Few statistically compelling correlations are found in pulsar timing data between the size of a rotational glitch and the time to the preceding glitch (backward waiting time) or the succeeding glitch ...(forward waiting time), except for a strong correlation between sizes and forward waiting times in PSR J0537−6910. This situation is counterintuitive if glitches are threshold-triggered events, as in standard theories (e.g., starquakes, superfluid vortex avalanches). Here, it is shown that the lack of correlation emerges naturally, when a threshold trigger is combined with secular stellar braking slower than a critical, calculable rate. The Pearson and Spearman correlation coefficients are computed and interpreted within the framework of a state-dependent Poisson process. Specific, falsifiable predictions are made regarding what objects currently targeted by long-term timing campaigns should develop strong size-waiting-time correlations as more data are collected in the future.
ABSTRACT
The elastic crust of a neutron star fractures repeatedly as it spins down electromagnetically. An idealized, macroscopic model of inhomogeneous crustal failure is presented based on a ...cellular automaton with nearest-neighbour tectonic interactions involving strain redistribution and thermal dissipation. Predictions are made of the size and waiting-time distributions of failure events, as well as the rate of failure as the star spins down. The last failure event typically occurs when the star spins down to $\approx 1{{\ \rm per\ cent}}$ of its birth frequency with implications for rotational glitch activity. Neutron stars are commonly suggested as sources of continuous gravitational waves. The output of the automaton is converted into predictions of the star’s mass ellipticity and gravitational wave strain as functions of its age, with implications for future observations with instruments such as the Laser Interferometer Gravitational Wave Observatory (LIGO).
Glitch size and waiting time probability density functions (PDFs) are estimated for the five pulsars that have glitched the most using the nonparametric kernel density estimator. Two objects exhibit ...decreasing size and waiting time PDFs. Their activity is Poisson-like, and their size statistics are approximately scale-invariant. Three objects exhibit a statistically significant local maximum in the PDFs, including one (PSR J1341−6220), which was classified as Poisson-like in previous analyses. Their activity is quasiperiodic, although the dispersion in waiting times is relatively broad. The classification is robust: it is preserved across a wide range of bandwidth choices. There is no compelling evidence for multimodality, but this issue should be revisited when more data become available. The implications for superfluid vortex avalanche models of pulsar glitches are explored briefly.
It is argued that the superfluid core of a neutron star super-rotates relative to the crust, because stratification prevents the core from responding to the electromagnetic braking torque, until the ...relevant dissipative (viscous or Eddington-Sweet) timescale, which can exceed ~10 super(3) yr and is much longer than the Ekman timescale, has elapsed. Hence, in some young pulsars, the rotation of the core today is a fossil record of its rotation at birth, provided that magnetic crust-core coupling is inhibited, e.g., by buoyancy, field-line topology, or the presence of uncondensed neutral components in the superfluid. Persistent core super-rotation alters our picture of neutron stars in several ways, allowing for magnetic field generation by ongoing dynamo action and enhanced gravitational wave emission from hydrodynamic instabilities.
Abstract
Superfluid vortex avalanches are one plausible cause of pulsar glitch activity. If they occur according to a state-dependent Poisson process, the measured long-term glitch rate is determined ...by the spin-down rate of the stellar crust,
Ω
̇
c
, and two phenomenological parameters quantifying the vortex−nucleus pinning force: a crust−superfluid angular velocity lag threshold,
X
cr
, and a reference unpinning rate,
λ
0
. A Bayesian analysis of 541 glitches in 177 pulsars, with
N
g
≥ 1 events per pulsar, yields
X
cr
=
0.15
−
0.04
+
0.09
rad
s
−
1
,
λ
ref
=
7.6
−
2.6
+
3.7
×
10
−
8
s
−
1
, and
a
=
−
0.27
−
0.03
+
0.04
assuming the phenomenological rate law
λ
0
=
λ
ref
τ
/(1 yr)
a
, where
τ
denotes the characteristic spin-down age. The results are broadly similar, whether one includes or excludes quasiperiodic glitch activity, giant glitches, or pulsars with
N
g
= 0, up to uncertainties about the completeness of the sample and the total observation time per pulsar. The
X
cr
and
λ
0
estimates are consistent with first-principles calculations based on nuclear theory, e.g., in the semiclassical local density approximation.
ABSTRACT
Many millisecond pulsars are thought to be old neutron stars spun up (‘recycled’) during an earlier accretion phase. They typically have relatively weak (≲109 G) dipole field strengths, ...consistent with accretion-induced magnetic burial. Recent data from the Neutron Star Interior Composition Explorer indicate that hotspots atop the recycled pulsar PSR J0030–0451 are not antipodal, so that the magnetic field cannot be that of a centred dipole. In this paper it is shown that multipolarity is naturally expected in the burial scenario because of equatorial field line compression. Grad–Shafranov equilibria are constructed to show how magnetic multipole moments can be calculated in terms of various properties, such as the amount of accreted mass and the crustal equation of state.
ABSTRACT
Among the five pulsars with the most recorded rotational glitches, only PSR J0534+2200 is found to have an autocorrelation between consecutive glitch sizes that differs significantly from ...zero (Spearman correlation coefficient ρ = −0.46, p-value = 0.046). No statistically compelling autocorrelations between consecutive waiting times are found. The autocorrelation observations are interpreted within the framework of a predictive meta-model describing stress release in terms of a state-dependent Poisson process. Specific combinations of size and waiting time autocorrelations are identified, alongside combinations of cross-correlations and size and waiting time distributions, that are allowed or excluded within the meta-model. For example, future observations of any ‘quasi-periodic’ glitching pulsar, such as PSR J0537–6910, should not reveal a positive waiting time autocorrelation. The implications for microphysical models of the stress-release process driving pulsar glitches are discussed briefly.
ABSTRACT
Three sudden spin-down events, termed ‘antiglitches’, were recently discovered in the accreting pulsar NGC 300 ULX-1 by the Neutron Star Interior Composition Explorer mission. Unlike ...previous antiglitches detected in decelerating magnetars, these are the first antiglitches recorded in an accelerating pulsar. One standard theory is that pulsar spin-up glitches are caused by avalanches of collectively unpinning vortices that transfer angular momentum from the superfluid interior to the crust of a neutron star. Here, we test whether vortex avalanches are also consistent with the antiglitches in NGC 300 ULX-1, with the angular momentum transfer reversed. We perform N-body simulations of up to 5 × 103 pinned vortices in two dimensions in secularly accelerating and decelerating containers. Vortex avalanches routinely occur in both scenarios, propagating inwards and outwards, respectively. The implications for observables, such as size and waiting time statistics, are considered briefly.
ABSTRACT
A microphysics-agnostic meta-model of rotational glitches in rotation-powered pulsars is developed, wherein the globally averaged internal stress accumulates as a Brownian process between ...glitches, and a glitch is triggered once a critical threshold is surmounted. Precise, falsifiable predictions are made regarding long-term event statistics in individual pulsars. For example, the Spearman cross-correlation coefficient between the size of a glitch and the waiting time until the next glitch should exceed 0.25 in all pulsars. Among the six pulsars with the most recorded glitches, PSR J0537−6910 and PSR J0835−4510 are consistent with the predictions of the meta-model, while PSR J1740−3015 and PSR J0631+1036 are not. PSR J0534+2200 and PSR J1341−6220 are only consistent with the meta-model, if there exists an undetected population of small glitches with small waiting times, which we do not resolve. The results are compared with a state-dependent Poisson process, another microphysics-agnostic meta-model in the literature. The results are also applied briefly to recent pulse-to-pulse observations of PSRJ0835−4510, which appear to reveal evidence for a negative fluctuation in rotation frequency just prior to the 2016 glitch.
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