62 research outputs found
Non-linear Kalman filters for calibration in radio interferometry
We present a new calibration scheme based on a non-linear version of Kalman
filter that aims at estimating the physical terms appearing in the Radio
Interferometry Measurement Equation (RIME). We enrich the filter's structure
with a tunable data representation model, together with an augmented
measurement model for regularization. We show using simulations that it can
properly estimate the physical effects appearing in the RIME. We found that
this approach is particularly useful in the most extreme cases such as when
ionospheric and clock effects are simultaneously present. Combined with the
ability to provide prior knowledge on the expected structure of the physical
instrumental effects (expected physical state and dynamics), we obtain a fairly
cheap algorithm that we believe to be robust, especially in low signal-to-noise
regime. Potentially the use of filters and other similar methods can represent
an improvement for calibration in radio interferometry, under the condition
that the effects corrupting visibilities are understood and analytically
stable. Recursive algorithms are particularly well adapted for pre-calibration
and sky model estimate in a streaming way. This may be useful for the SKA-type
instruments that produce huge amounts of data that have to be calibrated before
being averaged
Radio interferometric gain calibration as a complex optimization problem
Recent developments in optimization theory have extended some traditional
algorithms for least-squares optimization of real-valued functions
(Gauss-Newton, Levenberg-Marquardt, etc.) into the domain of complex functions
of a complex variable. This employs a formalism called the Wirtinger
derivative, and derives a full-complex Jacobian counterpart to the conventional
real Jacobian. We apply these developments to the problem of radio
interferometric gain calibration, and show how the general complex Jacobian
formalism, when combined with conventional optimization approaches, yields a
whole new family of calibration algorithms, including those for the polarized
and direction-dependent gain regime. We further extend the Wirtinger calculus
to an operator-based matrix calculus for describing the polarized calibration
regime. Using approximate matrix inversion results in computationally efficient
implementations; we show that some recently proposed calibration algorithms
such as StefCal and peeling can be understood as special cases of this, and
place them in the context of the general formalism. Finally, we present an
implementation and some applied results of CohJones, another specialized
direction-dependent calibration algorithm derived from the formalism.Comment: 18 pages; 6 figures; accepted by MNRA
The XXL Survey: : XXIX. GMRT 610 MHz continuum observations
Accepted for publication in a forthcoming issue of Astronomy & Astrophysics. Reproduced with permission from Astronomy & Astrophysics. © 2018 ESO.We present the 25 square-degree GMRT-XXL-N 610 MHz radio continuum survey, conducted at 50 cm wavelength with the Giant Metrewave Radio Telescope (GMRT) towards the XXL Northern field (XXL-N). We combined previously published observations of the XMM-Large Scale Structure (XMM-LSS) field, located in the central part of XXL-N, with newly conducted observations towards the remaining XXL-N area, and imaged the combined data-set using the Source Peeling and Atmospheric Modeling (SPAM) pipeline. The final mosaic encompasses a total area of 30:4 square degrees, with rms <150 μJy beam -1 over 60% of the area. The rms achieved in the inner 9.6 square degree area, enclosing the XMM-LSS field, is about 200 μJy beam -1, while that over the outer 12.66 square degree area (which excludes the noisy edges) is about 45 μJy beam -1. The resolution of the final mosaic is 6.5 arcsec. We present a catalogue of 5434 sources detected at ≥7 × rms. We verify, and correct the reliability of, the catalog in terms of astrometry, flux, and false detection rate. Making use of the (to date) deepest radio continuum survey over a relatively large (2 square degree) field, complete at the flux levels probed by the GMRT-XXL-N survey, we also assess the survey's incompleteness as a function of flux density. The radio continuum sensitivity reached over a large field with a wealth of multi-wavelength data available makes the GMRTXXL- N 610 MHz survey an important asset for studying the physical properties, environments and cosmic evolution of radio sources, in particular radio-selected active galactic nuclei (AGN).Peer reviewedFinal Accepted Versio
Faraday tomography of LoTSS-DR2 data: I. Faraday moments in the high-latitude outer Galaxy and revealing Loop III in polarisation
Observations of synchrotron emission at low radio frequencies reveal a
labyrinth of polarised Galactic structures. However, the explanation for the
wealth of structures remains uncertain due to the complex interactions between
the interstellar medium and the magnetic field. A multi-tracer approach to the
analysis of large sky areas is needed. This paper aims to use polarimetric
images from the LOFAR Two metre Sky Survey (LoTSS) to produce the biggest
mosaic of polarised emission in the northern sky at low radio frequencies (150
MHz) to date. The large area this mosaic covers allows for detailed
morphological and statistical studies of polarised structures in the
high-latitude outer Galaxy, including the well-known Loop III region. We
produced a 3100 square degree Faraday tomographic cube using a rotation measure
synthesis tool. We calculated the statistical moments of Faraday spectra and
compared them with data sets at higher frequencies (1.4 GHz) and with a map of
a rotation measure derived from extragalactic sources. The mosaic is dominated
by polarised emission connected to Loop III. Additionally, the mosaic reveals
an abundance of other morphological structures, mainly {narrow and extended}
depolarisation canals, which are found to be ubiquitous. We find a correlation
between the map of an extragalactic rotation measure and the LoTSS first
Faraday moment image. The ratio of the two deviates from a simple model of a
Burn slab (Burn 1966) along the line of sight, which highlights the high level
of complexity in the magnetoionic medium that can be studied at these
frequencies.Comment: 20 pages, 25 figures, accepted for publication in A&
Ram-pressure stripped radio tail and two ULXs in the spiral galaxy HCG 97b
We report LOFAR and VLA detections of extended radio emission in the spiral
galaxy HCG 97b, hosted by an X-ray bright galaxy group. The extended radio
emission detected at 144 MHz, 1.4 GHz and 4.8 GHz is elongated along the
optical disk and has a tail that extends 27 kpc in projection towards the
centre of the group at GHz frequencies or 60 kpc at 144 MHz. Chandra X-ray data
show two off-nuclear ultra-luminous X-ray sources (ULXs) with the more distant
one being a suitable candidate for an accreting intermediate-mass black hole
(IMBH) embedded in an environment with an increased density of molecular gas.
Given the observed morphology in optical, CO, and radio continuum, we propose
that the galaxy is undergoing ram-pressure stripping and the relativistic
plasma accelerated in star-forming regions is transported from the galactic
disc by galaxy-intragroup medium interaction. Finally, we also demonstrate that
the formation of the radio tail could, in principle, be the result of putative
IMBH-induced activity, which could facilitate the stripping or inject the radio
plasma via jets.Comment: 15 pages, 11 figures, submitted to MNRAS, comments are welcom
Lofar Low-Band Antenna Observations of the 3C 295 and Bootes Fields: Source Counts and Ultra-Steep Spectrum Sources
We present Low Frequency Array (LOFAR) Low Band observations of the Boötes and 3C 295 fields. Our images made at 34, 46, and 62 MHz reach noise levels of 12, 8, and 5 mJy beam-1, making them the deepest images ever obtained in this frequency range. In t
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