51 research outputs found
Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog
The polarization properties of radio sources at very low frequencies (h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing
Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog
The polarization properties of radio sources at very low frequencies (<200
MHz) have not been widely measured, but the new generation of low-frequency
radio telescopes, including the Low Frequency Array (LOFAR: a Square Kilometre
Array Low pathfinder), now gives us the opportunity to investigate these
properties. In this paper, we report on the preliminary development of a data
reduction pipeline to carry out polarization processing and Faraday tomography
for data from the LOFAR Two-meter Sky Survey (LOTSS) and present the results of
this pipeline from the LOTSS preliminary data release region (10h45m - 15h30m
right ascension, 45 - 57 degrees declination, 570 square degrees). We have
produced a catalog of 92 polarized radio sources at 150 MHz at 4.3 arcminute
resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized
sources at such low frequencies. We estimate a lower limit to the polarized
source surface density at 150 MHz, with our resolution and sensitivity, of 1
source per 6.2 square degrees. We find that our Faraday depth measurements are
in agreement with previous measurements and have significantly smaller errors.
Most of our sources show significant depolarization compared to 1.4 GHz, but
there is a small population of sources with low depolarization indicating that
their polarized emission is highly localized in Faraday depth. We predict that
an extension of this work to the full LOTSS data would detect at least 3400
polarized sources using the same methods, and probably considerably more with
improved data processing.Comment: 20 pages, 8 figures, 2 catalog tables (non-machine readable),
accepted for publication in A&
GLEAM : The GaLactic and Extragalactic All-sky MWA survey
© Astronomical Society of Australia 2015; published by Cambridge University Press. This is an Open Access article distributed in accordance with the terms of the Creative Commons Attribution (CC BY 4.0) license, which permits others to distribute, remix, adapt and build upon this work, for commercial use, provided the original work is properly cited. See: http://creativecommons.org/licenses/by/4.0/GLEAM, the GaLactic and Extragalactic All-sky MWA survey, is a survey of the entire radio sky south of declination +25 deg at frequencies between 72 and 231 MHz, made with the Murchison Widefield Array (MWA) using a drift scan method that makes efficient use of the MWA's very large field-of-view. We present the observation details, imaging strategies and theoretical sensitivity for GLEAM. The survey ran for two years, the first year using 40 kHz frequency resolution and 0.5 s time resolution; the second year using 10 kHz frequency resolution and 2 s time resolution. The resulting image resolution and sensitivity depends on observing frequency, sky pointing and image weighting scheme. At 154 MHz the image resolution is approximately 2.5 x 2.2/cos(DEC+26.7) arcmin with sensitivity to structures up to ~10 deg in angular size. We provide tables to calculate the expected thermal noise for GLEAM mosaics depending on pointing and frequency and discuss limitations to achieving theoretical noise in Stokes I images. We discuss challenges, and their solutions, that arise for GLEAM including ionospheric effects on source positions and linearly polarised emission, and the instrumental polarisation effects inherent to the MWA's primary beam.Peer reviewedFinal Published versio
Ionospheric modelling using GPS to calibrate the MWA. 1 : Comparison of first order ionospheric effects between GPS models and MWA observations
This document is the Accepted Manuscript version of the following article: B. S. Arora, et al, ‘Ionospheric Modelling using GPS to Calibrate the MWA. I: Comparison of First Order Ionospheric Effects between GPS Models and MWA Observations’, Publications of the Astronomical Society of Australia, Vol. 32, e029, August 2015. The final, published version is available online at doi: https://doi.org/10.1017/pasa.2015.29. COPYRIGHT: © Astronomical Society of Australia 2015.We compare first order (refractive) ionospheric effects seen by the Murchison Widefield Array (MWA) with the ionosphere as inferred from Global Positioning System (GPS) data. The first order ionosphere manifests itself as a bulk position shift of the observed sources across an MWA field of view. These effects can be computed from global ionosphere maps provided by GPS analysis centres, namely the Center for Orbit Determination in Europe (CODE), using data from globally distributed GPS receivers. However, for the more accurate local ionosphere estimates required for precision radio astronomy applications, data from local GPS networks needs to be incorporated into ionospheric modelling. For GPS observations, the ionospheric parameters are biased by GPS receiver instrument delays, among other effects, also known as receiver Differential Code Biases (DCBs). The receiver DCBs need to be estimated for any non-CODE GPS station used for ionosphere modelling, a requirement for establishing dense GPS networks in arbitrary locations in the vicinity of the MWA. In this work, single GPS station-based ionospheric modelling is performed at a time resolution of 10 minutes. Also the receiver DCBs are estimated for selected Geoscience Australia (GA) GPS receivers, located at Murchison Radio Observatory (MRO1), Yarragadee (YAR3), Mount Magnet (MTMA) and Wiluna (WILU). The ionospheric gradients estimated from GPS are compared with the ionospheric gradients inferred from radio source position shifts observed with the MWA. The ionospheric gradients at all the GPS stations show a correlation with the gradients observed with the MWA. The ionosphere estimates obtained using GPS measurements show promise in terms of providing calibration information for the MWA.Peer reviewe
The SUrvey for Pulsars and Extragalactic Radio Bursts - III. Polarization properties of FRBs 160102 and 151230
We report on the polarization properties of two fast radio bursts (FRBs): 151230 and 160102 discovered in the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes Radio Telescope. FRB 151230 is observed to be 6 ± 11 per cent circularly polarized and 35 ± 13 per cent linearly polarized with a rotation measure (RM) consistent with zero. Conversely, FRB160102 is observed to have a circular polarization fraction of 30±11 per cent, linear polarization fraction of 84 ± 15 per cent for RM = -221(6) radm-2, and the highest measured dispersion measure (2596.1±0.3 pc cm-3) for an FRB to date.We examine possible progenitor models for FRB 160102 in extragalactic, non-cosmological and cosmological scenarios. After accounting for the Galactic foreground contribution, we estimate the intrinsic RM to be -256(9) rad m-2in the low-redshift case and ~-2.4×102rad m-2in the highredshift case. We assess the relative likeliness of these scenarios and how each can be tested. We also place constraints on the scattering measure and study the impact of scattering on the signal's polarization position angle
An improved map of the Galactic Faraday sky
We aim to summarize the current state of knowledge regarding Galactic Faraday
rotation in an all-sky map of the Galactic Faraday depth. For this we have
assembled the most extensive catalog of Faraday rotation data of compact
extragalactic polarized radio sources to date. In the map making procedure we
use a recently developed algorithm that reconstructs the map and the power
spectrum of a statistically isotropic and homogeneous field while taking into
account uncertainties in the noise statistics. This procedure is able to
identify some rotation angles that are offset by an integer multiple of pi. The
resulting map can be seen as an improved version of earlier such maps and is
made publicly available, along with a map of its uncertainty. For the angular
power spectrum we find a power law behavior with a power law index of -2.14 for
a Faraday sky where an overall variance profile as a function of Galactic
latitude has been removed, in agreement with earlier work. We show that this is
in accordance with a 3D Fourier power spectrum P(k) proportional to k^-2.14 of
the underlying field n_e times B_r under simplifying geometrical and
statistical assumptions.Comment: 16 pages, 11 figures. Update in one data catalog. All results are
available at http://www.mpa-garching.mpg.de/ift/faraday
Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes (Corrigendum)
Positive and negative ionospheric disturbances prior to the 2016 christmas earthquake in Chile
Global ionospheric maps (GIMs) provided by the Center for Orbit Determination in Europe (CODE) served to analyze ionospheric disturbances prior to the December 25, 2016 earthquake in Chile. Generating global two-dimensional differential vertical total electron content (VTEC) maps it was possible to detect three ionospheric disturbances. Eleven days (December 14, 2016) before the seismic event a positive ionospheric anomaly was detected. On the other hand, 8, 7 and 6 days prior to the earthquake, two negative ionospheric anomalies were also observed. All three anomalies were seen to fall inside the earthquake preparation region. Because during the days in which all three anomalies were observed, the geomagnetic and solar conditions were rather quiet and also the anomalies were localized, it can be suggested that they are seismo-ionospheric signatures due to the Chiloé earthquake. Moreover, it could be also indicated that the possible physical mechanism that originates these three anomalies is air ionization due to the emanation of radon from the Earth’s crust. It is very likely that for the negative ionospheric anomaly observed between December 18 and December 19, 2016, the fountain effect of the Equatorial Ionization Anomaly (EIA) plays a role as well in its generation
Calibrating high-precision Faraday rotation measurements for LOFAR and the next generation of low-frequency radio telescopes (Corrigendum)
Contains fulltext :
147306.pdf (publisher's version ) (Open Access
- …