4,673 research outputs found
Formation of Double Neutron Stars, Millisecond Pulsars and Double Black Holes
The 1982 model for the formation of the Hulse-Taylor binary radio pulsar PSR
B1913+16 is described, which since has become the standard model for the
formation of double neutron stars, confirmed by the 2003 discovery of the
double pulsar system PSR J0737-3039AB. A brief overview is given of the present
status of our knowledge of the double neutron stars, of which 15 systems are
presently known. The binary-recycling model for the formation of millisecond
pulsars is described, as put forward independently by Alpar et al. (1982),
Radhakrishnan and Srinivasan (1982) and Fabian et al. (1983). This now is the
standard model for the formation of these objects, confirmed by the discovery
in 1998 of the accreting millisecond X-ray pulsars. It is noticed that the
formation process of close double black holes has analogies to that of close
double neutron stars, extended to binaries of larger iinitial component masses,
although there are also considerable differences in the physics of the binary
evolution at these larger masses.Comment: Has appeared in Journal of Astrophysics and Astronomy special issue
on 'Physics of Neutron Stars and Related Objects', celebrating the 75th birth
year of G. Srinivasa
Arbitrary phase control of X-ray pulses with zeptosecond phase stability
Until recently, advances in coherent quantum control have been restricted to the optical and other low-frequency domains, because of the lack of coherent light sources at higher photon energies. Now, coherent control has been demonstrated in the hard X-ray region for the first time, using a target of Mössbauer nuclei to convert a synchrotron pulse into a double pulse whose relative phase is tunable by mechanical motion of the target. This control scheme has the necessary fewzeptosecond phase stability. In this thesis, first, the data analysis procedure underlying this phase stability result is investigated, to ensure it is not a limiting factor in reaching the maximum possible phase stability. Second, a proof of principle of arbitrary control of the relative double pulse phase is demonstrated experimentally. This demonstration of arbitrary X-ray phase control enables more general manipulation of nuclear quantum dynamics and opens the door to the arbitrary shaping of X-ray pulses
Could 2S 0114+650 be a magnetar?
We investigate the spin evolution of the binary X-ray pulsar 2S 0114+650,
which possesses the slowest known spin period of hours. We argue
that, to interpret such long spin period, the magnetic field strength of this
pulsar must be initially \gsim 10^{14} G, that is, it was born as a magnetar.
Since the pulsar currently has a normal magnetic field G, our
results present support for magnetic field decay predicted by the magnetar
model.Comment: 7 pages, 1 figure, accepted for publication in ApJ
Long Gamma-Ray Burst Progenitors: Boundary Conditions and Binary Models
The observed association of Long Gamma-Ray Bursts (LGRBs) with peculiar Type
Ic supernovae gives support to Woosley`s collapsar/hypernova model, in which
the GRB is produced by the collapse of the rapidly rotating core of a massive
star to a black hole. The association of LGRBs with small star-forming galaxies
suggests low-metallicity to be a condition for a massive star to evolve to the
collapsar stage. Both completely-mixed single star models and binary star
models are possible. In binary models the progenitor of the GRB is a massive
helium star with a close companion. We find that tidal synchronization during
core-helium burning is reached on a short timescale (less than a few
millennia). However, the strong core-envelope coupling in the subsequent
evolutionary stages is likely to rule out helium stars with main-sequence
companions as progenitors of hypernovae/GRBs. On the other hand, helium stars
in close binaries with a neutron-star or black-hole companion can, despite the
strong core-envelope coupling in the post-helium burning phase, retain
sufficient core angular momentum to produce a hypernova/GRB.Comment: 8 pp., 2 figs, Proceedings of 5th Stromlo Symposiu
New Direct Observational Evidence for Kicks in SNe
We present an updated list of direct strong evidence in favour of kicks being
imparted to newborn neutron stars. In particular we discuss the new cases of
evidence resulting from recent observations of the X-ray binary Circinus X-1
and the newly discovered binary radio pulsar PSR J1141-6545. We conclude that
the assumption that neutron stars receive a kick velocity at their formation is
unavoidable (van den Heuvel & van Paradijs 1997).Comment: 2 pages, to appear in the proceedings of the IAU Colloq. 177 "Pulsar
Astronomy - 2000 and beyond
Beam Energy Considerations for Gold Nano-Particle Enhanced Radiation Treatment
Purpose: A novel approach using nano technology enhanced radiation modalities
is investigated. The proposed methodology uses antibodies labeled with
organically inert metals with a high atomic number. Irradiation using photons
with energies in the kilo--electron volt (keV) range show an increase in dose
due to a combination of an increase in photo-electric interactions and a
pronounced generation of Auger and/or Coster-Kronig (A-CK) electrons.
Methods: The dependency of the dose deposition on various factors is
investigated using Monte Carlo simulation models. The factors investigated
include: agent concentration, spectral dependency looking at mono--energetic
sources as well as classical bremsstrahlung sources. The optimization of the
energy spectrum is performed in terms of physical dose enhancement as well as
the dose deposited by Auger and/or Coster-Kronig electrons and their biological
effectiveness.
Results: A quasi-linear dependency on concentration and an exponential
decrease within the target medium is observed. The maximal dose enhancement is
dependent on the position of the target in the beam. Apart from irradiation
with low photon energies (10 - 20 keV) there is no added benefit from the
increase in generation of Auger electrons. Interestingly, a regular 110kVp
bremsstrahlung spectrum shows a comparable enhancement in comparison with the
optimized mono--energetic sources.
Conclusions: In conclusion we find that the use of nano-particle enhanced
shows promise to be implemented quite easily in regular clinic on a physical
level due to the advantageous properties in classical beams.Comment: Preprint submitted to Phys Med Bio
Navigation of brain networks
Understanding the mechanisms of neural communication in large-scale brain
networks remains a major goal in neuroscience. We investigated whether
navigation is a parsimonious routing model for connectomics. Navigating a
network involves progressing to the next node that is closest in distance to a
desired destination. We developed a measure to quantify navigation efficiency
and found that connectomes in a range of mammalian species (human, mouse and
macaque) can be successfully navigated with near-optimal efficiency (>80% of
optimal efficiency for typical connection densities). Rewiring network topology
or repositioning network nodes resulted in 45%-60% reductions in navigation
performance. Specifically, we found that brain networks cannot be progressively
rewired (randomized or clusterized) to result in topologies with significantly
improved navigation performance. Navigation was also found to: i) promote a
resource-efficient distribution of the information traffic load, potentially
relieving communication bottlenecks; and, ii) explain significant variation in
functional connectivity. Unlike prevalently studied communication strategies in
connectomics, navigation does not mandate biologically unrealistic assumptions
about global knowledge of network topology. We conclude that the wiring and
spatial embedding of brain networks is conducive to effective decentralized
communication. Graph-theoretic studies of the connectome should consider
measures of network efficiency and centrality that are consistent with
decentralized models of neural communication
Comment on "A non-interacting low-mass black hole -- giant star binary system"
Thompson et al. (Reports, 1 November 2019, p. 637, Science) interpreted the
unseen companion of the red giant star 2MASS J05215658+4359220 as most likely a
black hole. We argue that if the red giant is about one solar mass, its
companion can be a close binary consisting of two main-sequence stars. This
would explain why no X-ray emission is detected from the system.Comment: 3 pages, Author version of Technical Comment published in Science on
8 May, 202
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