2,049 research outputs found
A Search for Sub-millisecond Pulsations in Unidentified FIRST and NVSS Radio Sources
We have searched 92 unidentified sources from the FIRST and NVSS 1400 MHz
radio survey catalogs for radio pulsations at 610 MHz. The selected radio
sources are bright, have no identification with extragalactic objects, are
point-like and are more than 5% linearly polarized. Our search was sensitive to
sub-millisecond pulsations from pulsars with dispersion measures (DMs) less
than 500 pc cm-3 in the absence of scattering. We have detected no pulsations
from these sources and consider possible effects which might prevent detection.
We conclude that as a population, these sources are unlikely to be pulsars.Comment: 8 pages, including 2 tables and 1 figure. Accepted for publication in
A
One blind and three targeted searches for (sub)millisecond pulsars
We conducted one blind and three targeted searches for millisecond and
submillisecond pulsars. The blind search was conducted within 3deg of the
Galactic plane and at longitudes between 20 and 110deg. It takes 22073
pointings to cover this region, and 5487 different positions in the sky. The
first targeted search was aimed at Galactic globular clusters, the second one
at 24 bright polarized and pointlike radiosources with steep spectra, and the
third at 65 faint polarized and pointlike radiosources. The observations were
conducted at the large radiotelescope of Nancay Observatory, at a frequency
near 1400 MHz. Two successive backends were used, first a VLBI S2 system,
second a digital acquisition board and a PC with large storage capacity
sampling the signal at 50 Mb/s on one bit, over a 24-MHz band and in one
polarization. The bandwidth of acquisition of the second backend was later
increased to 48 MHz and the sampling rate to 100 Mb/s. The survey used the
three successive setups, with respective sensitivities of 3.5, 2.2, and 1.7
mJy. The targeted-search data were obtained with the third setup and reduced
with a method based on the Hough transform, yielding a sensitivity of 0.9 mJy.
The processing of the data was done in slightly differed time by
soft-correlation in all cases. No new short-period millisecond pulsars were
discovered in the different searches. To better understand the null result of
the blind survey, we estimate the probability of detecting one or more
short-period pulsars among a given Galactic population of synthetic pulsars
with our setup: 25% for the actual incomplete survey and 79% if we had
completed the whole survey with a uniform nominal sensitivity of 1.7 mJy. The
alternative of surveying a smaller, presumably more densely populated, region
with a higher sensitivity would have a low return and would be impractical at a
transit instrument. (abridged)Comment: accepted for publication in Astronomy & Astrophysic
Depolarization of Pulsar Radio Emission
We show that intensity dependent depolarization of single pulses (e.g.,
Xiluoris et al. 1994) may be due to the nonlinear decay of the "upper" ordinary
mode into an unpolarized extraordinary mode and a backward propagating wave.
The decay occurs in the innermost parts of the pulsar magnetosphere for
obliquely propagating O waves.Comment: 6 pages, 1 postscript figur
Observations of 20 millisecond pulsars in 47 Tucanae at 20 cm
We have used a new observing system on the Parkes radio telescope to carry
out a series of pulsar observations of the globular cluster 47 Tucanae at 20-cm
wavelength. We detected all 11 previously known pulsars, and have discovered
nine others, all of which are millisecond pulsars in binary systems. We have
searched the data for relatively short orbital period systems, and found one
pulsar with an orbital period of 96 min, the shortest of any known radio
pulsar.
The increased rate of detections with the new system resulted in improved
estimates of the flux density of the previously known pulsars, determination of
the orbital parameters of one of them, and a coherent timing solution for
another one. Five of the pulsars now known in 47 Tucanae have orbital periods
of a few hours and implied companion masses of only ~ 0.03 Msun. Two of these
are eclipsed at some orbital phases, while three are seen at all phases at 20
cm but not always at lower frequencies. Four and possibly six of the other
binary systems have longer orbital periods and companion masses ~ 0.2 Msun,
with at least two of them having relatively large orbital eccentricities. All
20 pulsars have rotation periods in the range 2-8 ms.Comment: 15 pages, 6 embedded EPS figures, to be published in The
Astrophysical Journa
A periodically active pulsar giving insight into magnetospheric physics
PSR B1931+24 (J1933+2421) behaves as an ordinary isolated radio pulsar during
active phases that are 5-10 days long. However, the radio emission switches off
in less than 10 seconds and remains undetectable for the next 25-35 days, then
it switches on again. This pattern repeats quasi-periodically. The origin of
this behaviour is unclear. Even more remarkably, the pulsar rotation slows down
50% faster when it is on than when it is off. This indicates a massive increase
in magnetospheric currents when the pulsar switches on, proving that pulsar
wind plays a substantial role in pulsar spin-down. This allows us, for the
first time, to estimate the currents in a pulsar magnetospheric during the
occurrence of radio emission.Comment: 12 pages, 2 figure
Are Supershells Powered by Multiple Supernovae? Modeling the Radio Pulsar Population Produced by OB Associations
Traditional searches for radio pulsars have targeted individual small regions
such as supernova remnants or globular clusters, or have covered large
contiguous regions of the sky. None of these searches has been specifically
directed towards giant supershells, some of which are likely to have been
produced by multiple supernova (SN) explosions from an OB association. Here we
perform a Montecarlo simulation of the pulsar population associated with
supershells powered by multiple SNe. We predict that several tens of radio
pulsars could be detected with current instruments associated with the largest
Galactic supershells (with kinetic energies >~ 10^{53} ergs), and a few pulsars
with the smaller ones. We test these predictions for some of the supershells
which lie in regions covered by past pulsar surveys. For the smaller
supershells, our results are consistent with the few detected pulsars per
bubble. For the giant supershell GSH 242-03+37, we find the multiple SN
hypothesis inconsistent with current data at the 95% level. We stress the
importance of undertaking deep pulsar surveys in correlation with supershells.
Failure to detect any pulsar enhancement in the largest of them would put
serious constraints on the multiple SN origin for them. Conversely, the
discovery of the pulsar population associated with a supershell would allow a
different/independent approach to the study of pulsar properties.Comment: accepted to ApJ; 17 pages, 2 figures, 1 tabl
Farming for the patchy Anthropocene: the spatial imaginaries of regenerative agriculture
With its focus on the species level of the Anthropos, there is growing concern that the Anthropocene analytic lacks the conceptual nuance needed to grapple with the unevenly distributed harms and responsibilities tied up with issues of biodiversity loss, global warming, and land use change. Conceptual variants like the patchy Anthropocene have been proposed to better capture the justice implications of these socio-ecological crises, directing attention to their spatially ubiquitous yet context-specific character. The figure of the plantation has come to play an important role in this scholarship due to the contribution intensive agriculture had made to these interlinking crises. Through empirical study of the regenerative agricultural movement, this paper reflects on how regenerative farmers use different sites (fields, soils, livestock stomachs) to apprehend their agro-ethical responsibilities to more-than-human actors both near to and far from the landscapes they manage. Our aims here are two-fold. First, we provide a more affirmative account of agricultural management than is currently offered by plantation farming: a model of food production that is not just âinâ the Anthropocene, but âforâ it. Second, we contribute to ongoing discussions unfolding in the social sciences around the tools needed to conceptualise the interlinking spatial and justice aspects of the Anthropocene transition. By bringing the patchy analytic into conversation with more established geographic writing on scale, volume, and horizontal connections, we show the merit of juxtaposing multiple models of spatial relation as a way of gaining ethical and conceptual traction on complex socio-ecological issues. We argue that the âpolymorphicâ spatial imaginaries of regenerative agriculturalists can offer some guidance on the tools needed to attend to the specificity of local Anthropocene outcomes in relation to socio-ecological forces actuating the world at much greater spatio-temporal scales
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