He-core) white dwarf (WD), while others indicate it is most likely
a high-mass carbon-oxygen (CO) WD. Binary evolution models predict PSR
J1640+2224 most likely formed in a low-mass X-ray binary (LMXB) based on the
pulsar's short spin period and long-period, low-eccentricity orbit, in which
case its companion should be a He-core WD with mass about , depending on metallicity. If it is instead a CO WD, that would
suggest the system has an unusual formation history. In this paper we present
the first astrometric parallax measurement for this system from observations
made with the Very Long Baseline Array (VLBA), from which we determine the
distance to be . We use this distance and a
reanalysis of archival optical observations originally taken in 1995 with the
Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) in
order to measure the WD's mass. We also incorporate improvements in
calibration, extinction model, and WD cooling models. We find that the existing
observations are not sufficient to tightly constrain the companion mass, but we
conclude the WD mass is with confidence. The limiting
factor in our analysis is the low signal-to-noise ratio of the original HST
observations.Comment: 6 pages, 5 figure
Spin frequency evolution and pulse profile variations of the recently re-activated radio magnetar XTE J1810-197
After spending almost a decade in a radio-quiet state, the Anomalous X-ray
Pulsar XTE J1810-197 turned back on in early December 2018. We have observed
this radio magnetar at 1.5 GHz with ~daily cadence since the first detection of
radio re-activation on 8 December 2018. In this paper, we report on the current
timing properties of XTE J1810-197 and find that the magnitude of the spin
frequency derivative has increased by a factor of 2.6 over our 48-day data set.
We compare our results with the spin-down evolution reported during its
previous active phase in the radio band. We also present total intensity pulse
profiles at five different observing frequencies between 1.5 and 8.4 GHz,
collected with the Lovell and the Effelsberg telescopes. The profile evolution
in our data set is less erratic than what was reported during the previous
active phase, and can be seen varying smoothly between observations. Profiles
observed immediately after the outburst show the presence of at least five
cycles of a very stable ~50-ms periodicity in the main pulse component that
lasts for at least tens of days. This remarkable structure is seen across the
full range of observing frequencies.Comment: 9 pages, 7 figures, updated with additional analysis of the 50-ms
oscillation, accepted for publication in MNRA
Chandra and RXTE studies of the X-ray/gamma-ray millisecond pulsar PSR J0218+4232
We report on high-resolution spatial and timing observations of the
millisecond pulsar PSR J0218+4232 performed with the Chandra X-ray Observatory
(CXO) and the Rossi X-ray Timing Explorer (RXTE). With these observations we
were able to study a) the possible spatial extent at X-ray energies of the DC
source coincident with PSR J0218+4232 in detail (CXO), b) the relative phasing
between the X-ray, radio and gamma-ray profiles (CXO and RXTE) and c) the
spectral properties at energies beyond 10 keV (RXTE). We found no indications
for extended emission at X-ray energies down to ~ 1 arcsec scales and confirmed
the presence of a point-like DC-component. The 2 non-thermal pulses in the
X-ray profile are found to be aligned with 2 of the 3 pulses visible at
radio-frequencies and more importantly with the two gamma-ray pulses seen in
the EGRET 100-1000 MeV pulse profile. The latter reduces now the random
occurrence probability for the detected gamma-ray signal to ~ 1.E-6, which
corresponds to a 4.9 sigma detection significance.Comment: 8 pages,7 figures, accepted for publication in Adv Sp Res:
Proceedings of the 34th COSPAR Scientific Assembly held in Housto
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