2,315 research outputs found
The 1997 event in the Crab pulsar revisited
A complex event observed in the radio pulses from the Crab pulsar in 1997
included echoes, a dispersive delay, and large changes in intensity. It is
shown that these phenomena were due to refraction at the edge of a plasma cloud
in the outer region of the Crab Nebula. Several similar events have been
observed, although in less detail. It is suggested that the plasma cloud is in
the form of filaments with diameter around 3 x 10^11m and electron density of
order 10^4 cm-3Comment: 5 pages 4 figs Accepted by MNRA
The Gamma Ray Pulsar Population
We apply a likelihood analysis to pulsar detections, pulsar upper limits, and
diffuse background measurements from the OSSE and EGRET instruments on the
Compton Gamma Ray Observatory to constrain the luminosity law for gamma-ray
pulsars and some properties of the gamma-ray pulsar population. We find that
the dependence of luminosity on spin period and dipole magnetic field is much
steeper at OSSE than at EGRET energies (50-200 keV and >100 MeV, respectively),
suggesting that different emission mechanisms are responsible for low- and
high-energy gamma-ray emission. Incorporating a spin-down model and assuming a
pulsar spatial distribution, we estimate the fraction of the Galactic gamma-ray
background due to unidentified pulsars and find that pulsars may be an
important component of the OSSE diffuse flux, but are most likely not important
at EGRET energies. Using measurements of the diffuse background flux from these
instruments, we are able to place constraints on the braking index, initial
spin period, and magnetic field of the Galactic pulsar population. We are also
able to constrain the pulsar birthrate to be between 1/(25 yr) and 1/(500 yr).
Our results are based on a large gamma-ray beam, but they do not scale in a
simple way with beam size. With our assumed beam size, the implied gamma-ray
efficiency for the EGRET detections is no more than 20%. We estimate that about
20 of the 169 unidentified EGRET sources are probably gamma-ray pulsars. We use
our model to predict the pulsar population that will be seen by future
gamma-ray instruments and estimate that GLAST will detect roughly 750 gamma-ray
pulsars as steady sources, only 120 of which are currently known radio pulsars.Comment: 32 pages, including figures. submitted to Ap
Absolute Timing of the Crab Pulsar with RXTE
We have monitored the phase of the main X-ray pulse of the Crab pulsar with
the Rossi X-ray Timing Explorer (RXTE) for almost eight years, since the start
of the mission in January 1996. The absolute time of RXTE's clock is
sufficiently accurate to allow this phase to be compared directly with the
radio profile. Our monitoring observations of the pulsar took place bi-weekly
(during the periods when it was at least 30 degrees from the Sun) and we
correlated the data with radio timing ephemerides derived from observations
made at Jodrell Bank. We have determined the phase of the X-ray main pulse for
each observation with a typical error in the individual data points of 50 us.
The total ensemble is consistent with a phase that is constant over the
monitoring period, with the X-ray pulse leading the radio pulse by
0.0102+/-0.0012 period in phase, or 344+/-40 us in time. The error estimate is
dominated by a systematic error of 40 us in the radio data, arising from
uncertainties in the variable amount of pulse delay due to interstellar
scattering and instrumental calibration. The statistical error is 0.00015
period, or 5 us. The separation of the main pulse and interpulse appears to be
unchanging at time scales of a year or less, with an average value of
0.4001+/-0.0002 period. There is no apparent variation in these values with
energy over the 2-30 keV range. The lag between the radio and X-ray pulses may
be constant in phase (rotational) or constant in time (linear pathlength). We
are not (yet) able to distinguish between these two interpretations.Comment: 11 pages, 2 figure
Unusual glitch activity in the RRAT J1819-1458: an exhausted magnetar?
We present an analysis of regular timing observations of the
high-magnetic-field Rotating Radio Transient (RRAT) J18191458 obtained using
the 64-m Parkes and 76-m Lovell radio telescopes over the past five years.
During this time, the RRAT has suffered two significant glitches with
fractional frequency changes of and .
Glitches of this magnitude are a phenomenon displayed by both radio pulsars and
magnetars. However, the behaviour of J18191458 following these glitches is
quite different to that which follows glitches in other neutron stars, since
the glitch activity resulted in a significant long-term net decrease in the
slow-down rate. If such glitches occur every 30 years, the spin-down rate, and
by inference the magnetic dipole moment, will drop to zero on a timescale of a
few thousand years. There are also significant increases in the rate of pulse
detection and in the radio pulse energy immediately following the glitches.Comment: accepted for publication in MNRAS, 7 pages, 7 figures, 1 tabl
The geometry of the double-pulsar system J0737-3039 from systematic intensity variations
The recent discovery of J0737-3039A & B-two pulsars in a highly relativistic
orbit around one another - offers an unprecedented opportunity to study the
elusive physics of pulsar radio emission. The system contains a rapidly
rotating pulsar with a spin period of 22.7 ms and a slow companion with a spin
period of 2.77 s, hereafter referred to as 'A' and 'B', respectively. A unique
property of the system is that the pulsed radio flux from B increases
systematically by almost two orders-of-magnitude during two short portions of
each orbit. Here, we describe a geometrical model of the system that
simultaneously explains the intensity variations of B and makes definitive and
testable predictions for the future evolution of the emission properties of
both stars. Our model assumes that B's pulsed radio flux increases when
illuminated by emission from A. This model provides constraints on the spin
axis orientation and emission geometry of A and predicts that its pulse profile
will evolve considerably over the next several years due to geodetic precession
until it disappears entirely in 15-20 years
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
Characterization of the Crab Pulsar's Timing Noise
We present a power spectral analysis of the Crab pulsar's timing noise,
mainly using radio measurements from Jodrell Bank taken over the period
1982-1989. The power spectral analysis is complicated by nonuniform data
sampling and the presence of a steep red power spectrum that can distort power
spectra measurement by causing severe power ``leakage''. We develop a simple
windowing method for computing red noise power spectra of uniformly sampled
data sets and test it on Monte Carlo generated sample realizations of red
power-law noise. We generalize time-domain methods of generating power-law red
noise with even integer spectral indices to the case of noninteger spectral
indices. The Jodrell Bank pulse phase residuals are dense and smooth enough
that an interpolation onto a uniform time series is possible. A windowed power
spectrum is computed revealing a periodic or nearly periodic component with a
period of about 568 days and a 1/f^3 power-law noise component with a noise
strength of 1.24 +/- 0.067 10^{-16} cycles^2/sec^2 over the analysis frequency
range 0.003 - 0.1 cycles/day. This result deviates from past analyses which
characterized the pulse phase timing residuals as either 1/f^4 power-law noise
or a quasiperiodic process. The analysis was checked using the Deeter
polynomial method of power spectrum estimation that was developed for the case
of nonuniform sampling, but has lower spectral resolution. The timing noise is
consistent with a torque noise spectrum rising with analysis frequency as f
implying blue torque noise, a result not predicted by current models of pulsar
timing noise. If the periodic or nearly periodic component is due to a binary
companion, we find a companion mass > 3.2 Earth masses.Comment: 53 pages, 9 figures, submitted to MNRAS, abstract condense
An empirical analysis of factors affecting the productivity of livestock in southern Botswana
This study attempts to identify factors responsible for differences in the productivity of cattle managed by private and communal livestock farmers in the southern region of Botswana during 1999/2000. Sample survey data are used to estimate the parameters of a block recursive regression model. Some of the equations postulated in the model are estimated with two-stage least squares (2SLS) to account for likely correlation between endogenous explanatory variables and the error term. The results show that (a) respondents with secure land tenure (private farms) and larger herds use more agricultural credit than do those who rely on open access communal grazing to raise cattle; (b) secure tenure and higher levels of liquidity from long-term credit and off-farm wage remittances promote investment in fixed improvements to land; (c) liquidity from short-term credit and wage remittances supports expenditure on operating inputs; and (d) herd productivity increases with greater investment in operating inputs and fixed improvements, and is therefore positively (but indirectly) influenced by secure land tenure. It can be inferred that government should (a) uphold private property rights to land where they already exist; (b) privatise open access grazing to individual owner-operators where this is politically, socially and economically feasible; and (c) where privatisation to individuals is not feasible, government should encourage users to convert the grazing into common property by subsidising the transaction costs of defining user groups and the boundaries of their resources, and of negotiating and enforcing rules limiting individual use of common property. This first-step in a gradual shift towards private property might be followed by a conversion of user-groups into non-user groups organised along the lines of investor-owned firms where members exchange use rights for benefits rights.Productivity Analysis,
Bumpy Spin-Down of Anomalous X-Ray Pulsars: The Link with Magnetars
The two anomalous X-ray pulsars (AXPs) with well-sampled timing histories, 1E
1048.1-5937 and 1E 2259+586, are known to spin down irregularly, with `bumps'
superimposed on an overall linear trend. Here we show that if AXPs are
non-accreting magnetars, i.e. isolated neutron stars with surface magnetic
fields B_0 > 10^{10} T, then they spin down electromagnetically in exactly the
manner observed, due to an effect called `radiative precession'. Internal
hydromagnetic stresses deform the star, creating a fractional difference
epsilon=(I_3-I_1)/I_1 ~ 10^{-8} between the principal moments of inertia I_1
and I_3; the resulting Eulerian precession couples to an oscillating component
of the electromagnetic torque associated with the near-zone radiation fields,
and the star executes an anharmonic wobble with period tau_pr ~ 2 pi / epsilon
Omega(t) ~ 10 yr, where Omega(t) is the rotation frequency as a function of
time t. We solve Euler's equations for a biaxial magnet rotating in vacuo; show
that the computed Omega(t) matches the measured timing histories of 1E
1048.1-5937 and 1E 2259+586; predict Omega(t) for the next 20 years for both
objects; predict a statistical relation between and tau_pr, to be
tested as the population of known AXPs grows; and hypothesize that radiative
precession will be observed in future X-ray timing of soft gamma-ray repeaters
(SGRs).Comment: 9 pages, 2 figures, to be published in The Astrophysical Journal
Letter
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