753 research outputs found
The influence of interpolation and station network density on the distributions and trends of climate variables in gridded daily data
We study the influence of station network density on the distributions and trends in indices of area-average daily precipitation and temperature in the E-OBS high resolution gridded dataset of daily climate over Europe, which was produced with the primary purpose of Regional Climate Model evaluation. Area averages can only be determined with reasonable accuracy from a sufficiently large number of stations within a grid-box. However, the station network on which E-OBS is based comprises only 2,316 stations, spread unevenly across approximately 18,000 0.22A degrees grid-boxes. Consequently, grid-box data in E-OBS are derived through interpolation of stations up to 500 km distant, with the distance of stations that contribute significantly to any grid-box value increasing in areas with lower station density. Since more dispersed stations have less shared variance, the resultant interpolated values are likely to be over-smoothed, and extreme daily values even more so. We perform an experiment over five E-OBS grid boxes for precipitation and temperature that have a sufficiently dense local station network to enable a reasonable estimate of the area-average. We then create a series of randomly selected station sub-networks ranging in size from four to all stations within the E-OBS interpolation search radii. For each sub-network realisation, we estimate the grid-box average applying the same interpolation methodology as used for E-OBS, and then evaluate the effect of network density on the distribution of daily values, as well as trends in extremes indices. The results show that when fewer stations have been used for the interpolation, both precipitation and temperature are over-smoothed, leading to a strong tendency for interpolated daily values to be reduced relative to the "true" area-average. The smoothing is greatest for higher percentiles, and therefore has a disproportionate effect on extremes and any derived extremes indices. For many regions of the E-OBS dataset, the station density is sufficiently low to expect this smoothing effect to be significant and this should be borne in mind by any users of the E-OBS dataset
pulsar_spectra: A pulsar flux density catalogue and spectrum fitting repository
We present the pulsar_spectra software repository, an open-source pulsar flux
density catalogue and automated spectral fitting software that finds the best
spectral model and produces publication-quality plots. The Python-based
software includes features that enable users in the astronomical community to
add newly published spectral measurements to the catalogue as they become
available. The spectral fitting software is an implementation of the method
described in Jankowski et al. (2018) which uses robust statistical methods to
decide on the best-fitting model for individual pulsar spectra. pulsar_spectra
is motivated by the need for a centralised repository for pulsar flux density
measurements to make published measurements more accessible to the astronomical
community and provide a suite of tools for measuring spectra
Testing the circularity of PSR B0818-41's carousel
The phenomenon of sub-pulse drifting is an important single-pulse phenomenon
that can potentially provide important insights into the elusive radio emission
mechanism in pulsars. We analyze the frequency behaviour of the single pulses
of B0818-41, observed from 300 to 500 MHz (Band 3 of the uGMRT), and compare it
to the evolution of the average profile to place constraints on the geometry of
the pulsar's emission beam. We show that a circular carousel of discrete
beamlets, where each beamlet has radial symmetry, is not consistent with the
observed behaviour, and describe an alternative, consistent range of possible
elliptical carousel geometries. We also combine the uGMRT data with some
archival MWA observations and several other published profiles to characterize
the profile evolution across a frequency range spanning ~170 MHz to ~1.4 GHz
PSR J0026-1955: A curious case of evolutionary subpulse drifting and nulling
PSR J0026-1955 was independently discovered by the Murchison Widefield Array
(MWA) recently. The pulsar exhibits subpulse drifting, where the radio emission
from a pulsar appears to drift in spin phase within the main pulse profile, and
nulling, where the emission ceases briefly. The pulsar showcases a curious case
of drift rate evolution as it exhibits rapid changes between the drift modes
and a gradual evolution in the drift rate within a mode. Here we report new
analysis and results from observations of J0026-1955 made with the upgraded
Giant Meterwave Radio Telescope (uGMRT) at 300-500 MHz. We identify two
distinct subpulse drifting modes: A and B, with mode A sub-categorised into A0,
A1, and A2, depending upon the drift rate evolutionary behaviour. Additionally,
the pulsar exhibits short and long nulls, with an estimated overall nulling
fraction of ~58%, which is lower than the previously reported value. Our
results also provide evidence of subpulse memory across nulls and a consistent
behaviour where mode A2 is often followed by a null. We investigate the drift
rate modulations of J0026-1955 and put forward two different models to explain
the observed drifting behaviour. We suggest that either a change in polar gap
screening or a slow relaxation in the spark configuration could possibly drive
the evolution in drift rates. J0026-1955 belongs to a rare subset of pulsars
which exhibit subpulse drifting, nulling, mode changing, and drift rate
evolution. It is, therefore, an ideal test bed for carousel models and to
uncover the intricacies of pulsar emission physics.Comment: 14 pages, 10 figures, 1 table. Accepted for publication in MNRA
Single-pulse analysis and average emission characteristics of PSR J1820-0427 from observations made with the MWA and uGMRT
We have studied the pulse-to-pulse variability in PSR J1820--0427 and its
frequency dependence using high-quality, wide-band observations made from the
upgraded Giant Metrewave Radio Telescope (uGMRT; 300-750 MHz) and the Murchison
Widefield Array (170-200 MHz). The low-frequency data reveal a previously
unreported feature in the average profile (at 185 MHz) after accounting for the
effects of temporal broadening arising from multi-path scattering due to the
Interstellar Medium (ISM). We advance a new method for flux density calibration
of beamformed data from the uGMRT and use it to measure the single pulse flux
densities across the uGMRT band. Combined with previously published
measurements, these flux densities are best fit with a power-law spectrum with
a low-frequency turnover. We also use calibrated flux densities to explore the
relationship between pulse-to-pulse variability and the spectral index of
individual pulses. Our analysis reveals a large scatter in the single-pulse
spectral indices and a general tendency for brighter pulses to show a
steepening of the spectral index. We also examine the frequency-dependence of
the pulse-fluence distribution and its relation to the Stochastic Growth
Theory.Comment: 13 pages, 9 figures, 2 tables. Accepted for publication in MNRA
MWA Tied-Array Processing IV: A Multi-Pixel Beamformer for Pulsar Surveys and Ionospheric Corrected Localisation
The Murchison Widefield Array (MWA) is a low-frequency aperture array capable
of high-time and frequency resolution astronomy applications such as pulsar
studies. The large field-of-view of the MWA (hundreds of square degrees) can
also be exploited to attain fast survey speeds for all-sky pulsar search
applications, but to maximise sensitivity requires forming thousands of
tied-array beams from each voltage-capture observation. The necessity of using
calibration solutions that are separated from the target observation both
temporally and spatially makes pulsar observations vulnerable to uncorrected,
frequency-dependent positional offsets due to the ionosphere. These offsets may
be large enough to move the source away from the centre of the tied-array beam,
incurring sensitivity drops of 30-50\% in Phase II extended array
configuration. We analyse these offsets in pulsar observations and develop a
method for mitigating them, improving both the source position accuracy and the
sensitivity. This analysis prompted the development of a multi-pixel
beamforming functionality that can generate dozens of tied-array beams
simultaneously, which runs a factor of ten times faster compared to the
original single-pixel version. This enhancement makes it feasible to observe
multiple pulsars within the vast field of view of the MWA and supports the
ongoing large-scale pulsar survey efforts with the MWA. We explore the extent
to which ionospheric offset correction will be necessary for the MWA Phase III
and the low-frequency Square Kilometre Array (SKA-Low).Comment: 10 pages, 5 figure
Cortical thickness and gyrification patterns in patients with psychogenic non-epileptic seizures
Psychogenic non-epileptic seizures (PNES) are often viewed as manifestations of altered motor and sensory function resulting from psychological responses to adverse experiences. Yet many patients and non-expert healthcare professionals find it difficult to understand how severe disturbances in normal neurological functioning can solely result from underlying psychological mechanisms to the exclusion of other physical causes. Perhaps importantly, recent advances using neuroimaging techniques point to possible structural and functional correlates in PNES. In an attempt to further our understanding of the neurobiological correlates of this condition, we compared the brain scans of 20 patients with PNES (14 females, mean age 41.05, range 19-62) and 20 age- and gender-matched healthy controls (14 females, mean age 40.65, range 21-61) to investigate group differences for cortical thickness and gyrification patterns using FreeSurfer. Compared to controls, patients with PNES showed cortical thickness increases in motor, sensory and occipital areas as well as cortical thickness decreases in temporal and frontal brain regions. In addition, we observed age-related changes in cortical thickness in the right lateral occipital area in PNES. However, contrary to our prediction that atypical gyrification may be present, we did not find any evidence of abnormalities on a measure thought to reflect prenatal and early childhood cortical development and organization. Nor did we find significant correlations between cortical thickness results and clinical features. These findings partly corroborate, but also differ from previous morphometric studies in PNES. These inconsistencies likely reflect the aetiology and phenomenological heterogeneity of PNES
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