80 research outputs found
The spark-associated soliton model for pulsar radio emission
We propose a new, self-consistent theory of coherent pulsar radio emission
based on the non-stationary sparking model of Ruderman & Sutherland (1975),
modified by Gil & Sendyk (2000) in the accompanying Paper I. According to these
authors, the polar cap is populated as densely as possible by a number of
sparks with a characteristic perpendicular dimension D approximately equal to
the polar gap height scale h, separated from each other also by about h. Each
spark reappears in approximately the same place on the polar cap for a time
scale much longer than its life-time and delivers to the open magnetosphere a
sequence of electron-positron clouds which flow orderly along a flux tube of
dipolar magnetic field lines. The overlapping of particles with different
momenta from consecutive clouds leads to effective two-stream instability,
which triggers electrostatic Langmuir waves at the altitudes of about 50
stellar radii. The electrostatic oscillations are modulationally unstable and
their nonlinear evolution results in formation of ``bunch-like'' charged
solitons. A characteristic soliton length along magnetic field lines is about
30 cm, so they are capable of emitting coherent curvature radiation at radio
wavelengths. The net soliton charge is about 10^21 fundamental charges,
contained within a volume of about 10^14 cm^3. For a typical pulsar, there are
about 10^5 solitons associated with each of about 25 sparks operating on the
polar cap at any instant. One soliton moving relativisticaly along dipolar
field lines with a Lorentz factor of the order of 100 generates a power of
about 10^21 erg/s by means of curvature radiation. Then the total power of a
typical radio pulsar can be estimated as being about 10^(27-28) erg/s.Comment: 27 pages, 5 figures, accepted by Ap
Drifting subpulses and inner acceleration regions in radio pulsars
The classical vacuum gap model of Ruderman & Sutherland, in which
spark-associated subbeams of subpulse emission circulate around the magnetic
axis due to the EB drift, provides a natural and plausible physical mechanism
of the subpulse drift phenomenon. Recent progress in the analysis of drifting
subpulses in pulsars has provided a strong support to this model by revealing a
number of subbeams circulating around the magnetic axis in a manner compatible
with theoretical predictions. However, a more detailed analysis revealed that
the circulation speed in a pure vacuum gap is too high when compared with
observations. Moreover, some pulsars demonstrate significant time variations of
the drift rate, including a change of the apparent drift direction, which is
obviously inconsistent with the EB drift scenario in a pure vacuum gap. We
resolved these discrepancies by considering a partial flow of iron ions from
the positively charged polar cap, coexisting with the production of outflowing
electron-positron plasmas. By fitting the observationally deduced drift-rates
to the theoretical values, we managed to estimate polar cap surface
temperatures in a number of pulsars. The estimated surface temperatures
correspond to a small charge depletion of the order of a few percent of the
corotational charge density. We also argue that if the thermionic electron
outflow from the surface of a negatively charged polar cap is slightly below
the Goldreich-Julian density, then the resulting small charge depletion will
have similar consequences as in the case of the ions outflow. We thus believe
that the sparking discharge of a partially shielded acceleration potential drop
occurs in all pulsars, with both positively (``pulsars'') and negatively
(``anti-pulsars'') charged polar caps
On the formation of inner vacuum gaps in radio pulsars
The problem of formation of the Ruderman-Sutherland type inner vacuum gap in
neutron stars with is considered. It is argued by
means of the condition (where is the critical temperature
above which Fe ions will not be bound at the surface and is
the actual temperature of the polar cap surface heated by the back-flow of
relativistic electrons) that the inner vacuum gap can form, provided that the
actual surface magnetic field is extremaly strong ( G) and
curved ( cm), irrespective of the value of dipolar component
measured from the pulsar spin down rate. Calculations are carried out for
pulsars with drifting subpulses and/or periodic intensity modulations, in which
the existence of the quasi steady vacuum gap discharging via drifting sparks is almost unavoidable. Using different pair-production
mechanisms and different estimates of the cohesive energies of surface iron
ions, we show that it is easier to form the vacuum gap controlled by the
resonant inverse Compton scaterring seed photons than by the curvature
radiation seed photons.Comment: 14 pages, 3 figure
Modelling of surface magnetic field in neutron stars: application to radio pulsars
We propose a vacuum gap (VG) model which can be applied uniformly for normal
and high magnetic field pulsars. The model requires strong and non-dipolar
surface magnetic field near the pulsar polar cap. We assume that the actual
surface magnetic field in pulsars results from a superposition of global dipole
field and crust-anchored small scale magnetic anomaly. We provide a numerical
formalism for modelling such structures of surface magnetic field and explore
it within the framework of VG model, which requires strong surface fields more
than 10^{13} G.Comment: Submitted to A&A, 11 pages, 9 figure
Vacuum Gap Model for PSR B0943+10
PSR B0943+10 is known to show remarkably stable drifting subpulses, which can
be interpreted in terms of a circumferential motion of 20 sparks, each
completing one circulation around the periphery of the polar cap in 37 pulsar
periods. We use this observational constraint and argue that the vacuum gap
model can adequately describe the observed drift patterns. Further we
demonstrate that {\em only} the presence of strong non-dipolar surface magnetic
field can favor such vacuum gap formation. Subsequently, for the first time we
are able to constrain the parameters of the surface magnetic field, and model
the expected magnetic structure on the polar cap of PSR B0943+10 considering
the inverse Compton scattering photon dominated vacuum gap.Comment: 6 pages, 3 figures, acccepted for publication in Astronomy and
Astrophysic
Constraining gamma-ray pulsar gap models with a simulated pulsar population
With the large sample of young gamma-ray pulsars discovered by the Fermi
Large Area Telescope (LAT), population synthesis has become a powerful tool for
comparing their collective properties with model predictions. We synthesised a
pulsar population based on a radio emission model and four gamma-ray gap models
(Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic) normalizing to the
number of detected radio pulsars in select group of surveys. The luminosity and
the wide beams from the outer gaps can easily account for the number of Fermi
detections in 2 years of observations. The wide slot-gap beams requires an
increase by a factor of ~10 of the predicted luminosity to produce a reasonable
number of gamma-ray pulsars. Such large increases in the luminosity may be
accommodated by implementing offset polar caps. The narrow polar-cap beams
contribute at most only a handful of LAT pulsars. Standard distributions in
birth location and pulsar spin-down power (Edot) fail to reproduce the LAT
findings: all models under-predict the number of LAT pulsars with high Edot,
and they cannot explain the high probability of detecting both the radio and
gamma-ray beams at high Edot. The beaming factor remains close to 1 over 4
decades in Edot evolution for the slot gap whereas it significantly decreases
with increasing age for the outer gaps. The evolution of the slot-gap
luminosity with Edot is compatible with the large dispersion of gamma-ray
luminosity seen in the LAT data. The stronger evolution predicted for the outer
gap, which is linked to the polar cap heating by the return current, is
apparently not supported by the LAT data. The LAT sample of gamma-ray pulsars
therefore provides a fresh perspective on the early evolution of the luminosity
and beam width of the gamma-ray emission from young pulsars, calling for thin
and more luminous gaps.Comment: 23 pages, 21 figures, accepted for publication in A&
The characteristics of millisecond pulsar emission: I. Spectra, pulse shapes and the beaming fraction
We have monitored a large sample of millisecond pulsars using the 100-m
Effelsberg radio telescope in order to compare their radio emission properties
to the slowly rotating population. With some notable exceptions, our findings
suggest that the two groups of objects share many common properties. A
comparison of the spectral indices between samples of normal and millisecond
pulsars demonstrates that millisecond pulsar spectra are not significantly
different from those of normal pulsars. There is evidence, however, that
millisecond pulsars are slightly less luminous and less efficient radio
emitters compared to normal pulsars. We confirm recent suggestions that a
diversity exists among the luminosities of millisecond pulsars with the
isolated millisecond pulsars being less luminous than the binary millisecond
pulsars. There are indications that old millisecond pulsars exhibit somewhat
flatter spectra than the presumably younger ones. We present evidence that
millisecond pulsar profiles are only marginally more complex than those found
among the normal pulsar population. Moreover, the development of the profiles
with frequency is rather slow, suggesting very compact magnetospheres. The
profile development seems to anti-correlate with the companion mass and the
spin period, again suggesting that the amount of mass transfer in a binary
system might directly influence the emission properties. The angular radius of
radio beams of millisecond pulsars does not follow the scaling predicted from a
canonical pulsar model which is applicable for normal pulsars. Instead they are
systematically smaller. The smaller inferred luminosity and narrower emission
beams will need to be considered in future calculations of the birth-rate of
the Galactic population.Comment: 40 pages, 14 figures, accepted for publication in Ap
Life satisfaction and risk of burnout among men and women working as physiotherapists
Objectives: Recently in Poland as a result of the high rate of aging population and high rates of morbidity, a growing demand for the physiotherapist profession is observed. The results of this study can be used to formulate principles for better organization of physiotherapist's workplace in order to prevent occurrence of burnout. The aim of this study is to investigate the effect of gender on satisfaction with life and burnout among active physiotherapists. Material and Methods: The survey was anonymous and voluntary, and involved a group of 200 active physiotherapists working in health care units and educational centers in Poland. The study group was selected randomly and incidentally. Each respondent received a demographic data sheet and a set of self-rating questionnaires (Life Satisfaction Questionnaire, Burnout Scale Inventory). Results: Burnout among men decreased along with increasing satisfaction with one's work and occupation, friends, relatives and acquaintances, sexuality, and increased due to greater satisfaction with one's housing status. Burnout among women decreased along with increasing satisfaction with one's health, free time and friends, relatives and acquaintances, and increased due to work at a setting other than a health care unit or educational center. Statistical analysis failed to reveal any significant differences with regard to the BSI domains and with regard to the overall burnout index as well as with regard to the assessment of satisfaction with life between female and male physiotherapists. Conclusions: Satisfaction with children, marriage and partnership, with one's work and occupation, interactions with friends, relatives and acquaintances and sexuality may contribute to reduction of burnout among men. Women who are satisfied with their children, family, health, free time and contacts with friends, relatives and acquaintances are less prone to burnout. Weak financial situation among women and deficiency of free time among men can induce burnout. Improving staff happiness may contribute to decreasing burnout
Differential Trends in the Codon Usage Patterns in HIV-1 Genes
Host-pathogen interactions underlie one of the most complex evolutionary phenomena resulting in continual adaptive genetic changes, where pathogens exploit the host's molecular resources for growth and survival, while hosts try to eliminate the pathogen. Deciphering the molecular basis of host–pathogen interactions is useful in understanding the factors governing pathogen evolution and disease propagation. In host-pathogen context, a balance between mutation, selection, and genetic drift is known to maintain codon bias in both organisms. Studies revealing determinants of the bias and its dynamics are central to the understanding of host-pathogen evolution. We considered the Human Immunodeficiency Virus (HIV) type 1 and its human host to search for evolutionary signatures in the viral genome. Positive selection is known to dominate intra-host evolution of HIV-1, whereas high genetic variability underlies the belief that neutral processes drive inter-host differences. In this study, we analyze the codon usage patterns of HIV-1 genomes across all subtypes and clades sequenced over a period of 23 years. We show presence of unique temporal correlations in the codon bias of three HIV-1 genes illustrating differential adaptation of the HIV-1 genes towards the host preferred codons. Our results point towards gene-specific translational selection to be an important force driving the evolution of HIV-1 at the population level
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