647 research outputs found
Detection of the radial velocity curve of the B5-A0 supergiant companion star of Cir X-1?
In this Paper we report on phase resolved I-band optical spectroscopic and
photometric observations of CirX-1 obtained with the Very Large Telescope. The
spectra are dominated by Paschen absorption lines at nearly all orbital phases
except near phase zero (coinciding with the X-ray dip) when the absorption
lines are filled-in by broad Paschen emission lines. The radial velocity curve
of the absorption lines corresponds to an eccentric orbit (e=0.45) whose period
and time of periastron passage are consistent with the period and phase
predicted by the most recent X-ray dip ephemeris. We found that the I-band
magnitude decreases from 17.6 to ~16.8 near phase 0.9-1.0, this brightening
coincides in phase with the X-ray dip. Even though it is likely that the
absorption line spectrum is associated with the companion star of CirX-1, we
cannot exclude the possibility that the spectrum originates in the accretion
disc. However, if the spectrum belongs to the companion star, it must be a
supergiant of spectral type B5-A0. If we assume that the compact object does
not move through the companion star at periastron, the companion star mass is
constrained to ~<10 Msun for a 1.4 Msun neutron star, whereas the inclination
has to be ~> 13.7 degrees. Alternatively, the measured absorption lines and
their radial velocity curve can be associated with the accretion disc
surrounding a 1.4 Msun neutron star and its motion around the centre of mass.
An absorption line spectrum from an accretion disc is typically found when our
line-of-sight passes through the accretion disc rim implying a high
inclination. However, from radio observations it was found that the angle
between the line-of-sight and the jet axis is smaller than 5 degrees implying
that the jet ploughs through the accretion disc in this scenario.Comment: 8 pages, 4 figures, 3 tables, accepted by MNRA
Enabling pulsar and fast transient searches using coherent dedispersion
We present an implementation of the coherent dedispersion algorithm capable
of dedispersing high-time-resolution radio observations to many different
dispersion measures (DMs). This approach allows the removal of the dispersive
effects of the interstellar medium and enables searches for pulsed emission
from pulsars and other millisecond-duration transients at low observing
frequencies and/or high DMs where time broadening of the signal due to
dispersive smearing would otherwise severely reduce the sensitivity. The
implementation, called 'cdmt', for Coherent Dispersion Measure Trials, exploits
the parallel processing capability of general-purpose graphics processing units
to accelerate the computations. We describe the coherent dedispersion algorithm
and detail how cdmt implements the algorithm to efficiently compute many
coherent DM trials. We present the concept of a semi-coherent dedispersion
search, where coherently dedispersed trials at coarsely separated DMs are
subsequently incoherently dedispersed at finer steps in DM. The software is
used in an ongoing LOFAR pilot survey to test the feasibility of performing
semi-coherent dedispersion searches for millisecond pulsars at 135MHz. This
pilot survey has led to the discovery of a radio millisecond pulsar -- the
first at these low frequencies. This is the first time that such a broad and
comprehensive search in DM-space has been done using coherent dedispersion, and
we argue that future low-frequency pulsar searches using this approach are both
scientifically compelling and feasible. Finally, we compare the performance of
cdmt with other available alternatives.Comment: 8 pages, 7 figures, submitted to Astronomy and Computin
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
Optical/Infrared Observations of the Anomalous X-ray Pulsar 1E 1048.1-5937 During Its 2007 X-Ray Flare
We report on optical and infrared observations of the anomalous X-ray pulsar
(AXP) 1E 1048.1-5937, made during its ongoing X-ray flare which started in 2007
March. We detected the source in the optical I and near-infrared Ks bands in
two ground-based observations and obtained deep flux upper limits from four
observations, including one with the Spitzer Space Telescope at 4.5 and 8.0
microns. The detections indicate that the source was approximately 1.3--1.6
magnitudes brighter than in 2003--2006, when it was at the tail of a previous
similar X-ray flare. Similar related flux variations have been seen in two
other AXPs during their X-ray outbursts, suggesting common behavior for large
X-ray flux variation events in AXPs. The Spitzer flux 1E 1048.1-5937 limits are
sufficiently deep that we can exclude mid-infrared emission similar to that
from the AXP 4U 0142+61, which has been interpreted as arising from a dust disk
around the AXP. The optical/near-infrared emission from probably has a
magnetospheric origin. The similarity in the flux spectra of 4U 0142+61 and 1E
1048.1-5937 challenges the dust disk model proposed for the latter.Comment: 5 pages, 1 figure, accepted by Ap
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
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
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