Unveiling optical signatures of outflows in accreting white dwarfs

Abstract

Accreting white dwarfs are known to show signatures of wind-type outflows in the ultraviolet. However, at optical wavelengths, wind detections have only been reported for a few sources. We present GTC-10.4 m optical spectroscopy of four accreting white dwarfs (BZ Cam, V751 Cyg, MV Lyr, and V425 Cas) observed during luminous epochs, when their optical emission is expected to be dominated by the accretion disc. Our analysis focuses on four emission lines: Hα and He Iλ5876, λ6678, and λ7065. Line profiles are complex and variable on short (minutes) and long (days to weeks) timescales, with transient absorption and emission components. Among them, we detect strong blueshifted absorptions at ≳1000 km s−1. These high-velocity components, present only in the blue wing of the emission lines, are observed in all four sources and could be associated with accretion disc winds. For MV Lyr and V425 Cas, these would represent the first detection of optical outflows in these objects, while in the cases of BZ Cam and V751 Cyg, the presence of outflows has been previously reported. This study suggests that, in addition to ultraviolet winds, optical outflows might also be common in accreting white dwarfs. We discuss the observational properties of these winds and their possible similarity to those detected in accreting black holes and neutrons stars.