2,439 research outputs found
RFI Identification and Mitigation Using Simultaneous Dual Station Observations
RFI mitigation is a critically important issue in radio astronomy using
existing instruments as well as in the development of next-generation radio
telescopes, such as the Square Kilometer Array (SKA). Most designs for the SKA
involve multiple stations with spacings of up to a few thousands of kilometers
and thus can exploit the drastically different RFI environments at different
stations. As demonstrator observations and analysis for SKA-like instruments,
and to develop RFI mitigation schemes that will be useful in the near term, we
recently conducted simultaneous observations with Arecibo Observatory and the
Green Bank Telescope (GBT). The observations were aimed at diagnosing RFI and
using the mostly uncorrelated RFI between the two sites to excise RFI from
several generic kinds of measurements such as giant pulses from Crab-like
pulsars and weak HI emission from galaxies in bands heavily contaminated by
RFI. This paper presents observations, analysis, and RFI identification and
excision procedures that are effective for both time series and spectroscopy
applications using multi-station data.Comment: 12 pages, 9 figures (4 in ps and 5 in jpg formats), Accepted for
publication in Radio Scienc
Prospects for Probing the Spacetime of Sgr A* with Pulsars
The discovery of radio pulsars in compact orbits around Sgr A* would allow an
unprecedented and detailed investigation of the spacetime of the supermassive
black hole. This paper shows that pulsar timing, including that of a single
pulsar, has the potential to provide novel tests of general relativity, in
particular its cosmic censorship conjecture and no-hair theorem for rotating
black holes. These experiments can be performed by timing observations with 100
micro-second precision, achievable with the Square Kilometre Array for a normal
pulsar at frequency above 15 GHz. Based on the standard pulsar timing
technique, we develop a method that allows the determination of the mass, spin,
and quadrupole moment of Sgr A*, and provides a consistent covariance analysis
of the measurement errors. Furthermore, we test this method in detailed mock
data simulations. It seems likely that only for orbital periods below ~0.3 yr
is there the possibility of having negligible external perturbations. For such
orbits we expect a ~10^-3 test of the frame dragging and a ~10^-2 test of the
no-hair theorem within 5 years, if Sgr A* is spinning rapidly. Our method is
also capable of identifying perturbations caused by distributed mass around Sgr
A*, thus providing high confidence in these gravity tests. Our analysis is not
affected by uncertainties in our knowledge of the distance to the Galactic
center, R0. A combination of pulsar timing with the astrometric results of
stellar orbits would greatly improve the measurement precision of R0.Comment: 12 pages, 10 Figures, accepted for publication in Ap
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Itaconate modulates tricarboxylic acid and redox metabolism to mitigate reperfusion injury.
ObjectivesCerebral ischemia/reperfusion (IR) drives oxidative stress and injurious metabolic processes that lead to redox imbalance, inflammation, and tissue damage. However, the key mediators of reperfusion injury remain unclear, and therefore, there is considerable interest in therapeutically targeting metabolism and the cellular response to oxidative stress.MethodsThe objective of this study was to investigate the molecular, metabolic, and physiological impact of itaconate treatment to mitigate reperfusion injuries in in vitro and in vivo model systems. We conducted metabolic flux and bioenergetic studies in response to exogenous itaconate treatment in cultures of primary rat cortical neurons and astrocytes. In addition, we administered itaconate to mouse models of cerebral reperfusion injury with ischemia or traumatic brain injury followed by hemorrhagic shock resuscitation. We quantitatively characterized the metabolite levels, neurological behavior, markers of redox stress, leukocyte adhesion, arterial blood flow, and arteriolar diameter in the brains of the treated/untreated mice.ResultsWe demonstrate that the "immunometabolite" itaconate slowed tricarboxylic acid (TCA) cycle metabolism and buffered redox imbalance via succinate dehydrogenase (SDH) inhibition and induction of anti-oxidative stress response in primary cultures of astrocytes and neurons. The addition of itaconate to reperfusion fluids after mouse cerebral IR injury increased glutathione levels and reduced reactive oxygen/nitrogen species (ROS/RNS) to improve neurological function. Plasma organic acids increased post-reperfusion injury, while administration of itaconate normalized these metabolites. In mouse cranial window models, itaconate significantly improved hemodynamics while reducing leukocyte adhesion. Further, itaconate supplementation increased survival in mice experiencing traumatic brain injury (TBI) and hemorrhagic shock.ConclusionsWe hypothesize that itaconate transiently inhibits SDH to gradually "awaken" mitochondrial function upon reperfusion that minimizes ROS and tissue damage. Collectively, our data indicate that itaconate acts as a mitochondrial regulator that controls redox metabolism to improve physiological outcomes associated with IR injury
Pulsar bow-shock nebulae. II. Hydrodynamical simulation
We present hydrodynamical simulations, using a 2-D two component model
(ambient medium and pul sar wind have different specific heat ratios), of bow
shocks in a representative regime for pu lsar wind driven bow-shock nebulae. We
also investigate the behaviour of a passive toroidal ma gnetic field wound
around the pulsar velocity direction. Moreover we estimate the opacity of t he
bow-shock to penetration of ISM neutral hydrogen: this quantity affects
observable properti es of the nebula, like its size, shape, velocity and
surface brightness distribution. Finally we compare these numerical results
with those from an analytical model. The development of mor e realistic models
is needed in order to tune the criteria for searches of new such objects, a s
well as to interpret data on the known objects.Comment: 17 pages, Latex, 6 Encapsulated PostScript figures, accepted for
publication in A&
Time-Correlated Structure in Spin Fluctuations in Pulsars
We study statistical properties of stochastic variations in pulse arrival
times, timing noise, in radio pulsars using a new analysis method applied in
the time domain. The method proceeds in two steps. First, we subtract
low-frequency wander using a high-pass filter. Second, we calculate the
discrete correlation function of the filtered data. As a complementary method
for measuring correlations, we introduce a statistic that measures the
dispersion of the data with respect to the data translated in time. The
analysis methods presented here are robust and of general usefulness for
studying arrival time variations over timescales approaching the average
sampling interval. We apply these methods to timing data for 32 pulsars. In two
radio pulsars, PSRs B1133+16 and B1933+16, we find that fluctuations in arrival
times are correlated over timescales of 10 - 20 d with the distinct signature
of a relaxation process. Though this relaxation response could be
magnetospheric in origin, we argue that damping between the neutron star crust
and interior liquid is a more likely explanation. Under this interpretation,
our results provide the first evidence independent from pulsar spin glitches of
differential rotation in neutron stars. PSR B0950+08, shows evidence for
quasi-periodic oscillations that could be related to mode switching.Comment: 25 pages, Final journal version (MNRAS
Prospects for probing strong gravity with a pulsar-black hole system
The discovery of a pulsar (PSR) in orbit around a black hole (BH) is expected
to provide a superb new probe of relativistic gravity and BH properties. Apart
from a precise mass measurement for the BH, one could expect a clean
verification of the dragging of space-time caused by the BH spin. In order to
measure the quadrupole moment of the BH for testing the no-hair theorem of
general relativity (GR), one has to hope for a sufficiently massive BH. In this
respect, a PSR orbiting the super-massive BH in the center of our Galaxy would
be the ultimate laboratory for gravity tests with PSRs. But even for gravity
theories that predict the same properties for BHs as GR, a PSR-BH system would
constitute an excellent test system, due to the high grade of asymmetry in the
strong field properties of these two components. Here we highlight some of the
potential gravity tests that one could expect from different PSR-BH systems,
utilizing present and future radio telescopes, like FAST and SKA.Comment: Proceedings of IAUS 291 "Neutron Stars and Pulsars: Challenges and
Opportunities after 80 years", J. van Leeuwen (ed.); 6 pages, 3 figure
Unusual glitch behaviours of two young pulsars
In this paper we report unusual glitches in two young pulsars, PSR J1825-0935
(B1822-09) and PSR J1835-1106. For PSR J1825-0935, a slow glitch characterised
by a temporary decrease in the slowdown rate occurred between 2000 December 31
to 2001 December 6. This event resulted in a permanent increase in frequency
with fractional size , however little
effect remained in slowdown rate. The glitch in PSR J1835-1106 occurred
abruptly in November 2001 (MJD 52220\pm3) with
and little or no change in the
slow-down rate. A significant change in apparently occurred at the
glitch with having opposite sign for the pre- and post-glitch data.Comment: Latex format, six files, 5 pages with 4 figues. accepted for MNRA
Optimizing Pulsar Timing Arrays to Maximize Gravitational Wave Single Source Detection: a First Cut
Pulsar Timing Arrays (PTAs) use high accuracy timing of a collection of low
timing noise pulsars to search for gravitational waves in the microhertz to
nanohertz frequency band. The sensitivity of such a PTA depends on (a) the
direction of the gravitational wave source, (b) the timing accuracy of the
pulsars in the array and (c) how the available observing time is allocated
among those pulsars. Here, we present a simple way to calculate the sensitivity
of the PTA as a function of direction of a single GW source, based only on the
location and root-mean-square residual of the pulsars in the array. We use this
calculation to suggest future strategies for the current North American
Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA in its goal of
detecting single GW sources. We also investigate the affects of an additional
pulsar on the array sensitivity, with the goal of suggesting where PTA pulsar
searches might be best directed. We demonstrate that, in the case of single GW
sources, if we are interested in maximizing the volume of space to which PTAs
are sensitive, there exists a slight advantage to finding a new pulsar near
where the array is already most sensitive. Further, the study suggests that
more observing time should be dedicated to the already low noise pulsars in
order to have the greatest positive effect on the PTA sensitivity. We have made
a web-based sensitivity mapping tool available at http://gwastro.psu.edu/ptasm.Comment: 14 pages, 3 figures, accepted by Ap
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