New supernova remnant candidates in the LOFAR Two Metre Sky Survey

Context. In spite of their key role in galaxy evolution and several decades of observational efforts, the census of supernova remnants (SNRs) in our Galaxy remains incomplete. Theoretical predictions based on the local supernova rate estimate the expected number of SNRs in the Galaxy to be greater than or similar to 1000. By contrast, the number of detected SNRs amounts to about 300. High-resolution, wide-area radio surveys at low frequencies are ideal tools with which to find missing SNRs, given the prominence of these sources at low radio frequencies. Aims. We aim to find missing SNRs using proprietary data from the LOFAR Two-Metre Sky Survey (LoTSS) at 144 MHz. Methods. We used LoTSS total intensity maps of two Galactic regions, one with 39 degrees < l < 66 degrees and |b| < 2.5 degrees, and the other with 145 degrees < l < 150 degrees and |b| < 3 degrees, in addition to mid-infrared (MIR) data from the Wide-field Infrared Survey Explorer (WISE) all-sky survey to search for SNR candidates. Results. We report the discovery of 14 new SNR candidates selected on the basis of their morphology at 144 MHz and a lack of MIR emission. We also follow up on 24 previously reported SNR candidates, inferring their spectral index between the LoTSS frequency (144 MHz) and the frequency at which they were reported. We confirm that 6 of these 24 sources have a non-thermal spectral index, whereas another 4 have a thermal index and are thus rejected as SNR candidates; our study is inconclusive regarding the remaining 14 sources. The majority of the new SNR candidates are small in angular size (<20') and have low surface brightness at 1 GHz (less than or similar to 10-21 W m-2 Hz-1 sr-1). Additionally, most of them are located in a Galactic region with 39 degrees < l < 60 degrees and |b| < 2.5 degrees. Conclusions. The high resolution and sensitivity of LoTSS observations has resulted in the detection of 14 new SNR candidates. In order to unambiguously confirm the SNR nature of these candidates, follow-up X-ray observations are required with facilities such as eROSITA.