Spectral features and variable circular polarisation in the radio emission from the pre-cataclysmic variable QS Vir

Context. QS Vir is a low-accretion rate cataclysmic variable (CV), or pre-CV, as the M dwarf companion is just filling its Roche lobe. We recently identified radio emission from QS Vir in the Very Large Array Sky Survey (VLASS), at a flux of ∼1 mJy. The origin of radio emission from CVs is not fully understood, with evidence for synchrotron emission from jets and other coherent plasma emission processes, such as electron cyclotron maser emission (ECME) or plasma radiation. Aims. Our aim is to constrain the radio emission mechanism for QS Vir, through spectroscopic, polarisation, and time variability measurements, all while checking for correlated X-ray variations. Methods. We took three epochs of new observations with the Karl G. Jansky Very Large Array (VLA) in S -, C -, and X -bands, with full Stokes polarisation information, complemented by near-simultaneous Swift /XRT X-ray data. Radio spectra were extracted to search for emission features characteristic of coherent plasma emission processes (e.g. high circular polarisation and narrow-band emission). We fit the X-ray spectra with absorbed power laws, finding no strong X-ray variability. Results. QS Vir showed a nearly flat radio spectrum, with fluxes of 0.4−0.6 mJy in all bands. Swift /XRT showed L X ∼ 5 × 10 29 erg/s in all observations. We identified strong, variable circular polarisation, ranging from 33 ± 3% in S -band in the last observation, to < 11% in the middle observation in all bands. Linear polarisation was not detected, with upper limits of at most 15%. Intriguingly, the S -band spectra show circularly polarised spectral bumps (width ∼0.5 GHz) that rise and decay within ≲5 minutes. Conclusions. We suggest that the radio emission from QS Vir consists of two components: a relatively constant, low-polarisation flat-spectrum component and a band-limited, rapidly variable, and strongly circularly polarised component. This latter coherent component may be associated with ECME or plasma radiation.