Limits on long-time-scale radio transients at 150 MHz using the TGSS ADR1 and LoTSS DR2 catalogues

Iris De Ruiter,Ralph A M J Wijers,Guillaume Leseigneur,Antonia Rowlinson,Alexander Drabent,Huib T Intema,Timothy W Shimwell

doi:10.1093/mnras/stab2695

Abstract

ABSTRACT We present a search for transient radio sources on time-scales of 2–9 yr at 150 MHz. This search is conducted by comparing the first Alternative Data Release of the TIFR GMRT Sky Survey (TGSS ADR1) and the second data release of the LOFAR Two-metre Sky Survey (LoTSS DR2). The overlapping survey area covers 5570 $\rm {deg}^2$ on the sky, or 14 per cent of the total sky. We introduce a method to compare the source catalogues that involves a pair match of sources, a flux density cutoff to meet the survey completeness limit and a newly developed compactness criterion. This method is used to identify both transient candidates in the TGSS source catalogue that have no counterpart in the LoTSS catalogue and transient candidates in LoTSS without a counterpart in TGSS. We find that imaging artefacts and uncertainties and variations in the flux density scales complicate the transient search. Our method to search for transients by comparing two different surveys, while taking into account imaging artefacts around bright sources and misaligned flux scales between surveys, is universally applicable to future radio transient searches. No transient sources were identified, but we are able to place an upper limit on the transient surface density of &lt;5.4 × 10−4 deg−2 at 150 MHz for compact sources with an integrated flux density over 100 mJy. Here we define a transient as a compact source with flux density greater than 100 mJy that appears in the catalogue of one survey without a counterpart in the other survey.

Highlights

In this paper we present a search for radio transients at 150 MHz, over timescales of 2 to 9 years, to an integrated flux density cutoff of 100 mJy. This search is conducted by comparing the first Alternative Data Release of the TIFR Giant Metrewave Radio Telescope (GMRT) Sky Survey (TGSS ADR1; Intema et al (2017)) at 150 MHz and the second data release of the Low Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS DR2; Shimwell et al in prep.) at 144 MHz
The search for transient sources in the TGSS survey leaves us with a sample of 10 transient candidates that are not found in the LOFAR TwoMetre Sky Survey (LoTSS) survey
We present the results of a blind transient search at low frequency by comparing TGSS ADR1 and LoTSS DR2

Summary

Introduction

There are several astrophysical phenomena that are known to be transient at low frequencies. In this study we focus on searching for previously unknown low-frequency (150 MHz) long-timescale (> year) radio transients and extreme variables. Our search is sensitive to various phenomena, for example, active galactic nuclei (AGNs) are known to be variable on these timescales (Figure 3 in Pietka et al (2015)) and at these frequencies (Hajela et al 2019). Both variable radio AGN (Williams & Berger 2016; Nyland et al 2020; KunertBajraszewska et al 2020) and changing-look AGN (Wołowska et al 2017, 2021) may be observed as transient radio emission.

Methods

Results

Discussion

Conclusion