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Mixed-morphology supernova remnants (MM SNRs) are a mysterious class ofobjects that display thermal X-ray emission within their radio shell. They arean older class of SNRs, and as such are profoundly affected by the environmentinto which they evolve. VRO 42.05.01 is a MM SNR of puzzling morphology in thedirection of the Galactic anticentre. Low-frequency radio observations ofsupernova remnants are sensitive to synchrotron electrons accelerated in theshock front. We aim to compare the low-frequency emission to higher frequencyobservations to understand the environmental and shock acceleration conditionsthat have given rise to the observed properties of this source. We present aLOFAR High Band Antenna map centred at 143 MHz of the region of the Galacticplane centred at $l = 166^\rm{o}, b = 3.5^\rm{o}$ at 143 MHz, with a resolutionof 148'' and an rms noise of 4.4 mJy bm$^{-1}$ . Our map is sensitive to scalesas large as $6^\rm{o}$. We compared the LOw Frequency ARay (LOFAR) observationsto archival higher frequency radio, infrared, and optical data to study theemission properties of the source in different spectral regimes. We did thisboth for the SNR and for OA 184, an H II region within our field of view. Wefind that the radio spectral index of VRO 42.05.01 increases at low radiofrequencies; i.e. the LOFAR flux is higher than expected from the measuredspectral index value at higher radio frequencies. This observed curvature inthe low-frequency end of the radio spectrum occurs primarily in the brightestregions of the source, while the fainter regions present a roughly constantpower-law behaviour between 143 MHz and 2695 MHz. We favour an explanation forthis steepening whereby radiative shocks have high compression ratios andelectrons of different energies probe different length scales across theshocks, therefore sampling regions of different compression ratios.

A low-frequency view of mixed-morphology supernova remnant VRO 42.05.01, and its neighbourhood