55 research outputs found
A Micro-glitch in the Millisecond Pulsar B1821-24 in M28
We report on the observation of a very small glitch observed for the first
time in a millisecond pulsar, PSR B1821-24 located in the globular cluster M28.
Timing observations were mainly conducted with the Nancay radiotelescope
(France) and confirmation comes from the 140ft radiotelescope at Green Bank and
the new Green Bank Telescope data. This event is characterized by a rotation
frequency step of 3 nHz, or 10^-11 in fractional frequency change along with a
short duration limited to a few days or a week. A marginally significant
frequency derivative step was also found. This glitch follows the main
characteristics of those in the slow period pulsars, but is two orders of
magnitude smaller than the smallest ever recorded. Such an event must be very
rare for millisecond pulsars since no other glitches have been detected when
the cumulated number of years of millisecond pulsar timing observations up to
2001 is around 500 for all these objects. However, pulsar PSR B1821-24 is one
of the youngest among the old recycled ones and there is likely a correlation
between age, or a related parameter, and timing noise. While this event happens
on a much smaller scale, the required adjustment of the star to a new
equilibrium figure as it spins down is a likely common cause for all glitches.Comment: Accepted by ApJ Letters, 5 pages, 2 figures, LaTex (uses
emulateapj.sty
Superfluid turbulence and pulsar glitch statistics
Experimental evidence is reviewed for the existence of superfluid turbulence
in a differentially rotating, spherical shell at high Reynolds numbers
(\Rey\gsim 10^3), such as the outer core of a neutron star. It is shown that
torque variability increases with \Rey, suggesting that glitch activity in
radio pulsars may be a function of \Rey as well. The \Rey distribution of
the 67 glitching radio pulsars with characteristic ages {\rm
yr} is constructed from radio timing data and cooling curves and compared with
the \Rey distribution of all 348 known pulsars with {\rm
yr}. The two distributions are different, with a Kolmogorov-Smirnov probability
. The conclusion holds for (modified) Urca and
nonstandard cooling, and for Newtonian and superfluid viscosities
Precision Timing of Two Anomalous X-Ray Pulsars
We report on long-term X-ray timing of two anomalous X-ray pulsars, 1RXS
J170849.0-400910 and 1E 2259+586, using the Rossi X-ray Timing Explorer. In
monthly observations made over 1.4 yr and 2.6 yr for the two pulsars,
respectively, we have obtained phase-coherent timing solutions which imply that
these objects have been rotating with great stability throughout the course of
our observations. For 1RXS J170849.0-400910, we find a rotation frequency of
0.0909169331(5) Hz and frequency derivative -15.687(4) x 10^(-14) Hz/s, for
epoch MJD 51215.931. For 1E 2259+586, we find a rotation frequency of
0.1432880613(2)Hz, and frequency derivative -1.0026(7) x 10^(-14) Hz/s, for
epoch MJD 51195.583. RMS phase residuals from these simple models are only
about 0.01 cycles for both sources. We show that the frequency derivative for
1E 2259+586 is inconsistent with that inferred from incoherent frequency
observations made over the last 20 yr. Our observations are consistent with the
magnetar hypothesis and make binary accretion scenarios appear unlikely.Comment: 12 pages including 3 figures. To appear in ApJ Letter
Gravitational radiation from nonaxisymmetric spherical Couette flow in a neutron star
The gravitational wave signal generated by global, nonaxisymmetric shear
flows in a neutron star is calculated numerically by integrating the
incompressible Navier--Stokes equation in a spherical, differentially rotating
shell. At Reynolds numbers \Rey \gsim 3 \times 10^{3}, the laminar Stokes
flow is unstable and helical, oscillating Taylor--G\"ortler vortices develop.
The gravitational wave strain generated by the resulting kinetic-energy
fluctuations is computed in both and polarizations as a function
of time. It is found that the signal-to-noise ratio for a coherent,
-{\rm s} integration with LIGO II scales as for a star at 1 {\rm kpc} with angular velocity
. This should be regarded as a lower limit: it excludes pressure
fluctuations, herringbone flows, Stuart vortices, and fully developed
turbulence (for \Rey \gsim 10^{6}).Comment: (1) School of Physics, University of Melbourne, Parkville, VIC 3010,
Australia. (2) Departamento de Fisica, Escuela de Ciencias,Universidad de
Oriente, Cumana, Venezuela, (3) Department of Mechanical Engineering,
University of Melbourne, Parkville, VIC 3010, Australia. Accepted for
publication in The Astrophysical Journal Letter
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 Second Glitch from the "Anomalous" X-ray Pulsar 1RXS J170849.0-4000910
We report on 5.4 yr of phase-coherent timing, using the Rossi X-ray Timing
Explorer, of the X-ray pulsar 1RXS 1708-4009, a member of the class known as
``anomalous X-ray pulsars.'' This object exhibited a rotational glitch in 1999.
Here we report a second much larger rotational glitch which occured ~1.5 yr
after the first. We show that the recoveries from the two glitches are
different, with the first showing only a possible slow, approximately linear
recovery, while the second showed a nearly complete recovery on a time scale of
\~50 days. The approximately exponential recovery time scale of the second
glitch is similar to that seen recently in 1E 2259+586 at the time of a major
outburst. This suggests 1RXS 1708-4009 undergoes similar bursting behavior,
although with our sparse observations we have detected no other evidence for
bursts from this source.Comment: 13 pages, 2 figures. Revised manuscript and Fig 1 following
refereeing. Accepted for publication in ApJL Aug 1
A Giant Glitch in the Energetic 69 ms X-ray Pulsar AXS J161730-505505
We present new results on the recently discovered 69 ms X-ray pulsar AXS
J161730-505505, the sixth youngest sample of all known pulsars. We have
undertaken a comprehensive X-ray observing campaign of AXS J161730-505505 with
the ASCA, SAX, and XTE observatories and follow its long term spin-down history
between 1989 and 1999, using these, archival GINGA and ASCA data sets, and the
radio ephemeris. The spin-down is not simply described by a linear function as
originally thought, but instead we find evidence of a giant glitch (|Delta P/P|
> 10E-6) between 1993 August and 1997 September, perhaps the largest yet
observed from a young pulsar. The glitch is well described by steps in the
period and its first derivative accompanied by a persistent second derivative
similar to those in the Vela pulsar. The pulse profile of AXS J161730-505505
presents a single asymmetric peak which is maintained over all observation
epochs. The energy spectrum is also steady over time, characterized by a highly
absorbed power-law with a photon index 1.4 +/- 0.2, consistent with that found
for other young rotation powered pulsars.Comment: 6 pages with 2 figures, LaTex, emulateapj.sty. To appear in the
Astrophysical Journal Letter
Global three-dimensional flow of a neutron superfluid in a spherical shell in a neutron star
We integrate for the first time the hydrodynamic
Hall-Vinen-Bekarevich-Khalatnikov equations of motion of a -paired
neutron superfluid in a rotating spherical shell, using a pseudospectral
collocation algorithm coupled with a time-split fractional scheme. Numerical
instabilities are smoothed by spectral filtering. Three numerical experiments
are conducted, with the following results. (i) When the inner and outer spheres
are put into steady differential rotation, the viscous torque exerted on the
spheres oscillates quasiperiodically and persistently (after an initial
transient). The fractional oscillation amplitude () increases
with the angular shear and decreases with the gap width. (ii) When the outer
sphere is accelerated impulsively after an interval of steady differential
rotation, the torque increases suddenly, relaxes exponentially, then oscillates
persistently as in (i). The relaxation time-scale is determined principally by
the angular velocity jump, whereas the oscillation amplitude is determined
principally by the gap width. (iii) When the mutual friction force changes
suddenly from Hall-Vinen to Gorter-Mellink form, as happens when a rectilinear
array of quantized Feynman-Onsager vortices is destabilized by a counterflow to
form a reconnecting vortex tangle, the relaxation time-scale is reduced by a
factor of compared to (ii), and the system reaches a stationary state
where the torque oscillates with fractional amplitude about a
constant mean value. Preliminary scalings are computed for observable
quantities like angular velocity and acceleration as functions of Reynolds
number, angular shear, and gap width. The results are applied to the timing
irregularities (e.g., glitches and timing noise) observed in radio pulsars.Comment: 6 figures, 23 pages. Accepted for publication in 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
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
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