Matching LOFAR sources across radio bands^★

¹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
e-mail: Lboehme@physik.uni-bielefeld.de
² Istituto di Radioastronomia, Istituto Nazionale di Astrofisica, Via Gobetti 101, 40127 Bologna, Italy
³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ SKA Observatory, Jodrell Bank, Lower Withington, Macclesfield SK11 9FT, UK

Received: 12 December 2022
Accepted: 27 April 2023

Abstract

Aims. The recent preliminary release of the LOFAR LBA Sky Survey (LoLSS) is the first wide-area, ultra-low frequency observation published from the Low Frequency Array (LOFAR). Our aim is to combine this data set with other surveys at higher frequencies to study the spectral properties of a large sample of radio sources.

Methods. We present a new cross-matching algorithm that takes into account the sizes of the radio sources. We applied it to the LoLSS-PR, LoTSS-DR1, LoTSS-DR2 (all LOFAR), TGSS-ADR1 (GMRT), WENSS (WSRT), and NVSS (VLA) catalogues. We then studied the number of matched counterparts for LoLSS radio sources and their spectral properties.

Results. We found counterparts for 22 607 (89.5%) LoLSS sources. The remaining 2640 sources (10.5%) were identified either as an artefact in the LoLSS survey (3.6%) or flagged due to their closeness to bright sources (6.9%). We found an average spectral index of α = −0.77 ± 0.18 between LoLSS and NVSS. Between LoLSS and LoTSS-DR2, we found α = −0.71 ± 0.31. The average spectral index is independent of the flux density above S₅₄ = 181 mJy. A comparison of the spectral slopes from LoLSS-LoTSS-DR2 with LoTSS-DR2–NVSS indicates that the probed population of radio sources shows evidence of a negative spectral curvature.