V-LoTSS: The Circularly-Polarised LOFAR Two-metre Sky Survey

TL;DR

This paper reports the detection of 68 circularly polarised radio sources from the LOFAR Two-metre Sky Survey, revealing diverse source classes and demonstrating the survey's sensitivity and potential for discovering new objects.

Contribution

It presents the most sensitive circular polarisation survey at 144 MHz, identifying various source classes and methods to classify unidentified sources.

Findings

Detected 68 circularly polarised sources.

Identified four source classes: stellar systems, pulsars, AGN, unidentified.

Survey completeness at flux densities ≥1 mJy.

Abstract

We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We use the second data release of the 144 MHz LOFAR Two-metre Sky Survey to produce circularly-polarised maps with median 140 $\mu$Jy beam$^{-1}$ noise and resolution of 20$''$ for $\approx$27% of the northern sky (5634 deg$^{2}$). The leakage of total intensity into circular polarisation is measured to be $\approx$0.06%, and our survey is complete at flux densities $\geq1$ mJy. A detection is considered reliable when the circularly-polarised fraction exceeds 1%. We find the population of circularly-polarised sources is composed of four distinct classes: stellar systems, pulsars, active galactic nuclei, and sources unidentified in the literature. The stellar systems can be further separated into chromospherically-active stars, M dwarfs, and brown dwarfs. Based on the circularly-polarised fraction and lack of an optical counterpart, we show it is possible to infer whether the unidentified sources are likely unknown pulsars or brown dwarfs. By the completion of this survey of the northern sky, we expect to detect 300$\pm$100 circularly-polarised sources.