1,629 research outputs found
Debye mass and heavy quark potential in a PNJL quark plasma
We calculate the Debye mass for the screening of the heavy quark potential in
a plasma of massless quarks coupled to the temporal gluon background governed
by the Polyakov loop potential within the PNJL model in RPA approximation. We
give a physical motivation for a recent phenomenological fit of lattice data by
applying the calculated Debye mass with its suppression in the confined phase
due to the Polyakov-loop to a description of the temperature dependence of the
singlet free energy for QCD with a heavy quark pair at infinite separation. We
compare the result to lattice data.Comment: 6 pages, 1 figure, contribution to Proceedings of the 6th
International Conference on "Critical Point and Onset of Deconfinement", to
appear in Phys. At. Nucl., vol. 7
On the number of solutions of a transcendental equation arising in the theory of gravitational lensing
The equation in the title describes the number of bright images of a point
source under lensing by an elliptic object with isothermal density. We prove
that this equation has at most 6 solutions. Any number of solutions from 1 to 6
can actually occur.Comment: 26 pages, 12 figure
The UTMOST pulsar timing programme I: overview and first results
We present an overview and the first results from a large-scale pulsar timing
programme that is part of the UTMOST project at the refurbished Molonglo
Observatory Synthesis Radio Telescope (MOST) near Canberra, Australia. We
currently observe more than 400 mainly bright southern radio pulsars with up to
daily cadences. For 205 (8 in binaries, 4 millisecond pulsars) we publish
updated timing models, together with their flux densities, flux density
variability, and pulse widths at 843 MHz, derived from observations spanning
between 1.4 and 3 yr. In comparison with the ATNF pulsar catalogue, we improve
the precision of the rotational and astrometric parameters for 123 pulsars, for
47 by at least an order of magnitude. The time spans between our measurements
and those in the literature are up to 48 yr, which allows us to investigate
their long-term spin-down history and to estimate proper motions for 60
pulsars, of which 24 are newly determined and most are major improvements. The
results are consistent with interferometric measurements from the literature. A
model with two Gaussian components centred at 139 and fits the transverse velocity distribution best. The pulse duty
cycle distributions at 50 and 10 per cent maximum are best described by
log-normal distributions with medians of 2.3 and 4.4 per cent, respectively. We
discuss two pulsars that exhibit spin-down rate changes and drifting subpulses.
Finally, we describe the autonomous observing system and the dynamic scheduler
that has increased the observing efficiency by a factor of 2-3 in comparison
with static scheduling.Comment: 31 pages, 14 figures, 6 tables, accepted for publication in MNRA
The first interferometric detections of Fast Radio Bursts
We present the first interferometric detections of Fast Radio Bursts (FRBs),
an enigmatic new class of astrophysical transient. In a 180-day survey of the
Southern sky we discovered 3 FRBs at 843 MHz with the UTMOST array, as part of
commissioning science during a major ongoing upgrade. The wide field of view of
UTMOST ( deg) is well suited to FRB searches. The primary beam
is covered by 352 partially overlapping fan-beams, each of which is searched
for FRBs in real time with pulse widths in the range 0.655 to 42 ms, and
dispersion measures 2000 pc cm. Detections of FRBs with the UTMOST
array places a lower limit on their distances of km (limit of
the telescope near-field) supporting the case for an astronomical origin.
Repeating FRBs at UTMOST or an FRB detected simultaneously with the Parkes
radio telescope and UTMOST, would allow a few arcsec localisation, thereby
providing an excellent means of identifying FRB host galaxies, if present. Up
to 100 hours of follow-up for each FRB has been carried out with the UTMOST,
with no repeating bursts seen. From the detected position, we present 3
error ellipses of 15 arcsec x 8.4 deg on the sky for the point of origin for
the FRBs. We estimate an all-sky FRB rate at 843 MHz above a fluence of 11 Jy ms of events sky d at the 95
percent confidence level. The measured rate of FRBs at 843 MHz is of order two
times higher than we had expected, scaling from the FRB rate at the Parkes
radio telescope, assuming that FRBs have a flat spectral index and a uniform
distribution in Euclidean space. We examine how this can be explained by FRBs
having a steeper spectral index and/or a flatter log-log
distribution than expected for a Euclidean Universe.Comment: 13 pages, 8 figures, 2 table
The enzymatic activity of the VEGFR2-receptor for the biosynthesis of dinucleoside polyphosphates
The group of dinucleoside polyphosphates encompasses a large number of molecules consisting of two nucleosides which are connected by a phosphate chain of variable length. While the receptors activated by dinucleoside polyphosphates as well as their degradation have been studied in detail, its biosynthesis has not been elucidated so far. Since endothelial cells released the dinucleoside polyphosphate uridine adenosine tetraphosphate (Up4A), we tested cytosolic proteins of human endothelial cells obtained from dermal vessels elicited for enzymatic activity. When incubated with ADP and UDP, these cells showed increasing concentrations of Up4A. The underlying enzyme was isolated by chromatography and the mass spectrometric analysis revealed that the enzymatic activity was caused by the vascular endothelial growth factor receptor 2 (VEGFR2). Since VEGFR2 but neither VEGFR1 nor VEGFR3 were capable to synthesise dinucleoside polyphosphates, Tyr-1175 of VEGFR2 is most likely essential for the enzymatic activity of interest. Further, VEGFR2-containing cells like HepG2, THP-1 and RAW264.7 were capable of synthesising dinucleoside polyphosphates. VEGFR2-transfected HEK 293T/17 but not native HEK 293T/17 cells synthesised dinucleoside polyphosphates in vivo too. The simultaneous biosynthesis of dinucleoside polyphosphates could amplify the response to VEGF, since dinucleoside polyphosphates induce cellular growth via P2Y purinergic receptors. Thus the biosynthesis of dinucleoside polyphosphates by VEGFR2 may enhance the proliferative response to VEGF. Given that VEGFR2 is primarily expressed in endothelial cells, the biosynthesis of dinucleoside polyphosphates is mainly located in the vascular system. Since the vasculature is also the main site of action of dinucleoside polyphosphates, activating vascular purinoceptors, blood vessels appear as an autocrine system with respect to dinucleoside polyphosphates. We conclude that VEGFR2 receptor is capable of synthesising dinucleoside polyphosphates. These mediators may modulate the effects of VEGFR2 due to their proliferative effects
The UTMOST: A hybrid digital signal processor transforms the MOST
The Molonglo Observatory Synthesis Telescope (MOST) is an 18,000 square meter
radio telescope situated some 40 km from the city of Canberra, Australia. Its
operating band (820-850 MHz) is now partly allocated to mobile phone
communications, making radio astronomy challenging. We describe how the
deployment of new digital receivers (RX boxes), Field Programmable Gate Array
(FPGA) based filterbanks and server-class computers equipped with 43 GPUs
(Graphics Processing Units) has transformed MOST into a versatile new
instrument (the UTMOST) for studying the dynamic radio sky on millisecond
timescales, ideal for work on pulsars and Fast Radio Bursts (FRBs). The
filterbanks, servers and their high-speed, low-latency network form part of a
hybrid solution to the observatory's signal processing requirements. The
emphasis on software and commodity off-the-shelf hardware has enabled rapid
deployment through the re-use of proven 'software backends' for its signal
processing. The new receivers have ten times the bandwidth of the original MOST
and double the sampling of the line feed, which doubles the field of view. The
UTMOST can simultaneously excise interference, make maps, coherently dedisperse
pulsars, and perform real-time searches of coherent fan beams for dispersed
single pulses. Although system performance is still sub-optimal, a pulsar
timing and FRB search programme has commenced and the first UTMOST maps have
been made. The telescope operates as a robotic facility, deciding how to
efficiently target pulsars and how long to stay on source, via feedback from
real-time pulsar folding. The regular timing of over 300 pulsars has resulted
in the discovery of 7 pulsar glitches and 3 FRBs. The UTMOST demonstrates that
if sufficient signal processing can be applied to the voltage streams it is
possible to perform innovative radio science in hostile radio frequency
environments.Comment: 12 pages, 6 figure
The UTMOST Survey for Magnetars, Intermittent pulsars, RRATs and FRBs I: System description and overview
We describe the ongoing `Survey for Magnetars, Intermittent pulsars, Rotating
radio transients and Fast radio bursts' (SMIRF), performed using the newly
refurbished UTMOST telescope. SMIRF repeatedly sweeps the southern Galactic
plane performing real-time periodicity and single-pulse searches, and is the
first survey of its kind carried out with an interferometer. SMIRF is
facilitated by a robotic scheduler which is capable of fully autonomous
commensal operations. We report on the SMIRF observational parameters, the data
analysis methods, the survey's sensitivities to pulsars, techniques to mitigate
radio frequency interference and present some early survey results. UTMOST's
wide field of view permits a full sweep of the Galactic plane to be performed
every fortnight, two orders of magnitude faster than previous surveys. In the
six months of operations from January to June 2018, we have performed
sweeps of the Galactic plane with SMIRF. Notable blind re-detections include
the magnetar PSR J16224950, the RRAT PSR J09413942 and the eclipsing
pulsar PSR J17482446A. We also report the discovery of a new pulsar, PSR
J170554. Our follow-up of this pulsar with the UTMOST and Parkes telescopes
at an average flux limit of mJy and mJy respectively,
categorizes this as an intermittent pulsar with a high nulling fraction of Comment: Submitted to MNRAS, comments welcom
Annexin A2 mediates apical trafficking of renal Na(+)-K(+)-2Cl(-)-cotransporter
The furosemide-sensitive Na(+)-K(+)-2Cl(-)-cotransporter (NKCC2) is responsible for urine concentration, and helps maintain systemic salt homeostasis. Its activity depends on trafficking to, and insertion into, the apical membrane, as well as on phosphorylation of conserved N-terminal serine and threonine residues. Vasopressin (AVP), signaling via PKA and other kinases, activates NKCC2. Association of NKCC2 with lipid rafts facilitates its AVP-induced apical translocation and activation at the surface. Lipid raft microdomains typically serve as platforms for membrane proteins to facilitate their interactions with other proteins, but little is known about partners that interact with NKCC2. Yeast two-hybrid screening identified an interaction between NKCC2 and the cytosolic protein, annexin A2 (AnxA2). Annexins mediate lipid raft-dependent trafficking of transmembrane proteins, including the AVP-regulated water channel, aquaporin 2. Here, we demonstrate that AnxA2, which binds to phospholipids in a Ca(2+)-dependent manner and may organize microdomains, is co-distributed with NKCC2 to promote its apical translocation in response to AVP stimulation and low chloride hypotonic stress. NKCC2 and AnxA2 interact in a phosphorylation-dependent manner. Phosphomimetic AnxA2 carrying a mutant, Src-dependent phosphoacceptor (AnxA2-Y24D-GFP), enhanced surface expression and raft association of NKCC2 by 5-fold upon AVP stimulation, whereas PKC-dependent AnxA2-S26D-GFP did not. As the AnxA2 effect involved only non-phosphorylated NKCC2, it appears to affect NKCC2 trafficking. Overexpression or knockdown experiments further supported the role of AnxA2 in the apical translocation and surface expression of NKCC2. In summary, this study identifies AnxA2 as a lipid raft-associated trafficking factor for NKCC2 and provides mechanistic insight into the regulation of this essential cotransporter
Image plane detection of FRB121102 with the MeerKAT radio telescope
We present the analysis of radio interferometric 2-s images from a MeerKAT
observation of the repeating fast radio burst FRB121102 on September 2019,
during which 11 distinct pulses have been previously detected using high time
and frequency resolution data cubes. In this work, we detected 6 out of the 11
bursts in the image plane at 1.48 GHz with a minimum peak signal-to-noise ratio
(S/N) of 5 {\sigma} and a fluence detection limit of 0.512 Jy ms. These
constitute the first detections of a fast radio burst (FRB) or a radio
transient using 2-s timescale images with MeerKAT data. Analysis of the fitted
burst properties revealed a weighted average precision of 1 arcsec in the
localization of the bursts. The accurate knowledge of FRB positions is
essential for identifying their host galaxy and understanding their mysterious
nature which is still unresolved to this day. We also produced 2-s images at
1.09 GHz but yielded no detection which we attributed to the spectral structure
of the pulses that are mostly higher in strength in the upper frequencies. We
also explore a new approach to difference imaging analysis (DIA) to search for
transients and find that our technique has the potential to reduce the number
of candidates and could be used to automate the detection of FRBs in the image
plane for future MeerKAT observations.Comment: The paper has already been accepted to MNRAS and we wait for proof of
review from the publisher for final publicatio
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