102 research outputs found
Neutron star glitches have a substantial minimum size
Glitches are sudden spin-up events that punctuate the steady spin down of
pulsars and are thought to be due to the presence of a superfluid component
within neutron stars. The precise glitch mechanism and its trigger, however,
remain unknown. The size of glitches is a key diagnostic for models of the
underlying physics. While the largest glitches have long been taken into
account by theoretical models, it has always been assumed that the minimum size
lay below the detectability limit of the measurements. In this paper we define
general glitch detectability limits and use them on 29 years of daily
observations of the Crab pulsar, carried out at Jodrell Bank Observatory. We
find that all glitches lie well above the detectability limits and by using an
automated method to search for small events we are able to uncover the full
glitch size distribution, with no biases. Contrary to the prediction of most
models, the distribution presents a rapid decrease of the number of glitches
below ~0.05 Hz. This substantial minimum size indicates that a glitch must
involve the motion of at least several billion superfluid vortices and provides
an extra observable which can greatly help the identification of the trigger
mechanism. Our study also shows that glitches are clearly separated from all
the other rotation irregularities. This supports the idea that the origin of
glitches is different to that of timing noise, which comprises the unmodelled
random fluctuations in the rotation rates of pulsars.Comment: 8 pages; 4 figures. Accepted for publication in MNRA
On the Apparent Nulls and Extreme Variability of PSR J1107-5907
We present an analysis of the emission behaviour of PSR J1107-5907, a source
known to exhibit separate modes of emission, using observations obtained over
approximately 10 yr. We find that the object exhibits two distinct modes of
emission; a strong mode with a broad profile and a weak mode with a narrow
profile. During the strong mode of emission, the pulsar typically radiates very
energetic emission over sequences of ~200-6000 pulses (~60 s-24 min), with
apparent nulls over time-scales of up to a few pulses at a time. Emission
during the weak mode is observed outside of these strong-mode sequences and
manifests as occasional bursts of up to a few clearly detectable pulses at a
time, as well as low-level underlying emission which is only detected through
profile integration. This implies that the previously described null mode may
in fact be representative of the bottom-end of the pulse intensity distribution
for the source. This is supported by the dramatic pulse-to-pulse intensity
modulation and rarity of exceptionally bright pulses observed during both modes
of emission. Coupled with the fact that the source could be interpreted as a
rotating radio transient (RRAT)-like object for the vast majority of the time,
if placed at a further distance, we advance that this object likely represents
a bridge between RRATs and extreme moding pulsars. Further to these emission
properties, we also show that the source is consistent with being a
near-aligned rotator and that it does not exhibit any measurable spin-down rate
variation. These results suggest that nulls observed in other intermittent
objects may in fact be representative of very weak emission without the need
for complete cessation. As such, we argue that longer (> 1 h) observations of
pulsars are required to discern their true modulation properties.Comment: 15 pages, 10 figures, accepted for publication in MNRA
X-ray Observations of XSS J12270-4859 in a New Low State: A Transformation to a Disk-Free Rotation-Powered Pulsar Binary
We present XMM-Newton and Chandra observations of the low-mass X-ray binary
XSS J12270--4859, which experienced a dramatic decline in optical/X-ray
brightness at the end of 2012, indicative of the disappearance of its accretion
disk. In this new state, the system exhibits previously absent
orbital-phase-dependent, large-amplitude X-ray modulations with a decline in
flux at superior conjunction. The X-ray emission remains predominantly
non-thermal but with an order of magnitude lower mean luminosity and
significantly harder spectrum relative to the previous high flux state. This
phenomenology is identical to the behavior of the radio millisecond pulsar
binary PSR J1023+0038 in the absence of an accretion disk, where the X-ray
emission is produced in an intra-binary shock driven by the pulsar wind. This
further demonstrates that XSS J12270-4859 no longer has an accretion disk and
has transformed to a full-fledged eclipsing "redback" system that hosts an
active rotation-powered millisecond pulsar. There is no evidence for diffuse
X-ray emission associated with the binary that may arise due to outflows or a
wind nebula. An extended source situated 1.5' from XSS J12270--4859 is unlikely
to be associated, and is probably a previously uncatalogued galaxy cluster.Comment: 8 pages, 6 figures; accepted for publication in the Astrophysical
Journa
Correlated emission and spin-down variability in radio pulsars
The recent revelation that there are correlated period derivative and pulse
shape changes in pulsars has dramatically changed our understanding of timing
noise as well as the relationship between the radio emission and the properties
of the magnetosphere as a whole. Using Gaussian processes we are able to model
timing and emission variability using a regression technique that imposes no
functional form on the data. We revisit the pulsars first studied by Lyne et
al. (2010). We not only confirm the emission and rotational transitions
revealed therein, but reveal further transitions and periodicities in 8 years
of extended monitoring. We also show that in many of these objects the pulse
profile transitions between two well-defined shapes, coincident with changes to
the period derivative. With a view to the SKA and other telescopes capable of
higher cadence we also study the detection limitations of period derivative
changes.Comment: 4 pages, 2 Figures, Proceedings of IAU Symposium 337 "Pulsar
Astrophysics - The Next 50 Years" held at Jodrell Bank Observatory, UK Sept.
4-8 201
X-ray Observations of High-B Radio Pulsars
The study of high-magnetic-field pulsars is important for examining the
relationships between radio pulsars, magnetars, and X-ray-isolated neutron
stars (XINSs). Here we report on X-ray observations of three such
high-magnetic-field radio pulsars. We first present the results of a deep
XMM-Newton observation of PSR J1734-3333, taken to follow up on its initial
detection in 2009. The pulsar's spectrum is well fit by a blackbody with a
temperature of 300 +/- 60 eV, with bolometric luminosity L_bb = 2.0(+2.2
-0.7)e+32 erg/s = 0.0036E_dot for a distance of 6.1 kpc. We detect no X-ray
pulsations from the source, setting a 1 sigma upper limit on the pulsed
fraction of 60% in the 0.5-3 keV band. We compare PSR J1734-3333 to other
rotation-powered pulsars of similar age and find that it is significantly
hotter, supporting the hypothesis that the magnetic field affects the observed
thermal properties of pulsars. We also report on XMM-Newton and Chandra
observations of PSRs B1845-19 and J1001-5939. We do not detect either pulsar,
setting 3 sigma upper limits on their blackbody temperatures of 48 and 56 eV,
respectively. Despite the similarities in rotational properties, these sources
are significantly cooler than all but one of the XINSs, which we attribute to
the two groups having been born with different magnetic fields and hence
evolving differently.Comment: 18 pages, 2 tables, 5 figures, accepted for publication in the
Astrophysical Journa
Very long baseline astrometry of PSR J1012+5307 and its implications on alternative theories of gravity
PSR J1012+5307, a millisecond pulsar in orbit with a helium white dwarf (WD),
has been timed with high precision for about 25 years. One of the main
objectives of this long-term timing is to use the large asymmetry in
gravitational binding energy between the neutron star and the WD to test
gravitational theories. Such tests, however, will be eventually limited by the
accuracy of the distance to the pulsar. Here, we present VLBI (very long
baseline interferometry) astrometry results spanning approximately 2.5 years
for PSR J1012+5307, obtained with the Very Long Baseline Array as part of the
MSPSRPI project. These provide the first proper motion and absolute position
for PSR J1012+5307 measured in a quasi-inertial reference frame. From the VLBI
results, we measure a distance of kpc (all the estimates
presented in the abstract are at 68% confidence) for PSR J1012+5307, which is
the most precise obtained to date. Using the new distance, we improve the
uncertainty of measurements of the unmodeled contributions to orbital period
decay, which, combined with three other pulsars, places new constraints on the
coupling constant for dipole gravitational radiation
and the fractional time derivative of
Newton's gravitational constant in the local universe. As the uncertainties of the
observed decays of orbital period for the four leading pulsar-WD systems become
negligible in years, the uncertainties for and
will be improved to and
, respectively, predominantly limited by the distance
uncertainties.Comment: published in ApJ (2020ApJ...896...85D
Reconciling optical and radio observations of the binary millisecond pulsar PSR J1640+2224
Previous optical and radio observations of the binary millisecond pulsar PSR
J1640+2224 have come to inconsistent conclusions about the identity of its
companion, with some observations suggesting the companion is a low-mass
helium-core (He-core) white dwarf (WD), while others indicate it is most likely
a high-mass carbon-oxygen (CO) WD. Binary evolution models predict PSR
J1640+2224 most likely formed in a low-mass X-ray binary (LMXB) based on the
pulsar's short spin period and long-period, low-eccentricity orbit, in which
case its companion should be a He-core WD with mass about , depending on metallicity. If it is instead a CO WD, that would
suggest the system has an unusual formation history. In this paper we present
the first astrometric parallax measurement for this system from observations
made with the Very Long Baseline Array (VLBA), from which we determine the
distance to be . We use this distance and a
reanalysis of archival optical observations originally taken in 1995 with the
Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope (HST) in
order to measure the WD's mass. We also incorporate improvements in
calibration, extinction model, and WD cooling models. We find that the existing
observations are not sufficient to tightly constrain the companion mass, but we
conclude the WD mass is with confidence. The limiting
factor in our analysis is the low signal-to-noise ratio of the original HST
observations.Comment: 6 pages, 5 figure
Model-based asymptotically optimal dispersion measure correction for pulsar timing
In order to reach the sensitivity required to detect gravitational waves,
pulsar timing array experiments need to mitigate as much noise as possible in
timing data. A dominant amount of noise is likely due to variations in the
dispersion measure. To correct for such variations, we develop a statistical
method inspired by the maximum likelihood estimator and optimal filtering. Our
method consists of two major steps. First, the spectral index and amplitude of
dispersion measure variations are measured via a time-domain spectral analysis.
Second, the linear optimal filter is constructed based on the model parameters
found in the first step, and is used to extract the dispersion measure
variation waveforms. Compared to current existing methods, this method has
better time resolution for the study of short timescale dispersion variations,
and generally produces smaller errors in waveform estimations. This method can
process irregularly sampled data without any interpolation because of its
time-domain nature. Furthermore, it offers the possibility to interpolate or
extrapolate the waveform estimation to regions where no data is available.
Examples using simulated data sets are included for demonstration.Comment: 15 pages, 15 figures, submitted 15th Sept. 2013, accepted 2nd April
2014 by MNRAS. MNRAS, 201
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