971 research outputs found
An algorithm for determining the rotation count of pulsars
We present here a simple, systematic method for determining the correct
global rotation count of a radio pulsar; an essential step for the derivation
of an accurate phase-coherent ephemeris. We then build on this method by
developing a new algorithm for determining the global rotational count for
pulsars with sparse timing data sets. This makes it possible to obtain
phase-coherent ephemerides for pulsars for which this has been impossible until
now. As an example, we do this for PSR J0024-7205aa, an extremely faint MSP
recently discovered in the globular cluster 47 Tucanae. This algorithm has the
potential to significantly reduce the number of observations and the amount of
telescope time needed to follow up on new pulsar discoveries.Comment: 13 pages in MNRAS emulation format, 7 figures. Accepted for
publication in MNRA
Chandra studies of the globular cluster 47 Tucanae: A deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars, and new constraints to r-mode instability window
We combined Chandra ACIS observations of the globular cluster 47 Tucanae
(hereafter, 47 Tuc) from 2000, 2002, and 2014-15 to create a deeper X-ray
source list, and study some of the faint radio millisecond pulsars (MSPs)
present in this cluster. We have detected 370 X-ray sources within the
half-mass radius (2.79) of the cluster, 81 of which are newly identified, by
including new data and using improved source detection techniques. The majority
of the newly identified sources are in the crowded core region, indicating
cluster membership. We associate five of the new X-ray sources with
chromospherically active BY Dra or W UMa variables identified by Albrow et al.
(2001). We present alternative positions derived from two methods, centroiding
and image reconstruction, for faint, crowded sources. We are able to extract
X-ray spectra of the recently discovered MSPs 47 Tuc aa, 47 Tuc ab, the newly
timed MSP 47 Tuc Z, and the newly resolved MSPs 47 Tuc S and 47 Tuc F.
Generally, they are well fit by black body or neutron star atmosphere models,
with temperatures, luminosities and emitting radii similar to those of other
known MSPs in 47 Tuc, though 47 Tuc aa and 47 Tuc ab reach lower X-ray
luminosities. We limit X-ray emission from the full surface of the rapidly
spinning (542 Hz) MSP 47 Tuc aa, and use this limit to put an upper bound for
amplitude of r-mode oscillations in this pulsar as
and constrain the shape of the r-mode instability window.Comment: 17 pages, 11 figures, 6 tables, Accepted for publication in MNRA
Noise-induced phenomena in riparian vegetation dynamics
Random forcing due to the river streamflow is a key element in riparian vegetation ecosystems. It influences several aspects of the riparian landscape, the most important being the morphology and water availability. In this letter, we analytically solve a stochastic model to show how hydrological random fluctuations are able to induce both statistically stable states and bimodality in vegetation behavior. These noise-induced results can contribute to explain two well-documented features of several riparian landscapes: the bell-shaped biomass distribution along riparian transects, and spatial vegetation patchiness along a river
A tight-binding model for MoS monolayers
We propose an accurate tight-binding parametrization for the band structure
of MoS monolayers near the main energy gap. We introduce a generic and
straightforward derivation for the band energies equations that could be
employed for other monolayer dichalcogenides. A parametrization that includes
spin-orbit coupling is also provided. The proposed set of model parameters
reproduce both the correct orbital compositions and location of valence and
conductance band in comparison with ab initio calculations. The model gives a
suitable starting point for realistic large-scale atomistic electronic
transport calculations.Comment: 35 pages, 8 figure
Noise-driven cooperative dynamics between vegetation and topography in riparian zones
Riparian ecosystems exhibit complex biotic and abiotic dynamics, where the triad vegetation-sediments-stream determines the ecogeomorphological features of the river landscape. Random fluctuations of the water stage are a key trait of this triad, and a number of behaviors of the fluvial environment can be understood only taking into consideration the role of noise. In order to elucidate how randomness shape riparian transects, a stochastic model that takes into account the main links between vegetation, sediments, and the stream is adopted, emphasizing the capability of vegetation to alter the plot topography. A minimalistic approach is pursued, and the probability density function of vegetation biomass is analytically evaluated in any transect plot. This probability density function strongly depends on the vegetation-topography feedback. We demonstrate how the vegetation-induced modifications of the bed topography create more suitable conditions for the survival of vegetation in a stochastically dominated environment
Effects of Atrial Fibrillation on the Coronary Flow at Different Heart Rates: A Computational Approach
Atrial fibrillation (AF) has several effects on the cardiovascular system responses. This study focuses on the consequences of AF on the coronary blood flow, by exploiting a computational approach. 2000 heartbeat periods (RR) were simulated for 5 different mean heart rates (HR), ranging from 50 to 130 bpm. The resulting flow rate signals at the coronary level were analysed through a specific set of hemodynamic parameters. Three main results emerge during AF: (i) maximal coronary flow rates modify with HR, (ii) the coronary perfusion begins to be impaired when exceeding 90-110 bpm, and (iii) the coronary perfusion pressure is not a good estimate of the coronary blood flow at HRs higher than 90-110 bpm
Long-Term Observations Of The Pulsars In 47 Tucanae – I. A Study Of Four Elusive Binary Systems
For the past couple of decades, the Parkes radio telescope has been regularly observing the millisecond pulsars in 47 Tucanae (47 Tuc). This long-term timing program was designed to address a wide range of scientific issues related to these pulsars and the globular cluster where they are located. In this paper, the first of a series, we address one of these objectives: the characterization of four previously known binary pulsars for which no precise orbital parameters were known, namely 47 Tuc P, V, W and X (pulsars 47 Tuc R and Y are discussed elsewhere). We determined the previously unknown orbital parameters of 47 Tuc V and X and greatly improved those of 47 Tuc P and W. For pulsars W and X we obtained, for the first time, full coherent timing solutions across the whole data span, which allowed a much more detailed characterization of these systems. 47 Tuc W, a well-known tight eclipsing binary pulsar, exhibits a large orbital period variability, as expected for a system of its class. 47 Tuc X turns out to be in a wide, extremely circular, 10.9-day long binary orbit and its position is ~3.8 arcmin away from the cluster center, more than three times the distance of any other pulsar in 47 Tuc. These characteristics make 47 Tuc X a very different object with respect to the other pulsars of the cluster
Characterizing the cardiovascular functions during atrial fibrillation through lumped-parameter modeling
Atrial fibrillation (AF), causing irregular and rapid heartbeats, is the most common
arrhythmia. Due to the widespread impact on the population and the disabling symptoms
related to rapid heart rate, AF is a subject of growing interest under several aspects:
statistical analyses on the heartbeat distributions, risk factors, impact on quality of life,
correlation with other cardiac pathologies. However, several key points on the
consequences induced by AF on the cardiovascular system are still not completely
understood. The proposed work aims at quantifying the impact of AF on the most relevant
cardiovascular parameters by means of a lumped-parameter modeling, paying particular
attention to the stochastic nature of the irregular heartbeats and the reduced contractility of
the heart. The global response leads to a rather impressive overall agreement with the
clinical state-of-the-art measures regarding AF: reduced cardiac output with correlated
arterial hypotension, as well as higher left atrial volume and pressure values are some of
the most representative outcomes emerging during AF. Moreover, new insights on
hemodynamic parameters such as cardiac flow rates, which are difficult to measure and
almost never offered in literature, are here provided
Pulsar J0453+1559: A Double Neutron Star System with a Large Mass Asymmetry
To understand the nature of supernovae and neutron star (NS) formation, as
well as binary stellar evolution and their interactions, it is important to
probe the distribution of NS masses. Until now, all double NS (DNS) systems
have been measured to have a mass ratio close to unity (q 0.91). Here we
report the measurement of the individual masses of the 4.07-day binary pulsar
J0453+1559 from measurements of the rate of advance of periastron and Shapiro
delay: The mass of the pulsar is 1.559(5) and that of its companion
is 1.174(4) ; q = 0.75. If this companion is also a neutron star
(NS), as indicated by the orbital eccentricity of the system (e=0.11), then its
mass is the smallest precisely measured for any such object. The pulsar has a
spin period of 45.7 ms and a spin derivative of 1.8616(7) x; from these
we derive a characteristic age of ~ 4.1 x years and a magnetic field of
~ 2.9 x G,i.e, this pulsar was mildly recycled by accretion of matter
from the progenitor of the companion star. This suggests that it was formed
with (very approximately) its current mass. Thus NSs form with a wide range of
masses, which is important for understanding their formation in supernovae. It
is also important for the search for gravitational waves released during a
NS-NS merger: it is now evident that we should not assume all DNS systems are
symmetric
Long-term observations of the pulsars in 47 Tucanae - II. Proper motions, accelerations and jerks
This paper is the second in a series where we report the results of the
long-term timing of the millisecond pulsars (MSPs) in 47 Tucanae with the
Parkes 64-m radio telescope. We obtain improved timing parameters that provide
additional information for studies of the cluster dynamics: a) the pulsar
proper motions yield an estimate of the proper motion of the cluster as a whole
(, ) and the motion of the pulsars
relative to each other. b) We measure the second spin-period derivatives caused
by the change of the pulsar line-of-sight accelerations; 47 Tuc H, U and
possibly J are being affected by nearby objects. c) For ten binary systems we
now measure changes in the orbital period caused by their acceleration in the
gravitational field of the cluster. From all these measurements, we derive a
cluster distance no smaller than 4.69 kpc and show that the
characteristics of these MSPs are very similar to their counterparts in the
Galactic disk. We find no evidence in favour of an intermediate mass black hole
at the centre of the cluster. Finally, we describe the orbital behaviour of the
four "black widow" systems. Two of them, 47 Tuc J and O, exhibit orbital
variability similar to that observed in other such systems, while for 47 Tuc I
and R the orbits seem to be remarkably stable. It appears, therefore, that not
all "black widows" have unpredictable orbital behaviour.Comment: 21 pages in journal format, 9 figures, 4 tables, accepted for
publication in MNRAS, several clarifications made and typos fixe
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