111 research outputs found
Accreting Millisecond X-Ray Pulsars
Accreting Millisecond X-Ray Pulsars (AMXPs) are astrophysical laboratories
without parallel in the study of extreme physics. In this chapter we review the
past fifteen years of discoveries in the field. We summarize the observations
of the fifteen known AMXPs, with a particular emphasis on the multi-wavelength
observations that have been carried out since the discovery of the first AMXP
in 1998. We review accretion torque theory, the pulse formation process, and
how AMXP observations have changed our view on the interaction of plasma and
magnetic fields in strong gravity. We also explain how the AMXPs have deepened
our understanding of the thermonuclear burst process, in particular the
phenomenon of burst oscillations. We conclude with a discussion of the open
problems that remain to be addressed in the future.Comment: Review to appear in "Timing neutron stars: pulsations, oscillations
and explosions", T. Belloni, M. Mendez, C.M. Zhang Eds., ASSL, Springer;
[revision with literature updated, several typos removed, 1 new AMXP added
A structured review of long-term care demand modelling
Long-term care (LTC) represents a significant and substantial proportion of healthcare spends across the globe. Its main aim is to assist individuals suffering with more or more chronic illnesses, disabilities or cognitive impairments, to carry out activities associated with daily living. Shifts in several economic, demographic and social factors have raised concerns surrounding the sustainability of current systems of LTC. Substantial effort has been put into modelling the LTC demand process itself so as to increase understanding of the factors driving demand for LTC and its related services. Furthermore, such modeling efforts have also been used to plan the operation and future composition of the LTC system itself. The main aim of this paper is to provide a structured review of the literature surrounding LTC demand modeling and any such industrial application, whilst highlighting any potential direction for future researchers
Optical and infrared flares from a transient Galactic soft gamma-ray repeater
Soft gamma-ray repeaters (SGRs) are a rare type of gamma-ray transient
sources that are ocasionally detected as bursts in the high-energy sky. They
are thought to be produced by magnetars, young neutron stars with very strong
magnetic fields of the order of 10^(14-15) G. Only three such objects are known
in our Galaxy, and a fourth one is associated with the supernova remnant N49 in
the Large Magellanic Cloud. In none of these cases has an optical counterpart
to either the gamma-ray flares or the quiescent source been identified. Here we
present multi-wavelength observations of a puzzling source, SWIFT
J195509+261406, for which we detected more than 40 flaring episodes in the
optical band over a time span of 3 days, plus a faint infrared flare 11 days
later, after which it returned to quiescence. We propose that SWIFT
J195509+261406 is a member of a subgroup of SGRs for which the long-term X-ray
emission is transient in nature. Furthermore, it is the first SGR for which
bursts have been detected in the optical and near-infrared bands and maybe the
link between the "persistent" SGRs and the dim isolated neutron stars.Comment: Version submitted to Nature on 31 Jan 2008. A substantially revised
version of this work has been published in Nature, vol. 455 issue 7212 pp
506-509 under the title "Flares from a Galactic magnetar suggest a missing
link to dim isolated neutron stars
Understanding interactions in face-to-face and remote undergraduate science laboratories
This paper reviews the ways in which interactions have been studied, and the findings of such studies, in science
education in both face-to-face and remote laboratories. Guided by a systematic selection process, 27 directly
relevant articles were analysed based on three categories: the instruments used for measuring interactions, the
research findings on student interactions, and the theoretical frameworks used in the studies of student
interactions. In face-to-face laboratories, instruments for measuring interactions and the characterisation of the
nature of interactions were prominent. For remote laboratories, the analysis of direct interactions was found to be
lacking. Instead, studies of remote laboratories were mainly concerned with their practical scope. In addition, it is
found that only a limited number of theoretical frameworks have been developed and applied in the research
design. Existent theories are summarised and possible theoretical frameworks that may be implemented in studies
of interactions in undergraduate laboratories are proposed. Finally, future directions for research on the interrelationship between student interactions and laboratory learning are suggested
Conformational Changes and Slow Dynamics through Microsecond Polarized Atomistic Molecular Simulation of an Integral Kv1.2 Ion Channel
Structure and dynamics of voltage-gated ion channels, in particular the motion of
the S4 helix, is a highly interesting and hotly debated topic in current
membrane protein research. It has critical implications for insertion and
stabilization of membrane proteins as well as for finding how transitions occur
in membrane proteins—not to mention numerous applications in drug
design. Here, we present a full 1 µs atomic-detail molecular dynamics
simulation of an integral Kv1.2 ion channel, comprising 120,000 atoms. By
applying 0.052 V/nm of hyperpolarization, we observe structural rearrangements,
including up to 120° rotation of the S4 segment, changes in
hydrogen-bonding patterns, but only low amounts of translation. A smaller
rotation (∼35°) of the extracellular end of all S4 segments is
present also in a reference 0.5 µs simulation without applied field,
which indicates that the crystal structure might be slightly different from the
natural state of the voltage sensor. The conformation change upon
hyperpolarization is closely coupled to an increase in 310 helix
contents in S4, starting from the intracellular side. This could support a model
for transition from the crystal structure where the hyperpolarization
destabilizes S4–lipid hydrogen bonds, which leads to the helix
rotating to keep the arginine side chains away from the hydrophobic phase, and
the driving force for final relaxation by downward translation is partly
entropic, which would explain the slow process. The coordinates of the
transmembrane part of the simulated channel actually stay closer to the recently
determined higher-resolution Kv1.2 chimera channel than the starting structure
for the entire second half of the simulation (0.5–1 µs).
Together with lipids binding in matching positions and significant thinning of
the membrane also observed in experiments, this provides additional support for
the predictive power of microsecond-scale membrane protein simulations
Multi-messenger observations of a binary neutron star merger
On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta
Localization and broadband follow-up of the gravitational-wave transient GW150914
A gravitational-wave (GW) transient was identified in data recorded by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) detectors on 2015 September 14. The event, initially designated G184098 and later given the name GW150914, is described in detail elsewhere. By prior arrangement, preliminary estimates of the time, significance, and sky location of the event were shared with 63 teams of observers covering radio, optical, near-infrared, X-ray, and gamma-ray wavelengths with ground- and space-based facilities. In this Letter we describe the low-latency analysis of the GW data and present the sky localization of the first observed compact binary merger. We summarize the follow-up observations reported by 25 teams via private Gamma-ray Coordinates Network circulars, giving an overview of the participating facilities, the GW sky localization coverage, the timeline, and depth of the observations. As this event turned out to be a binary black hole merger, there is little expectation of a detectable electromagnetic (EM) signature. Nevertheless, this first broadband campaign to search for a counterpart of an Advanced LIGO source represents a milestone and highlights the broad capabilities of the transient astronomy community and the observing strategies that have been developed to pursue neutron star binary merger events. Detailed investigations of the EM data and results of the EM follow-up campaign are being disseminated in papers by the individual teams
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