Low(er) frequency follow-up of 28 candidate, large-scale synchrotron sources

TL;DR

This study re-examines 28 candidate large-scale synchrotron sources near galaxy clusters using lower frequency radio observations, finding no evidence that these sources are related to the cosmic web's synchrotron emission.

Contribution

It introduces a novel technique to assess surface brightness sensitivity for detecting large-scale synchrotron emission at low frequencies.

Findings

Lower frequency observations did not confirm the candidate sources as synchrotron emission.

The sources are unlikely to be associated with the cosmic web.

The technique improves sensitivity assessment for extended radio sources.

Abstract

We follow up on a report by Vacca et al. (2018) of 28 candidate large-scale diffuse synchrotron sources in an 8{\deg}$\times$8{\deg} area of the sky (centred at RA 5h0m0s Dec 5{\deg}48'0''). These sources were originally observed at 1.4 GHz using a combination of the single-dish Sardinia Radio Telescope (SRT) and archival NRAO VLA Sky Survey (NVSS) data. They are in an area with nine massive galaxy clusters at z $\approx$ 0.1, and are candidates for the first detection of filaments of the synchrotron cosmic web. We attempt to verify these candidate sources with lower frequency observations at 154 MHz with the Murchison Widefield Array (MWA) and at 887 MHz with the Australian Square Kilometre Array Pathfinder (ASKAP). We use a novel technique to calculate the surface brightness sensitivity of these instruments to show that our lower frequency observations, and in particular those by ASKAP, are ideally suited to detect large-scale, extended synchrotron emission. Nonetheless, we are forced to conclude that none of these sources are likely to be synchrotron in origin or associated with the cosmic web.