Accretion disc winds imprint distinct signatures in the optical variability spectrum of black hole transients

TL;DR

This study applies rms spectral analysis to optical data of a black hole binary, revealing wind signatures and variability features that can help identify outflows in accreting systems.

Contribution

It introduces the first use of rms spectra in optical wavelengths for low-mass X-ray binaries, uncovering wind-related features and variability signatures.

Findings

Rms spectra show spectral features distinct from flux spectra.

Absorption components in rms spectra correlate with emission lines.

Inverted P-Cygni profiles indicate wind-related variability.

Abstract

Quantifying the variability, measured as the root mean square (rms), of accreting systems as a function of energy is a powerful tool for constraining the physical properties of these objects. Here, we present the first application of this method to optical spectra of low-mass X-ray binaries. We use high-time-resolution data of the black hole transient V404 Cygni, obtained with the \textit{Gran Telescopio Canarias} during its 2015 outburst. During this event, conspicuous wind-related features, such as P-Cygni profiles, were detected in the flux spectra. We find that rms spectra are rich in spectral features, although they are typically morphologically different from their flux counterparts. Specifically, we typically observe absorption components in correspondence to the presence of emission lines in the flux spectra. Similarly, when analysing segments with significant variability in the optical flux, P-Cygni line profiles appear inverted in the rms spectra (i.e., enhanced variability in the blue-shifted region, accompanied by a decrease in that associated with the red component). We discuss the possible origin of these features, which resemble those found in other objects, such as active galactic nuclei. Finally, we highlight the potential of this technique for future searches for wind-type outflows in accreting compact objects.