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

¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
e-mail: maria.arias.de.saavedra@gmail.com
² GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³ SRON, Netherlands Institute for Space Research, Utrech, The Netherlands
⁴ ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA, Leiden, The Netherlands
⁶ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
⁷ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL, Nijmegen, The Netherlands
⁸ SURFsara, PO Box 94613 1090 GP, Amsterdam, The Netherlands
⁹ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK
¹⁰ RAL Space, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK

Received: 16 July 2018
Accepted: 7 October 2018

Abstract

Context. Mixed-morphology supernova remnants (MM SNRs) are a mysterious class of objects that display thermal X-ray emission within their radio shell. They are an older class of SNRs, and as such are profoundly affected by the environment into which they evolve. VRO 42.05.01 is a MM SNR of puzzling morphology in the direction of the Galactic anticentre.

Aims. Low-frequency radio observations of supernova remnants are sensitive to synchrotron electrons accelerated in the shock front. We aim to compare the low-frequency emission to higher frequency observations to understand the environmental and shock acceleration conditions that have given rise to the observed properties of this source.

Methods. We present a LOFAR High Band Antenna map centred at 143 MHz of the region of the Galactic plane centred at l = 166 ° ,  b = 3.5° at 143 MHz, with a resolution of 148″ and an rms noise of 4.4 mJy bm⁻¹. Our map is sensitive to scales as large as 6°. We compared the LOw Frequency ARay (LOFAR) observations to archival higher frequency radio, infrared, and optical data to study the emission properties of the source in different spectral regimes. We did this both for the SNR and for OA 184, an H II region within our field of view.

Results. We find that the radio spectral index of VRO 42.05.01 increases at low radio frequencies; i.e. the LOFAR flux is higher than expected from the measured spectral index value at higher radio frequencies. This observed curvature in the low-frequency end of the radio spectrum occurs primarily in the brightest regions of the source, while the fainter regions present a roughly constant power-law behaviour between 143 MHz and 2695 MHz. We favour an explanation for this steepening whereby radiative shocks have high compression ratios and electrons of different energies probe different length scales across the shocks, therefore sampling regions of different compression ratios.