Accretion of matter onto black holes is universally associated with strong radiative feedback 1 and powerful outflows 2 . In particular, black hole transients 3 show outflows whose properties 4 are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disc encircling the black hole, and collimated radio jets 5,6 . Very recently, a distinct optical variability pattern has been reported in the transient black hole transient V404 Cyg, and interpreted as disrupted mass flow into the inner regions of its large accretion disc 7 . Here, we report on the discovery of a sustained outer accretion disc wind in V404 Cyg, which is unlike any seen previously. We find that the outflowing wind is neutral, has a large covering factor, expands at 1% of the speed of light and triggers a nebular phase once accretion sharply drops and the ejecta become optically thin. The large expelled mass (> l0 -8 M ¤ ) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disc was depleted by the wind, detaching the inner regions from the rest of the disc. The luminous, but brief, accretion phases shown by transients with large accretion discs 2 imply that this outflow is most likely a new fundamental ingredient regulating mass accretion onto black holes.The X-ray binary V404 Cyg (GS 2023+338) is a confirmed stellar-mass BH 8 with a precisely determined distance of 2.4 kpc 9 . Following 25 years of quiescence, the Swift mission detected renewed activity on Jun 15, 2015 10 , initiating a 2-week period of intensely violently variable emission across all wavelengths 11,12 . Our high signal-to-noise GTC optical spectra covering the entire X-ray/radio active phase (~15 days) show that, contemporaneously with radio jet emission, continuous ejections of neutral material at ~0.01c are present from low-level accretion phases (<1% of the Eddington luminosity; L EDD ) to the Xray peak (Methods; Fig. 1, ED Fig. 1). These are observed in hydrogen (Balmer) and helium (He I) emission lines as deep P-Cyg profiles throughout the outburst 13 , and extremely broad wings once the Xray and radio fluxes decay. P-Cyg profiles result from resonant scattering in an expanding outflow with a spherical geometry or at least sustaining a large solid angle 14, 15 (Methods). Among a dozen transitions showing this feature, the deepest are seen in the He I-5876 emission line, which is used as a reference for this study (see ED Fig. 2.).The strongest P-Cyg profiles are witnessed during days 1 to 6 (Fig. 1 and Fig. 2 for the evolution of the profiles during day 2; Methods), when the X-ray luminosity is typically 10 3 times fainter than the ~L EDD flares displayed later in the outburst 7,11 (ED Fig.1). Blue-shifted absorptions are as deep as 30% below the continuum level and we measure terminal velocities in the range V T =1,500 -3,000 km s -1 (Fig. 1, Fig 2, ED Fig 2; ED Fig. 3). Symmetric red-shifted (i.e. positive velocity) outflow emission, completely detached from the accre...

We search the literature for reports on the spectral properties of neutron-star lowmass X-ray binaries when they have accretion luminosities between 10 34 and 10 36 ergs s −1 , corresponding to roughly 0.01% -1% of the Eddington accretion rate for a neutron star. We found that in this luminosity range the photon index (obtained from fitting a simple absorbed power-law in the 0.5-10 keV range) increases with decreasing 0.5-10 keV X-ray luminosity (i.e., the spectrum softens). Such behaviour has been reported before for individual sources, but here we demonstrate that very likely most (if not all) neutron-star systems behave in a similar manner and possibly even follow a universal relation. When comparing the neutron-star systems with blackhole systems, it is clear that most black-hole binaries have significantly harder spectra at luminosities of 10 34 − 10 35 erg s −1 . Despite a limited number of data points, there are indications that these spectral differences also extend to the 10 35 − 10 36 erg s −1 range, but above a luminosity of 10 35 erg s −1 the separation between neutron-star and black-hole systems is not as clear as below. In addition, the black-hole spectra only become softer below luminosities of 10 34 erg s −1 compared to 10 36 erg s −1 for the neutron-star systems. This observed difference between the neutron-star binaries and black-hole ones suggests that the spectral properties (between 0.5-10 keV) at 10 34 − 10 35 erg s −1 can be used to tentatively determine the nature of the accretor in unclassified X-ray binaries. More observations in this luminosity range are needed to determine how robust this diagnostic tool is and whether or not there are (many) systems that do not follow the general trend. We discuss our results in the context of properties of the accretion flow at low luminosities and we suggest that the observed spectral differences likely arise from the neutron-star surface becoming dominantly visible in the X-ray spectra. We also suggest that both the thermal component and the non-thermal component might be caused by low-level accretion onto the neutronstar surface for luminosities below a few times 10 34 erg s −1 .