674 research outputs found
Filamentation Instability of Interacting Current Sheets in Striped Relativistic Winds: The Origin of Low Sigma?
I outline a mechanism, akin to Weibel instabilities of interpenetrating
beams, in which the neighboring current sheets in a striped wind from an
oblique rotator interact through a two stream-like mechanism (a Weibel
instability in flatland), to create an anomalous resistivity that heats the
sheets and causes the magnetic field to diffusively annihilate in the wind
upstream of the termination shock. The heating has consequences for observable
unpulsed emission from pulsars.Comment: 7 pages, 9 figures. To be published in the proceedings of ``40 Years
of Pulsars'
Theory of Pulsar Wind Nebulae
Our understanding of Pulsar Wind Nebulae (PWNe), has greatly improved in the
last years thanks to unprecedented high resolution images taken from the
HUBBLE, CHANDRA and XMM satellites. The discovery of complex but similar inner
features, with the presence of unexpected axisymmetric rings and jets, has
prompted a new investigation into the dynamics of the interaction of the pulsar
winds with the surrounding SNR, which, thanks to the improvement in the
computational resources, has let to a better understanding of the properties of
these objects. On the other hand the discovery of non-thermal emission from bow
shock PWNe, and of systems with a complex interaction between pulsar and SNR,
has led to the development of more reliable evolutionary models. I will review
the standard theory of PWNe, their evolution, and the current status in the
modeling of their emission properties, in particular I will show that our
evolutionary models are able to describe the observations, and that the X-ray
emission can now be reproduced with sufficient accuracy, to the point that we
can use these nebulae to investigate fundamental issues as the properties of
relativistic outflows and particle acceleration.Comment: 9 page, 5 figures, Proceeding of the conference "40 Years of
Pulsars", 12-17 August 2007, Montreal, Canada. (figures are not properly
displayed in .ps or .pdf version please download archive for them
Forced oscillations in relativistic accretion disks and QPOs
In this work we explore the idea that the high frequency QPOs observed in
LMXBs may be explained as a resonant coupling between the neutron star spin and
epicyclic modes of accretion disk oscillations. We propose a new model for
these QPOs based on forced oscillations induced in the accretion disk due to a
stellar asymmetric rotating gravitational or magnetic field. It is shown that
particles evolving in a rotating non-axisymmetric field are subject to three
kinds of resonances: a corotation resonance, a Lindblad resonance due to a
driving force, and a parametric resonance due to the time varying epicyclic
frequencies. These results are extends by means of 2D numerical simulations of
a simplified version of the accretion disk. The simulations are performed for
the Newtonian gravitational potential, as well as for a pseudo-general
relativistic potential, which enables us to explore the behavior of the
resonances around both rotating neutron stars and black holes. Density
perturbations are only significant in the region located close to the inner
edge of the disk near the ISCO where the gravitational or magnetic perturbation
is maximal. It is argued that the nearly periodic motion induced in the disk
will produce high quality factor QPOs.
Finally, applying this model to a typical neutron star, we found that the
strongest response occurs when the frequency difference of the two modes equals
either the spin frequency (for "slow rotators") or half of it (for "fast
rotators"). The two main excited modes may both be connected to vertical
oscillations of the disk. We emphasize that strong gravity is not needed to
excite the modes.Comment: Proceedings of the 363. WE-Heraeus Seminar on: Neutron Stars and
Pulsars (Posters and contributed talks) Physikzentrum Bad Honnef, Germany,
May.14-19, 2006, eds. W.Becker, H.H.Huang, MPE Report 291, pp.189-19
Twenty Years of Searching for (and Finding) Globular Cluster Pulsars
Globular clusters produce orders of magnitude more millisecond pulsars per
unit mass than the Galactic disk. Since the first cluster pulsar was uncovered
twenty years ago, at least 138 have been identified - most of which are binary
millisecond pulsars. Because of their origins involving stellar encounters,
many of these systems are exotic objects that would never be observed in the
Galactic disk. Examples include pulsar-main sequence binaries, extremely rapid
rotators (including the current record holder), and millisecond pulsars in
highly eccentric orbits. These systems are allowing new probes of the
interstellar medium, the equation of state of material at supra-nuclear
density, the mass distribution of neutron stars, and the dynamics of globular
clusters.Comment: 9 pages, 6 figures. Submitted review for the "40 Years of Pulsars"
conference in Montreal, Aug 2007. To be published by the AI
The Parkes Pulsar Timing Array
Detection and study of gravitational waves from astrophysical sources is a
major goal of current astrophysics. Ground-based laser-interferometer systems
such as LIGO and VIRGO are sensitive to gravitational waves with frequencies of
order 100 Hz, whereas space-based systems such as LISA are sensitive in the
millihertz regime. Precise timing observations of a sample of millisecond
pulsars widely distributed on the sky have the potential to detect
gravitational waves at nanohertz frequencies. Potential sources of such waves
include binary super-massive black holes in the cores of galaxies, relic
radiation from the inflationary era and oscillations of cosmic strings. The
Parkes Pulsar Timing Array (PPTA) is an implementation of such a system in
which 20 millisecond pulsars have been observed using the Parkes radio
telescope at three frequencies at intervals of two -- three weeks for more than
two years. Analysis of these data has been used to limit the gravitational wave
background in our Galaxy and to constrain some models for its generation. The
data have also been used to investigate fluctuations in the interstellar and
Solar-wind electron density and have the potential to investigate the stability
of terrestrial time standards and the accuracy of solar-system ephemerides.Comment: 9 pages, 6 figures, Proceedings of "40 Years of Pulsars: Millisecond
Pulsars, Magnetars and More", Montreal, August 2007. Corrected SKA detection
limi
Nearby, Thermally Emitting Neutron Stars
We describe a sample of thermally emitting neutron stars discovered in the
ROSAT All-Sky Survey. We discuss the basic observational properties of these
objects and conclude that they are nearby, middle-aged pulsars with moderate
magnetic fields that we see through their cooling radiation. While these
objects are potentially very useful as probes of matter at very high densities
and magnetic fields, our lack of understanding of their surface emission limits
their current utility. We discuss this and other outstanding problems: the
spectral evolution of one sources and the relation of this population to the
overall pulsar population.Comment: 9 pages, one table, 3 figures. To appear in the proceedings of "40
Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17,
2007, McGill University, Montreal, Canad
What makes the Crab pulsar shine?
Our high time resolution observations of individual pulses from the Crab
pulsar show that the main pulse and interpulse differ in temporal behavior,
spectral behavior, polarization and dispersion. The main pulse properties are
consistent with one current model of pulsar radio emission, namely, soliton
collapse in strong plasma turbulence. The high-frequency interpulse is quite
another story. Its dynamic spectrum cannot easily be explained by any current
emission model; its excess dispersion must come from propagation through the
star's magnetosphere. We suspect the high-frequency interpulse does not follow
the ``standard model'', but rather comes from some unexpected region within the
star's magnetosphere. Similar observations of other pulsars will reveal whether
the radio emission mechanisms operating in the Crab pulsar are unique to that
star, or can be identified in the general population.Comment: 5 pages, 2 figures, to appear in proceedings of meeting "Forty Years
of Pulsars: Millisecond Pulsars, Magnetars and More", Montreal, August 200
Constraining the neutron star equation of state using quiescent low-mass X-ray binaries
Chandra or XMM-Newton observations of quiescent low-mass X-ray binaries can
provide important constraints on the equation of state of neutron stars. The
mass and radius of the neutron star can potentially be determined from fitting
a neutron star atmosphere model to the observed X-ray spectrum. For a radius
measurement it is of critical importance that the distance to the source is
well constrained since the fractional uncertainty in the radius is at least as
large as the fractional uncertainty in the distance. Uncertainties in modelling
the neutron star atmosphere remain. At this stage it is not yet clear if the
soft thermal component in the spectra of many quiescent X-ray binaries is
variable on timescales too short to be accommodated by the cooling neutron star
scenario. This can be tested with a long XMM-Newton observation of the neutron
star X-ray transient CenX-4 in quiescence. With such an observation one can use
the Reflection Grating Spectrometer spectrum to constrain the interstellar
extinction to the source. This removes this parameter from the X-ray spectral
fitting of the EPIC pn and MOS spectra and allows one to investigate whether
the variability observed in the quiescent X-ray spectrum of this source is due
to variations in the soft thermal spectral component or variations in the power
law spectral component coupled with variations in N_H. This will test whether
the soft thermal component can indeed be due to the hot thermal glow of the
neutron star. Irrespective of the outcome of such a study, the observed cooling
in quiescence in sources for which the crust is significantly out of thermal
equilibrium with the core due to a prolonged outburst, such as KS 1731-260,
seem excellent candidates for mass and radius determinations through modelling
the observed X-rays with a neutron star atmosphere model.Comment: 7 pages, 3 figures, proceedings "40 years of pulsars" conferenc
Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision
We have measured variations in scattering time scales in the Crab Pulsar over
a 30-year period, using observations made at 610 MHz with the 42-ft telescope
at Jodrell Bank Observatory. Over more recent years, where regular Lovell
Telescope observations at frequencies around 1400 MHz were available, we have
also determined the dispersion measure variations, after disentangling the
scattering delay from the dispersive delay. We demonstrate a relationship
between scattering and dispersion measure variations, with a correlation
coefficient of . The short time scales over which these quantities
vary, the size of the variations, and the close correlation between scattering
and dispersion measure all suggest that the effects are due to discrete
structures within the Crab Nebula, with size scales of AU
(corresponding to an angular size of mas at an assumed distance of 2200
pc). We mitigate the effects of scattering on the observed pulse shape by using
the measured scattering information to modify the template used for generating
the pulse arrival times, thus improving the precision to which the pulsar can
be timed. We test this on timing data taken during periods of high scattering,
and obtain a factor of two improvement in the root mean square of the timing
residuals.Comment: 10 pages, 7 figures. Accepted for publication in MNRA
The Connection Between Low-Mass X-ray Binaries and (Millisecond) Pulsars: A Binary Evolution Perspective
I review the evolutionary connection between low-mass X-ray binaries (LMXBs)
and pulsars with binary companions (bPSRs) from a stellar binary evolution
perspective. I focus on the evolution of stellar binaries with end-states
consisting of a pulsar with a low-mass (<1.0 solar mass) companion, starting at
the point the companion's progenitor first initiates mass transfer onto the
neutron star. Whether this mass transfer is stable and the physics driving
ongoing mass transfer partitions the phase space of the companions's initial
mass and initial orbital period into five regions. The qualitative nature of
the mass-transfer process and the binary's final end-state differ between
systems in each region; four of these regions each produce a particular class
of LMXBs. I compare the theoretical expectations to the populations of galactic
field LMXBs with companion-mass constraints and field bPSRs. I show that the
population of accreting millisecond pulsars are all identified with only two of
the four LMXB classes and that these systems do not have readily identifiable
progeny in the bPSR population. I discuss which sub-populations of bPSRs can be
explained by binary evolution theory and those that currently are not. Finally
I discuss some outstanding questions in this field.Comment: 9 pages, 5 figures, to appear in the proceedings of the 40 Years of
Pulsars conference held at McGill University in August 200
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