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

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.

Figures (8)

• Figure 1: Hardness-intensity diagram of J1727 during its discovery outburst. The 11 spectroscopic epochs gathered with X-Shooter are marked as indicated in the legend. The open circles represent the peak of the two bright radio flares reported by Hughes2025ApJ...988..109H. The open square highlights the onset of the hard-to-soft state transition Bollemeijer2023Bollemeijer2023a while the open diamond indicates the onset soft-to-hard state transition Podgorny20204atel
• Figure 2: Spectral evolution of J1727 as observed by X-Shooter's blue-arm. The colour and number of each observation match those in Figure \ref{['fig:HID']}. A vertical offset is applied for clarity. The rest wavelengths of key transitions are marked by vertical ticks. Shaded regions indicate the presence of interestellar lines (except for Ca ii H & K lines at 3933 and 3968 Å respectively).
• Figure 3: Continuum‐normalized line profiles of J1727 across the 11 X-Shooter observing epochs in velocity space. The figure contain transitions spanning different ionisation potentials, from left to right, the panels show H$\alpha$, H$\delta$, He i$\lambda10830$, He ii$\lambda4686$, and O vi + He ii. Each colour trace corresponds to the same epoch shown in Figure \ref{['fig: spec evo']}. Dashed lines indicate zero velocity, while dotted lines mark the approximate positions of transient absorption features on either side of the emission profiles, intended as a qualitative guide to the eye. In the last panel, the zero velocity of O vi corresponds to a wavelength of 5284.1 Å. Dash-dotted lines in the third and last panels indicate the relative central velocity of Pa$\gamma$ and He ii, respectively.
• Figure 4: Temporal evolution of X-ray, radio and optical lines through the outburst. From top to bottom, the first pannel shows the X-ray light curve, the second panel shows the $10$ GHz radio light curve from Hughes2025ApJ...988..109H, the third and fourth panels shows the Flux and FWHM of He ii$\lambda4686$ and H$\alpha$, while the last shows the peak-to-peak separation of the disc-like profiles from He ii$\lambda4686$. The vertical dashed lines mark the two radio flares associated with the bipolar ejections observed at the onset of the hard-to-soft transition. Dotted lines indicate the state transitions. We note that the error bars in the flux density are smaller than the symbol size. Given the asymmetric line profiles during the last epoch (see sec. \ref{['sec: STH results']}), the open symbols are estimated by mirroring the red component about the line centre to represent the missing blue component.
• Figure 5: Blue end of the J1727 optical spectrum before he soft-to-hard state transition (Epoch 10) and at the onset of the dim-hard state (Epoch 11). Before the state-transition, during Epoch 10, the spectrum is dominated by emission lines. By Epoch 11, at the onset of the dim-hard state, He ii is still observed in emission but with a much narrower red-shifted component (See the comparison in Fig. \ref{['fig: epochs10-11']}). During this last epoch, the overall spectral shape exhibit broad absorption lines (partially filled with narrow emission), somewhat resembling a stellar atmosphere, and a stronger Balmer absorption edge. The broad absorption lines, most notably from $\mathrm{H_\gamma}$ to $\mathrm{H_\zeta}$, are filled with a relatively narrow emission component on the red side of the rest wavelength. The contrast in some of the lines allows us to estimate the centroid of the absorption component. The inset shows a zoom-in of H$\gamma$ profile, overlaid with the best-fit Lorentzian model obtained by masking the central emission cores (see Table \ref{['tab: abs fit']}). These results are consistent with an optically thick component moving in the line sight with a bulk velocity of $v_w\simeq-750\,\mathrm{km\,s^{-1}}$ (see Sections \ref{['sec: STH results']} and \ref{['sec: outflow discussion']} for details).