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

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.

The environment of supernova remnant VRO 42.05.01 as probed with IRAM 30m molecular line observations

“…In our earlier work on VRO 42.05.01 (Arias et al 2019), we followed this claim up by examining the H I cubes from the CGPS (Taylor et al 2003). We did not see some of the purported features of interaction listed in Landecker et al (1989), including the alleged H I cavity.…”

“…In fact, the radius of the small shell delimits the shock front; if the remnant is right at the edge of the close molecular cloud, the stellar wind would have had to blow a cavity slightly larger than 8.4 pc. In light of our new suggested distance for VRO 42.05.01, we revisited the H I cubes from the CGPS for features at velocities closer than those around the range of −22 km s −1 to −54 km s −1 studied by Landecker et al (1989), and that we followed up on in Arias et al (2019). The H I environment at the velocity ranges around our alleged systemic velocity for VRO 41.05.01, −6 km s −1 , is quite crowded.…”

“…1). It has been studied in the X-rays (Burrows & Guo 1994;Guo & Burrows 1997;Bocchino et al 2009;Matsumura et al 2017), in the optical (van den Bergh et al 1973;Fesen et al 1985;Boumis et al 2016), in the radio continuum (Landecker et al 1982;Pineault et al 1985;Leahy & Tian 2005;Arias et al 2019), and in H I (Landecker et al 1989). It has also been detected in GeV γ-rays (Araya 2013), although this detec-SHELL WING Fig.…”

“…The most comprehensive model of the source to date is that of Pineault et al (1987), who proposed that the supernova occurred in a warm medium of intermediate density (which surrounds the shell) and that a part of the remnant (the wing) broke out into a diffuse interstellar cavity, possibly blown by a stellar wind or one or more earlier SNRs. In Arias et al (2019) we elaborated on this model, suggesting that the sharp triangular cavity of the wing could be due to the progenitor star exploding as it was moving supersonically into the more rarefied medium, forming a bow shock whose structure remains visible in the SNR.…”

Candidate radio supernova remnants observed by the GLEAM survey over 345° <l< 60° and 180° <l< 240°

“…Like Landecker et al (1982), we may estimate the density discontinuity between the two slabs by assuming the remnant is in the adiabatic phase and the SNR energy has been divided equally between the two components. The ratio of radii of the two components is 2:1, and the Sedov-Taylor relation (Arias et al, 2019) postulate an alternative explanation for the shape of VRO42.05.01, where the progenitor star was moving at supersonic velocities from a high to low-density environment, creating a bow-shock in the latter. After forming a supernova, the "wing" expands outward into the bow-shock cavity, while the circular shell expands in the cavity left behind in the higherdensity medium.…”