13 research outputs found
Avalanche dynamics of radio pulsar glitches
We test statistically the hypothesis that radio pulsar glitches result from
an avalanche process, in which angular momentum is transferred erratically from
the flywheel-like superfluid in the star to the slowly decelerating, solid
crust via spatially connected chains of local, impulsive, threshold-activated
events, so that the system fluctuates around a self-organised critical state.
Analysis of the glitch population (currently 285 events from 101 pulsars)
demonstrates that the size distribution in individual pulsars is consistent
with being scale invariant, as expected for an avalanche process. The
waiting-time distribution is consistent with being exponential in seven out of
nine pulsars where it can be measured reliably, after adjusting for
observational limits on the minimum waiting time, as for a constant-rate
Poisson process. PSR J05376910 and PSR J08354510 are the exceptions;
their waiting-time distributions show evidence of quasiperiodicity. In each
object, stationarity requires that the rate equals , where is the angular acceleration of the
crust, is the mean glitch size, and is the
relative angular acceleration of the crust and superfluid. There is no evidence
that changes monotonically with spin-down age. The rate distribution
itself is fitted reasonably well by an exponential for . For , its exact form is unknown; the
exponential overestimates the number of glitching pulsars observed at low
, where the limited total observation time exercises a selection bias.Comment: Accepted for publication in the Astrophysical Journa
A Comparison of Measured Crab and Vela Glitch Healing Parameters with Predictions of Neutron Star Models
There are currently two well-accepted models that explain how pulsars exhibit
glitches, sudden changes in their regular rotational spin-down. According to
the starquake model, the glitch healing parameter, Q, which is measurable in
some cases from pulsar timing, should be equal to the ratio of the moment of
inertia of the superfluid core of a neutron star (NS) to its total moment of
inertia. Measured values of the healing parameter from pulsar glitches can
therefore be used in combination with realistic NS structure models as one test
of the feasibility of the starquake model as a glitch mechanism. We have
constructed NS models using seven representative equations of state of
superdense matter to test whether starquakes can account for glitches observed
in the Crab and Vela pulsars, for which the most extensive and accurate glitch
data are available. We also present a compilation of all measured values of Q
for Crab and Vela glitches to date which have been separately published in the
literature. We have computed the fractional core moment of inertia for stellar
models covering a range of NS masses and find that for stable NSs in the
realistic mass range 1.4 +/- 0.2 solar masses, the fraction is greater than
0.55 in all cases. This range is not consistent with the observational
restriction Q < 0.2 for Vela if starquakes are the cause of its glitches. This
confirms results of previous studies of the Vela pulsar which have suggested
that starquakes are not a feasible mechanism for Vela glitches. The much larger
values of Q observed for Crab glitches (Q > 0.7) are consistent with the
starquake model predictions and support previous conclusions that starquakes
can be the cause of Crab glitches.Comment: 8 pages, including 3 figures and 1 table. 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
Observations of a Series of Six Recent Glitches in the Crab Pulsar
From 1995 to 1999, daily monitoring of the radio emission from the Crab
pulsar at the Green Bank and Jodrell Bank observatories revealed a series of
six sudden rotational spinups or glitches, doubling the number of glitches
observed for this pulsar since 1969. With these observations, the range of time
intervals between significant Crab glitches has widened considerably,
indicating that the occurrence of Crab glitches may be more random than
previously thought. The new glitch amplitudes () span an order
of magnitude from 2e-9 to 3e-8. Except in one case, which we suggest may
represent an ``aftershock'' event, the frequency jumps display an exponential
recovery with a timescale of 3 days for the smaller glitches and 10
days for the largest (1996) glitch. In the largest event, a portion of the
spinup was resolved in time, as was previously reported for the 1989 glitch. A
pronounced change in frequency derivative also occurs after each glitch and is
correlated with the size of the initial frequency jump, although for some of
the smaller glitches this appears to be a temporary effect. We discuss the
properties of the ensemble of observed Crab glitches and compare them with the
properties of Vela glitches, highlighting those differences which must be
explained by evolutionary models.Comment: 15 pages, 14 embedded figures, to appear in Ap
Pulsar Constraints on Neutron Star Structure and Equation of State
With the aim of constraining the structural properties of neutron stars and
the equation of state of dense matter, we study sudden spin-ups, glitches,
occurring in the Vela pulsar and in six other pulsars. We present evidence that
glitches represent a self-regulating instability for which the star prepares
over a waiting time. The angular momentum requirements of glitches in Vela
indicate that at least 1.4% of the star's moment of inertia drives these
events. If glitches originate in the liquid of the inner crust, Vela's
`radiation radius' must exceed ~12 km for a mass of 1.4 solar masses.
Observational tests of whether other neutron stars obey this constraint will be
possible in the near future.Comment: 5 pages, including figures. To appear in Physical Review Letter
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Measurement of RF surface efficiency at cryogenic temperatures using a resonant cavity
Exploiting the potential efficiency gain of a normal conducting rf accelerator operated at cryogenic temperatures requires careful preparation of the rf conducting surface. Experimental apparatus has been assembled to study the surface conductivity to rf currents at 425 MHz and 850 MHz through a temperature range from room temperature to 14 K. The apparatus is built around an open-ended coaxial cavity with the cavity tubular ends below the cutoff frequency at resonance. The center conductor in the coaxial cavity is the test sample, and the use of a dielectric stand-off for the center conductor precludes the need for an rf contact joint and facilitates sample changes. The rf testing is conducted under vacuum with low-power rf. A CTI-Cryogenics cryopump coldhead is used for cryogenic temperature cycling of the test cavity. A detailed description of the apparatus and measurement procedures are presented