1,615 research outputs found
Are We Seeing Magnetic Axis Reorientation in the Crab and Vela Pulsars?
Variation in the angle between a pulsar's rotational and magnetic
axes would change the torque and spin-down rate. We show that sudden increases
in , coincident with glitches, could be responsible for the persistent
increases in spin-down rate that follow glitches in the Crab pulsar. Moreover,
changes in at a rate similar to that inferred for the Crab pulsar
account naturally for the very low braking index of the Vela pulsar. If
increases with time, all pulsar ages obtained from the conventional
braking model are underestimates. Decoupling of the neutron star liquid
interior from the external torque cannot account for Vela's low braking index.
Variations in the Crab's pulse profile due to changes in might be
measurable.Comment: 14 pages and one figure, Latex, uses aasms4.sty. Accepted to ApJ
Letter
Neutron star magnetic field evolution, crust movement and glitches
Spinning superfluid neutrons in the core of a neutron star interact strongly
with co-existing superconducting protons. One consequence is that the
outward(inward) motion of core superfluid neutron vortices during spin-down(up)
of a neutron star may alter the core's magnetic field. Such core field changes
are expected to result in movements of the stellar crust and changes in the
star's surface magnetic field which reflect those in the core below. Observed
magnitudes and evolution of the spin-down indices of canonical pulsars are
understood as a consequence of such surface field changes. If the growing
crustal strains caused by the changing core magnetic field configuration in
canonical spinning-down pulsars are relaxed by large scale crust-cracking
events, special properties are predicted for the resulting changes in
spin-period. These agree with various glitch observations, including glitch
activity, permanent shifts in spin-down rates after glitches in young pulsars,
the intervals between glitches, families of glitches with different magnitudes
in the same pulsar, the sharp drop in glitch intervals and magnitudes as pulsar
spin-periods approach 0.7s, and the general absence of glitching beyond this
period.Comment: LaTex, 28 pages, 8 figs, accepted for publication in Ap
Time-Correlated Structure in Spin Fluctuations in Pulsars
We study statistical properties of stochastic variations in pulse arrival
times, timing noise, in radio pulsars using a new analysis method applied in
the time domain. The method proceeds in two steps. First, we subtract
low-frequency wander using a high-pass filter. Second, we calculate the
discrete correlation function of the filtered data. As a complementary method
for measuring correlations, we introduce a statistic that measures the
dispersion of the data with respect to the data translated in time. The
analysis methods presented here are robust and of general usefulness for
studying arrival time variations over timescales approaching the average
sampling interval. We apply these methods to timing data for 32 pulsars. In two
radio pulsars, PSRs B1133+16 and B1933+16, we find that fluctuations in arrival
times are correlated over timescales of 10 - 20 d with the distinct signature
of a relaxation process. Though this relaxation response could be
magnetospheric in origin, we argue that damping between the neutron star crust
and interior liquid is a more likely explanation. Under this interpretation,
our results provide the first evidence independent from pulsar spin glitches of
differential rotation in neutron stars. PSR B0950+08, shows evidence for
quasi-periodic oscillations that could be related to mode switching.Comment: 25 pages, Final journal version (MNRAS
Radio pulsar populations
The goal of this article is to summarize the current state of play in the
field of radio pulsar statistics. Simply put, from the observed sample of
objects from a variety of surveys with different telescopes, we wish to infer
the properties of the underlying sample and to connect these with other
astrophysical populations (for example supernova remnants or X-ray binaries).
The main problem we need to tackle is the fact that, like many areas of
science, the observed populations are often heavily biased by a variety of
selection effects. After a review of the main effects relevant to radio
pulsars, I discuss techniques to correct for them and summarize some of the
most recent results. Perhaps the main point I would like to make in this
article is that current models to describe the population are far from complete
and often suffer from strong covariances between input parameters. That said,
there are a number of very interesting conclusions that can be made concerning
the evolution of neutron stars based on current data. While the focus of this
review will be on the population of isolated Galactic pulsars, I will also
briefly comment on millisecond and binary pulsars as well as the pulsar content
of globular clusters and the Magellanic Clouds.Comment: 16 pages, 6 figures, to appear in Proceedings of ICREA Workshop on
The High-Energy Emission from Pulsars and their Systems, Sant Cugat, Spain,
2010 April 12-16 (Springer
Optical Observations of the Binary Millisecond Pulsars J2145-0750 and J0034-0534
We report on optical observations of the low-mass binary millisecond pulsar
systems J0034-0534 and J2145-0750. A faint (I=23.5) object was found to be
coincident with the timing position of PSR J2145-0750. While a galaxy or
distant main-sequence star cannot be ruled out, its magnitude is consistent
with an ancient white dwarf, as expected from evolutionary models. For PSR
J0034-0534 no objects were detected to a limiting magnitude of R=25.0,
suggesting that the white dwarf in this system is cold. Using white dwarf
cooling models, the limit on the magnitude of the PSR J0034-0534 companion
suggests that at birth the pulsar in this system may have rotated with a period
as short as 0.6 ms. These observations provide further evidence that the
magnetic fields of millisecond pulsars do not decay on time scales shorter than
1 Gyr.Comment: 6 pages, uuencoded, gz -9 compressed postscript, accepted by ApJ
Birth and Evolution of Isolated Radio Pulsars
We investigate the birth and evolution of Galactic isolated radio pulsars. We
begin by estimating their birth space velocity distribution from proper motion
measurements of Brisken et al. (2002, 2003). We find no evidence for
multimodality of the distribution and favor one in which the absolute
one-dimensional velocity components are exponentially distributed and with a
three-dimensional mean velocity of 380^{+40}_{-60} km s^-1. We then proceed
with a Monte Carlo-based population synthesis, modelling the birth properties
of the pulsars, their time evolution, and their detection in the Parkes and
Swinburne Multibeam surveys. We present a population model that appears
generally consistent with the observations. Our results suggest that pulsars
are born in the spiral arms, with a Galactocentric radial distribution that is
well described by the functional form proposed by Yusifov & Kucuk (2004), in
which the pulsar surface density peaks at radius ~3 kpc. The birth spin period
distribution extends to several hundred milliseconds, with no evidence of
multimodality. Models which assume the radio luminosities of pulsars to be
independent of the spin periods and period derivatives are inadequate, as they
lead to the detection of too many old simulated pulsars in our simulations.
Dithered radio luminosities proportional to the square root of the spin-down
luminosity accommodate the observations well and provide a natural mechanism
for the pulsars to dim uniformly as they approach the death line, avoiding an
observed pile-up on the latter. There is no evidence for significant torque
decay (due to magnetic field decay or otherwise) over the lifetime of the
pulsars as radio sources (~100 Myr). Finally, we estimate the pulsar birthrate
and total number of pulsars in the Galaxy.Comment: 27 pages, including 15 figures, accepted by Ap
Timing models for the long-orbital period binary pulsar PSR B1259-63
The pulsar PSR B1259-63 is in a highly eccentric 3.4-yr orbit with the Be
star SS 2883. Timing observations of this pulsar, made over a 7-yr period using
the Parkes 64-m radio telescope, cover two periastron passages, in 1990 August
and 1994 January. The timing data cannot be fitted by the normal pulsar and
Keplerian binary parameters. A timing solution including a (non-precessing)
Keplerian orbit and timing noise (represented as a polynomial of fifth order in
time) provide a satisfactory fit to the data. However, because the Be star
probably has a significant quadrupole moment, we prefer to interpret the data
by a combination of timing noise, dominated by a cubic phase term, and
and terms. We show that the and are
likely to be a result of a precessing orbit caused by the quadrupole moment of
the tilted companion star. We further rule out a number of possible physical
effects which could contribute to the timing data of PSR B1259-63 on a
measurable level.Comment: LaTeX, 9 pages, 8 figures, accepted for publication in MNRA
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
A Search for Pulsars in Quiescent Soft X-Ray Transients. I
We have carried out a deep search at 1.4 GHz for radio pulsed emission from
six soft X-ray transient sources observed during their X-ray quiescent phase.
The commonly accepted model for the formation of the millisecond radio pulsars
predicts the presence of a rapidly rotating, weakly magnetized neutron star in
the core of these systems. The sudden drop in accretion rate associated with
the end of an X-ray outburst causes the Alfv\`en surface to move outside the
light cylinder, allowing the pulsar emission process to operate. No pulsed
signal was detected from the sources in our sample. We discuss several
mechanisms that could hamper the detection and suggest that free-free
absorption from material ejected from the system by the pulsar radiation
pressure could explain our null result.Comment: accepted by Ap
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