Unveiling optical signatures of outflows in accreting white dwarfs

TL;DR

This study uses optical spectroscopy to detect high-velocity outflows in four accreting white dwarfs, revealing that optical outflows are more common than previously observed, and may be similar to winds in other compact objects.

Contribution

First detection of optical outflows in MV Lyr and V425 Cas, expanding understanding of wind signatures in accreting white dwarfs.

Findings

High-velocity blue-shifted absorption features detected in all four sources.

Optical outflows may be as common as ultraviolet winds in these systems.

Potential similarity between white dwarf outflows and those in black holes and neutron stars.

Abstract

Accreting white dwarfs are known to show signatures of wind-type outflows in the ultraviolet. At optical wavelengths, however, wind detections have only been reported for a few sources. We present GTC-10.4m 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. We focused the analysis on four emission lines: H$\alpha$ and He I $\lambda$5876, $\lambda$6678, $\lambda$7065. Line profiles are complex and variable on short (minutes) and long (days to weeks) time scales, with transient absorption and emission components. Among them, we detect strong blue-shifted absorptions at $\gtrsim 1000$ 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 case 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 be also 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.