Optical outburst evolution of the transient black hole X-ray binary Swift J1727.8-1613: Disc response to jet ejections and late-outburst emergence of powerful disc winds

TL;DR

This study investigates the optical spectral evolution of the transient black hole binary Swift J1727.8-1613 during its 2023 outburst, revealing disc wind responses to jet ejections and state transitions, with implications for system evolution.

Contribution

It provides the first detailed optical spectroscopic analysis of Swift J1727.8-1613's outburst, linking disc wind features to state changes and jet activity, and estimates wind mass-loss rates.

Findings

Detection of He II flux increase during radio flares.

Appearance of broad Balmer absorption lines in the dim-hard state.

Estimated wind mass-loss rate comparable to accretion rate.

Abstract

Swift J1727.8$-$1613 is a newly discovered transient low-mass X-ray binary harbouring a stellar-mass ($\sim 10M_\odot$) black hole. We present state-resolved VLT/X-Shooter optical spectroscopy of its 2023 outburst, sampling the luminous hard-to-soft and late soft-to-hard transitions. During the onset of the brightest radio flare, He\,\textsc{ii} flux rises relative to adjacent epochs, with reduced peak-to-peak separation and full-width-half-maximum, consistent with enhanced irradiation shifting line emissivity to larger radii. We detect no contemporaneous change in the line base tracing the inner disc. The most dramatic change occurs at the onset of the dim-hard state, when strong, broad (higher-order) Balmer lines appear in absorption, and He\,\textsc{ii} remains in emission, but becomes highly asymmetric. While the hardening of the X-ray spectrum likely promotes the reappearance of an underlying disc photosphere, the kinematic alignment between the Balmer absorption ($v_w\sim-750\,\mathrm{km\,s^{-1}}$) and the suppressed blue peak of He\,\textsc{ii} suggests a unified origin in a massive, cool ($T\lesssim10^{4}\,\mathrm{K}$) accretion disc wind. Radiative transfer simulations demonstrate that such asymmetric He\,\textsc{ii} profiles are naturally produced in a rotating and accelerating outflow. Using the Sobolev approximation, we estimate the wind mass-loss rate to be $\dot{M}_w\gtrsim10^{-9}\,M_\odot\,\mathrm{yr^{-1}}$, comparable to the instantaneous accretion rate and a significant fraction of the secular mass-transfer rate from the donor. If persistent at quiescent-level X-ray luminosities, this outflow could strongly impact the system's secular evolution.