243 research outputs found
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
PSR J1909-3744, a Binary Millisecond Pulsar with a Very Small Duty Cycle
We report the discovery of PSR J1909-3744, a 2.95 millisecond pulsar in a
nearly circular 1.53 day orbit. Its narrow pulse width of 43 microseconds
allows pulse arrival times to be determined with great accuracy. We have
spectroscopically identified the companion as a moderately hot (T = 8500 K)
white dwarf with strong absorption lines. Radial velocity measurements of the
companion will yield the mass ratio of the system. Our timing data suggest the
presence of Shapiro delay; we expect that further timing observations, combined
with the mass ratio, will allow the first accurate determination of a
millisecond pulsar mass. We have measured the timing parallax and proper motion
for this pulsar which indicate a transverse velocity of 140 (+80/-40) km/s.
This pulsar's stunningly narrow pulse profile makes it an excellent candidate
for precision timing experiments that attempt to detect low frequency
gravitational waves from coalescing supermassive black hole binaries.Comment: 12 pages, 4 figures. Accepted for publication in ApJ
Pulsars in Globular Clusters with the SKA
Globular clusters are highly efficient radio pulsar factories. These pulsars
can be used as precision probes of the clusters' structure, gas content,
magnetic field, and formation history; some of them are also highly interesting
in their own right because they probe exotic stellar evolution scenarios as
well as the physics of dense matter, accretion, and gravity. Deep searches with
SKA1-MID and SKA1-LOW will plausibly double to triple the known population.
Such searches will only require one to a few tied-array beams, and can be done
during early commissioning of the telescope - before an all-sky pulsar survey
using hundreds to thousands of tied-array beams is feasible. With SKA2 it will
be possible to observe most of the active radio pulsars within a large fraction
of the Galactic globular clusters, an estimated population of 600 - 3700
observable pulsars (those beamed towards us). This rivals the total population
of millisecond pulsars that can be found in the Galactic field; fully
characterizing it will provide the best-possible physical laboratories as well
as a rich dynamical history of the Galactic globular cluster system.Comment: 15 pages, 5 figures, to be published in: "Advancing Astrophysics with
the Square Kilometre Array", Proceedings of Science, PoS(AASKA14)04
Discovery of Five Recycled Pulsars in a High Galactic Latitude Survey
We present five recycled pulsars discovered during a 21-cm survey of
approximately 4,150 deg^2 between 15 deg and 30 deg from the galactic plane
using the Parkes radio telescope. One new pulsar, PSR J1528-3146, has a 61 ms
spin period and a massive white dwarf companion. Like many recycled pulsars
with heavy companions, the orbital eccentricity is relatively high (~0.0002),
consistent with evolutionary models that predict less time for circularization.
The four remaining pulsars have short spin periods (3 ms < P < 6 ms); three of
these have probable white dwarf binary companions and one (PSR J2010-1323) is
isolated. PSR J1600-3053 is relatively bright for its dispersion measure of
52.3 pc cm^-3 and promises good timing precision thanks to an intrinsically
narrow feature in its pulse profile, resolvable through coherent dedispersion.
In this survey, the recycled pulsar discovery rate was one per four days of
telescope time or one per 600 deg^2 of sky. The variability of these sources
implies that there are more millisecond pulsars that might be found by
repeating this survey.Comment: 15 pages, 3 figures, accepted for publication in Ap
Implications of the PSR 1257+12 Planetary System for Isolated Millisecond Pulsars
The first extrasolar planets were discovered in 1992 around the millisecond
pulsar PSR 1257+12. We show that recent developments in the study of accretion
onto magnetized stars, plus the existence of the innermost, moon-sized planet
in the PSR 1257+12 system, suggest that the pulsar was born with approximately
its current rotation frequency and magnetic moment. If so, this has important
implications for the formation and evolution of neutron star magnetic fields as
well as for the formation of planets around pulsars. In particular, it suggests
that some and perhaps all isolated millisecond pulsars may have been born with
high spin rates and low magnetic fields instead of having been recycled by
accretion.Comment: 17 pages including one figure, uses aaspp4, accepted by Ap
Simian immunodeficiency virus infection in wild-caught chimpanzees from Cameroon
Simian immunodeficiency viruses (SIVcpz) infecting chimpanzees (Pan troglodytes) in west central Africa are the closest relatives to all major variants of human immunodeficiency virus type 1 ([HIV-1]; groups M, N and O), and have thus been implicated as the source of the human infections; however, information concerning the prevalence, geographic distribution, and subspecies association of SIVcpz still remains limited. In this study, we tested 71 wild-caught chimpanzees from Cameroon for evidence of SIVcpz infection. Thirty-nine of these were of the central subspecies (Pan troglodytes troglodytes), and 32 were of the Nigerian subspecies (Pan troglodytes vellerosus), as determined by mitochondrial DNA analysis. Serological analysis determined that one P. t. troglodytes ape (CAM13) harbored serum antibodies that cross-reacted strongly with HIV-1 antigens; all other apes were seronegative. To characterize the newly identified virus, 14 partially overlapping viral fragments were amplified from fecal virion RNA and concatenated to yield a complete SIVcpz genome (9,284 bp). Phylogenetic analyses revealed that SIVcpzCAM13 fell well within the radiation of the SIVcpzPtt group of viruses, as part of a clade including all other SIVcpzPtt strains as well as HIV-1 groups M and N. However, SIVcpzCAM13 clustered most closely with SIVcpzGAB1 from Gabon rather than with SIVcpzCAM3 and SIVcpzCAM5 from Cameroon, indicating the existence of divergent SIVcpzPtt lineages within the same geographic region. These data, together with evidence of recombination among ancestral SIVcpzPtt lineages, indicate long-standing endemic infection of central chimpanzees and reaffirm a west central African origin of HIV-1. Whether P. t. vellerosus apes are naturally infected with SIVcpz requires further study
Gravitational-radiation losses from the pulsar-white-dwarf binary PSR J1141-6545
Pulsars in close binary orbit around another neutron star or a massive white
dwarf make ideal laboratories for testing the predictions of gravitational
radiation and self-gravitational effects. We report new timing measurements of
the pulsar-white-dwarf binary PSR J1141-6545, providing strong evidence that
such asymmetric systems have gravitational wave losses that are consistent with
general relativity. The orbit is found to be decaying at a rate of
times the general relativistic prediction and the Shapiro delay
is consistent with the orbital inclination angle derived from scintillation
measurements. The system provides a unique test-bed for tensor-scalar theories
of gravity; our current measurements place stringent constraints in the theory
space, with a limit of for weakly non-linear
coupling and an asymptotic limit of for
strongly non-linear coupling, where is the linear coupling strength
of matter to an underlying scalar field. This asymptotic limit is nearly three
times smaller than the Cassini bound ().Comment: 4 pages, 2 figures, To Appear in Physical Review
Green Bank Telescope Studies of Giant Pulses from Millisecond Pulsars
We have conducted a search for giant pulses from four millisecond pulsars
using the 100m Green Bank Telescope. Coherently dedispersed time-series from
PSR J0218+4232 were found to contain giant pulses of very short intrinsic
duration whose energies follow power-law statistics. The giant pulses are in
phase with the two minima of the radio integrated pulse profile but are phase
aligned with the peaks of the X-ray profile. Historically, individual pulses
more than 10-20 times the mean pulse energy have been deemed to be ``giant
pulses''. As only 4 of the 155 pulses had energies greater than 10 times the
mean pulse-energy, we argue the emission mechanism responsible for giant pulses
should instead be defined through: (a) intrinsic timescales of microsecond or
nanosecond duration; (b) power-law energy statistics; and (c) emission
occurring in narrow phase-windows coincident with the phase windows of
non-thermal X-ray emission. Four short-duration pulses with giant-pulse
characteristics were also observed from PSR B1957+20. As the inferred magnetic
fields at the light cylinders of the millisecond pulsars that emit giant pulses
are all very high, this parameter has previously been considered to be an
indicator of giant pulse emissivity. However, the frequency of giant pulse
emission from PSR~B1957+20 is significantly lower than for other millisecond
pulsars that have similar magnetic fields at their light cylinders. This
suggests that the inferred magnetic field at the light cylinder is a poor
indicator of the rate of emission of giant pulses.Comment: 26 pages, 4 figures, accepted to Ap
General Relativistic Geodetic Spin Precession in Binary Pulsar B1913+16: Mapping the Emission Beam in Two Dimensions
We have carefully measured the pulse profile of the binary pulsar PSR
B1913+16 at 21 cm wavelength for twenty years, in order to search for
variations that result from general relativistic geodetic precession of the
spin axis. The profile width is found to decrease with time in its inner
regions, while staying essentially constant on its outer skirts. We fit these
data to a model of the beam shape and precession geometry. Four equivalent
solutions are found, but evolutionary considerations and polarization data
select a single preferred model. While the current data sample only a limited
range of latitudes owing to the long precessional cycle, the preferred model
shows a beam elongated in the latitude direction and hourglass--shaped.Comment: Accepted by AP
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