559 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
Long-term Radio Observations of the Intermittent Pulsar B1931+24
We present an analysis of approximately 13-yr of observations of the
intermittent pulsar B1931+24 to further elucidate its behaviour. We find that
while the source exhibits a wide range of nulling (~4-39 d) and radio-emitting
(~1-19 d) timescales, it cycles between its different emission phases over an
average timescale of approximately 38 d, which is remarkably stable over many
years. On average, the neutron star is found to be radio emitting for 26 +- 6 %
of the time. No evidence is obtained to suggest that the pulsar undergoes any
systematic, intrinsic variations in pulse intensity during the radio-emitting
phases. In addition, we find no evidence for any correlation between the length
of consecutive emission phases. An analysis of the rotational behaviour of the
source shows that it consistently assumes the same spin-down rates, i.e. nudot
= -16 +- 1 x 10^-15 s^-2 when emitting and nudot = -10.8 +- 0.4 x 10^-15 s^-2
when not emitting, over the entire observation span. Coupled with the stable
switching timescale, this implies that the pulsar retains a high degree of
magnetospheric memory, and stability, in spite of comparatively rapid (~ms)
dynamical plasma timescales. While this provides further evidence to suggest
that the behaviour of the neutron star is governed by magnetospheric-state
switching, the underlying trigger mechanism remains illusive. This should be
elucidated by future surveys with next generation telescopes such as LOFAR,
MeerKAT and the SKA, which should detect similar sources and provide more clues
to how their radio emission is regulated.Comment: 12 pages, 12 figures, accepted for publication in MNRA
A Study of Giant Pulses from PSR J1824-2452A
We have searched for microsecond bursts of emission from millisecond pulsars
in the globular cluster M28 using the Parkes radio telescope. We detected a
total of 27 giant pulses from the known emitter PSR J1824-2452A. At wavelengths
around 20 cm the giant pulses are scatter-broadened to widths of around 2
microseconds and follow power-law statistics. The pulses occur in two narrow
phase-windows which correlate in phase with X-ray emission and trail the peaks
of the integrated radio pulse-components. Notably, the integrated radio
emission at these phase windows has a steeper spectral index than other
emission. The giant pulses exhibit a high degree of polarization, with many
being 100% elliptically polarized. Their position angles appear random.
Although the integrated emission of PSR J1824-2452A is relatively stable for
the frequencies and bandwidths observed, the intensities of individual giant
pulses vary considerably across our bands. Two pulses were detected at both
2700 and 3500 MHz. The narrower of the two pulses is 20 ns wide at 3500 MHz. At
2700 MHz this pulse has an inferred brightness temperature at maximum of 5 x
10^37 K. Our observations suggest the giant pulses of PSR J1824-2452A are
generated in the same part of the magnetosphere as X-ray emission through a
different emission process to that of ordinary pulses.Comment: Accepted by Ap
Simultaneous Absolute Timing of the Crab Pulsar at Radio and Optical Wavelengths
The Crab pulsar emits across a large part of the electromagnetic spectrum.
Determining the time delay between the emission at different wavelengths will
allow to better constrain the site and mechanism of the emission. We have
simultaneously observed the Crab Pulsar in the optical with S-Cam, an
instrument based on Superconducting Tunneling Junctions (STJs) with s time
resolution and at 2 GHz using the Nan\c{c}ay radio telescope with an instrument
doing coherent dedispersion and able to record giant pulses data. We have
studied the delay between the radio and optical pulse using simultaneously
obtained data therefore reducing possible uncertainties present in previous
observations. We determined the arrival times of the (mean) optical and radio
pulse and compared them using the tempo2 software package. We present the most
accurate value for the optical-radio lag of 255 21 s and suggest the
likelihood of a spectral dependence to the excess optical emission asociated
with giant radio pulses.Comment: 8 pages; accepted for publication in Astronomy and Astrophysic
Radio disappearance of the magnetar XTE J1810-197 and continued X-ray timing
We report on timing, flux density, and polarimetric observations of the
transient magnetar and 5.54 s radio pulsar XTE J1810-197 using the GBT, Nancay,
and Parkes radio telescopes beginning in early 2006, until its sudden
disappearance as a radio source in late 2008. Repeated observations through
2016 have not detected radio pulsations again. The torque on the neutron star,
as inferred from its rotation frequency derivative f-dot, decreased in an
unsteady manner by a factor of 3 in the first year of radio monitoring. In
contrast, during its final year as a detectable radio source, the torque
decreased steadily by only 9%. The period-averaged flux density, after
decreasing by a factor of 20 during the first 10 months of radio monitoring,
remained steady in the next 22 months, at an average of 0.7+/-0.3 mJy at 1.4
GHz, while still showing day-to-day fluctuations by factors of a few. There is
evidence that during this last phase of radio activity the magnetar had a steep
radio spectrum, in contrast to earlier behavior. There was no secular decrease
that presaged its radio demise. During this time the pulse profile continued to
display large variations, and polarimetry indicates that the magnetic geometry
remained consistent with that of earlier times. We supplement these results
with X-ray timing of the pulsar from its outburst in 2003 up to 2014. For the
first 4 years, XTE J1810-197 experienced non-monotonic excursions in f-dot by
at least a factor of 8. But since 2007, its f-dot has remained relatively
stable near its minimum observed value. The only apparent event in the X-ray
record that is possibly contemporaneous with the radio shut-down is a decrease
of ~20% in the hot-spot flux in 2008-2009, to a stable, minimum value. However,
the permanence of the high-amplitude, thermal X-ray pulse, even after the radio
demise, implies continuing magnetar activity.Comment: ApJ, accepted, 12 pages, 9 figure
An improved solar wind electron-density model for pulsar timing
Variations in the solar wind density introduce variable delays into pulsar
timing observations. Current pulsar timing analysis programs only implement
simple models of the solar wind, which not only limit the timing accuracy, but
can also affect measurements of pulsar rotational, astrometric and orbital
parameters. We describe a new model of the solar wind electron density content
which uses observations from the Wilcox Solar Observatory of the solar magnetic
field. We have implemented this model into the tempo2 pulsar timing package. We
show that this model is more accurate than previous models and that these
corrections are necessary for high precision pulsar timing applications.Comment: Accepted by ApJ, 13 pages, 4 figure
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