Browsing by Author "Sekhar, Srikrishna"

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    Direction-dependent corrections in polarimetric radio imaging. Iii. A-to-Z solver— Modeling the full jones antenna aperture illumination pattern
    (IOP Publishing, 2022) Sekhar, Srikrishna; Jagannathan, Preshanth; Kirk, Brian
    In this third paper of a series describing direction-dependent corrections for polarimetric radio imaging, we present the the A-to-Z solver methodology to model the full Jones antenna aperture illumination pattern (AIP) using Zernike polynomials. In order to achieve accurate, thermal noise-limited imaging with modern radio interferometers, it is necessary to correct for the instrumental effects of the antenna primary beam (PB) as a function of time, frequency, and polarization. The algorithm employs the orthonormal, circular Zernike polynomial basis to model the full Jones AIP response, which is obtained by a Fourier transform of corresponding antenna holography measurements. These full Jones models are then used to reconstruct the full Mueller AIP response of an antenna, in principle accounting for all the off-axis frequency-dependent leakage effects of the PB. The A-to-Z solver is general enough to accommodate any interferometer for which holographic measurements exist, and we have successfully modeled the AIP of the VLA, MeerKAT, and ALMA as a demonstration of its versatility. We show that our models capture the PB morphology to high accuracy within the first two side lobes, and show the viability of full Mueller gridding and deconvolution for any telescope given high-quality holographic measurements.
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    MIGHTEE polarization early science fields: The deep polarized sky
    (Oxford University Press, 2024) Taylor, Andrew Russell; Sekhar, Srikrishna; Collier, Jordan D.
    The MeerKAT International GigaHertz Tiered Extragalactic Exploration (MIGHTEE) is one of the MeerKAT large survey projects, designed to pathfind SKA key science. MIGHTEE is undertaking deep radio imaging of four well-observed fields (COSMOS, XMM-LSS, ELAIS S1, and CDFS) totaling 20 square degrees to μJy sensitivities. Broad-band imaging observations between 880 and1690 MHz yield total intensity continuum, spectro-polarimetry, and atomic hydrogen spectral imaging. Early science data from MIGHTEE are being released from initial observations of COSMOS and XMM–LSS. This paper describes the spectro-polarimetric observations, the polarization data processing of the MIGHTEE early science fields, and presents polarization data images and catalogues. The catalogues include radio spectral index, redshift information, and faraday rotation measure synthesis results for 13 267 total intensity radio sources down to a polarized intensity detection limit of ∼20 μJy bm−1. Polarized signals were detected from 324 sources. For the polarized detections, we include a catalogue of faraday depth from both faraday synthesis and Q, U fitting, as well as total intensity and polarization spectral indices. The distribution of redshift of the total radio sources and detected polarized sources are the same, with median redshifts of 0.86 and 0.82, respectively. Depolarization of the emission at longer-wavelengths is seen to increase with decreasing total-intensity spectral index, implying that depolarization is intrinsic to the radio sources. No evidence is seen for a redshift dependence of the variance of faraday depth.
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    Pks 1830-211: oh and hi at z = 0.89 and the first meerkat uhf spectrum
    (EDP Sciences, 2021) Combes, Françoise; Gupta, Neeraj; Sekhar, Srikrishna
    The Large Survey Project (LSP) "MeerKAT Absorption Line Survey"(MALS) is a blind H I 21 cm and OH 18 cm absorption line survey in the L- and UHF-bands, primarily designed to better determine the occurrence of atomic and molecular gas in the circumgalactic and intergalactic medium, and its redshift evolution. Here we present the first results using the UHF band obtained towards the strongly lensed radio source PKS 1830-211, revealing the detection of absorption produced by the lensing galaxy. With merely 90 min of data acquired on-source for science verification and processed using the Automated Radio Telescope Imaging Pipeline (ARTIP), we detect in absorption the known H I 21 cm and OH 18 cm main lines at z = 0.89 at an unprecedented signal-to-noise ratio (4000 in the continuum, in each 6 km s-1 wide channel). For the first time we report the detection of OH satellite lines at z = 0.89, which until now have not been detected at z > 0.25.