2,116 research outputs found
The 2000 Periastron Passage of PSR B1259-63
We report here on a sequence of 28 observations of the binary pulsar system
PSR B1259-63/SS2883 at four radio frequencies made with the Australia Telescope
Compact Array around the time of the 2000 periastron passage. Observations made
on 2000 Sep 1 show that the pulsar's apparent rotation measure (RM) reached a
maximum of rad m, some 700 times the value measured
away from periastron, and is the largest astrophysical RM measured. This value,
combined with the dispersion measure implies a magnetic field in the Be star's
wind of 6 mG. We find that the light curve of the unpulsed emission is similar
to that obtained during the 1997 periastron but that differences in detail
imply that the emission disc of the Be star is thicker and/or of higher
density. The behaviour of the light curve at late times is best modelled by the
adiabatic expansion of a synchrotron bubble formed in the pulsar/disc
interaction. The expansion rate of the bubble km s is
surprisingly low but the derived magnetic field of 1.6 G close to that
expected.Comment: 8 pages, 6 figures, 3 tables, LaTeX (mn.sty). Accepted for
publication in the Monthly Notices of the Royal Astronomical Society. Also
available at http://astronomy.swin.edu.au/staff/tconnors/publications.htm
Pulsar magnetic alignment and the pulsewidth-age relation
Using pulsewidth data for 872 isolated radio pulsars we test the hypothesis
that pulsars evolve through a progressive narrowing of the emission cone
combined with progressive alignment of the spin and magnetic axes. The new data
provide strong evidence for the alignment over a time-scale of about 1 Myr with
a log standard deviation of around 0.8 across the observed population. This
time-scale is shorter than the time-scale of about 10 Myr found by previous
authors, but the log standard deviation is larger. The results are inconsistent
with models based on magnetic field decay alone or monotonic counter-alignment
to orthogonal rotation. The best fits are obtained for a braking index
parameter n_gamma approximately equal to 2.3, consistent the mean of the six
measured values, but based on a much larger sample of young pulsars. The
least-squares fitted models are used to predict the mean inclination angle
between the spin and magnetic axes as a function of log characteristic age.
Comparing these predictions to existing estimates it is found that the model in
which pulsars are born with a random angle of inclination gives the best fit to
the data. Plots of the mean beaming fraction as a function of characteristic
age are presented using the best-fitting model parameters.Comment: 13 pages, 11 figures, Accepted for publication in MNRA
Timing, glitches and braking index of PSR B0540-69
We report a pulse-time history of PSR B054069 based on the analysis of an
extended Data set including ASCA, BeppoSAX and RXTE observations spanning a
time interval of about 8 years. This interval includes also the epoch of the
glitch episode reported by Zhang et al. (2001). Our analysis shows the presence
of a relevant timing noise and does not give a clear evidence of the glitch
occurrence. We performed an accurate evaluation of the main timing parameters,
, and and derived a mean braking index of
quite different from the lower value found by Zhang et al.
(2001), but in rather good agreement with other several values reported in the
literature.Comment: 9 pages 5 figures, accepted by A&A, main journa
Are highâlatitude forwardâreverse shock pairs driven by CME overexpansion?
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95129/1/jgra18211.pd
Radio Emission Signatures in the Crab Pulsar
Our high time resolution observations of individual pulses from the Crab
pulsar show that both the time and frequency signatures of the interpulse are
distinctly different from those of the main pulse. Main pulses can occasionally
be resolved into short-lived, relatively narrow-band nanoshots. We believe
these nanoshots are produced by soliton collapse in strong plasma turbulence.
Interpulses at centimeter wavelengths are very different. Their dynamic
spectrum contains regular, microsecond-long emission bands. We have detected
these bands, proportionately spaced in frequency, from 4.5 to 10.5 GHz. The
bands cannot easily be explained by any current theory of pulsar radio
emission; we speculate on possible new models.Comment: 26 pages, 10 figures, to appear in Ap
High-energy Emission from Pulsar Outer Magnetospheres
We investigate a stationary pair production cascade in the outer
magnetosphere of an isolated, spinning neutron star. The charge depletion due
to global flows of charged particles, causes a large electric field along the
magnetic field lines. Migratory electrons and/or positrons are accelerated by
this field to radiate gamma-rays via curvature and inverse-Compton processes.
Some of such gamma-rays collide with the X-rays to materialize as pairs in the
gap. The replenished charges partially screen the electric field, which is
self-consistently solved together with the energy distribution of particles and
gamma-rays at each point along the field lines. By solving the set of Maxwell
and Boltzmann equations, we demonstrate that an external injection of charged
particles at nearly Goldreich-Julian rate does not quench the gap but shifts
its position and that the particle energy distribution cannot be described by a
power-law. The injected particles are accelerated in the gap and escape from it
with large Lorentz factors. We show that such escaping particles migrating
outside of the gap contribute significantly to the gamma-ray luminosity for
young pulsars and that the soft gamma-ray spectrum between 100 MeV and 3 GeV
observed for the Vela pulsar can be explained by this component. We also
discuss that the luminosity of the gamma-rays emitted by the escaping particles
is naturally proportional to the square root of the spin-down luminosity.Comment: 24 pages, 11 figures; to appear in the inaugural (Sep) issue of
Progress in Astrophysics Researches (a new book series
Representations of Time Coordinates in FITS
In a series of three previous papers, formulation and specifics of the
representation of World Coordinate Transformations in FITS data have been
presented. This fourth paper deals with encoding time. Time on all scales and
precisions known in astronomical datasets is to be described in an unambiguous,
complete, and self-consistent manner. Employing the well--established World
Coordinate System (WCS) framework, and maintaining compatibility with the FITS
conventions that are currently in use to specify time, the standard is extended
to describe rigorously the time coordinate. World coordinate functions are
defined for temporal axes sampled linearly and as specified by a lookup table.
The resulting standard is consistent with the existing FITS WCS standards and
specifies a metadata set that achieves the aims enunciated above.Comment: FITS WCS Paper IV: Time. 27 pages, 11 table
Adaptive Filters Revisited - RFI Mitigation in pulsar observations
Pulsar detection and timing experiments are applications where adaptive
filters seem eminently suitable tools for radio-frequency-interference (RFI)
mitigation. We describe a novel variant which works well in field trials of
pulsar observations centred on an observing frequency of 675 MHz, a bandwidth
of 64 MHz and with 2-bit sampling. Adaptive filters have generally received bad
press for RFI mitigation in radio astronomical observations with their most
serious drawback being a spectral echo of the RFI embedded in the filtered
signals. Pulsar observations are intrinsically less sensitive to this as they
operate in the (pulsar period) time domain. The field trials have allowed us to
identify those issues which limit the effectiveness of the adaptive filter. We
conclude that adaptive filters can significantly improve pulsar observations in
the presence of RFI.Comment: Accepted for publication in Radio Scienc
Reply to comment by P. Riley and J. T. Gosling on âAre highâlatitude forwardâreverse shock pairs driven by overexpansion?â
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95455/1/jgra18795.pd
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