692 research outputs found
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
Very Long Baseline Interferometry Measured Proper Motion and Parallax of the -ray Millisecond Pulsar PSR J0218+4232
PSR J02184232 is a millisecond pulsar (MSP) with a flux density 0.9
mJy at 1.4 GHz. It is very bright in the high-energy X-ray and -ray
domains. We conducted an astrometric program using the European VLBI Network
(EVN) at 1.6 GHz to measure its proper motion and parallax. A model-independent
distance would also help constrain its -ray luminosity. We achieved a
detection of signal-to-noise ratio S/N > 37 for the weak pulsar in all five
epochs. Using an extragalactic radio source lying 20 arcmin away from the
pulsar, we estimate the pulsar's proper motion to be
mas yr and mas yr, and a parallax of mas. The very long
baseline interferometry (VLBI) proper motion has significantly improved upon
the estimates from long-term pulsar timing observations. The VLBI parallax
provides the first model-independent distance constraints:
kpc, with a corresponding lower-limit of
kpc. This is the first pulsar trigonometric parallax measurement based
solely on EVN observations. Using the derived distance, we believe that PSR
J02184232 is the most energetic -ray MSP known to date. The
luminosity based on even our 3 lower-limit distance is high enough to
pose challenges to the conventional outer gap and slot gap models.Comment: 5 pages, 2 figures, 2 tables; published in the Astrophysical Journal
Letters on 2014 Feb. 1
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
Long-term Observations of Three Nulling Pulsars
We present an analysis of approximately 200 hours of observations of the
pulsars J16345107, J17174054 and J18530505, taken over the course of
14.7 yr. We show that all of these objects exhibit long term nulls and
radio-emitting phases (i.e. minutes to many hours), as well as considerable
nulling fractions (NFs) in the range . PSR J17174054 is
also found to exhibit short timescale nulls () and burst phases
() during its radio-emitting phases. This behaviour acts to
modulate the NF, and therefore the detection rate of the source, over
timescales of minutes. Furthermore, PSR J18530505 is shown to exhibit a weak
emission state, in addition to its strong and null states, after sufficient
pulse integration. This further indicates that nulls may often only represent
transitions to weaker emission states which are below the sensitivity
thresholds of particular observing systems. In addition, we detected a
peak-to-peak variation of in the spin-down rate of PSR
J17174054, over timescales of hundreds of days. However, no long-term
correlation with emission variation was found.Comment: 10 pages, 8 figures, accepted for publication in MNRA
Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision
We have measured variations in scattering time scales in the Crab Pulsar over
a 30-year period, using observations made at 610 MHz with the 42-ft telescope
at Jodrell Bank Observatory. Over more recent years, where regular Lovell
Telescope observations at frequencies around 1400 MHz were available, we have
also determined the dispersion measure variations, after disentangling the
scattering delay from the dispersive delay. We demonstrate a relationship
between scattering and dispersion measure variations, with a correlation
coefficient of . The short time scales over which these quantities
vary, the size of the variations, and the close correlation between scattering
and dispersion measure all suggest that the effects are due to discrete
structures within the Crab Nebula, with size scales of AU
(corresponding to an angular size of mas at an assumed distance of 2200
pc). We mitigate the effects of scattering on the observed pulse shape by using
the measured scattering information to modify the template used for generating
the pulse arrival times, thus improving the precision to which the pulsar can
be timed. We test this on timing data taken during periods of high scattering,
and obtain a factor of two improvement in the root mean square of the timing
residuals.Comment: 10 pages, 7 figures. Accepted for publication in MNRA
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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