Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni

Abstract

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...

Evidence for Simultaneous Jets and Disk Winds in Luminous Low-Mass X-Ray Binaries

“…The intrinsic luminosities during these Chandra observations, while probably still sub-Eddington, were likely well above the 0.005-0.05 L Edd luminosities implied by the (non-absorption-corrected) 2-10 keV fluxes. During the same outburst Muñoz-Darias et al (2016) observed P Cygni profiles in various optical H and He emission lines, indicating a disk wind with terminal outflow velocities in the range of ∼1500-3000 km s −1 . V404 Cyg was active in the radio at the time of these optical and X-ray observations (Trushkin et al 2015;Tsubono et al 2015), which suggests that the source was producing disk winds and jets at the same time, as already pointed out by King et al (2015).…”

Rapid optical variations correlated with X-rays in the 2015 second outburst of V404 Cygni (GS 2023+338)

“…This would indicate that the effect of reprocessing was weaker in the December outburst than that in the June outburst. We suggest the reason is that the outer disc would be depleted due to ionisation during the June outburst and/or the strong outflow discussed in Muñoz-Darias et al (2016). On the other hand, in the June outburst, the outer disc is thought to have been optically thick ).…”

An elevation of 0.1 light-seconds for the optical jet base in an accreting Galactic black hole system

“…In summary, our estimates provide robust upper limits to the elevation of optical jet emission zone, to within correction factors of order unity. We finally note the presence of a significant disc wind that has been observed in V404 Cygni 24,25 . Detailed analysis of the cold (optically--emitting) phase of this wind places its formation region at the outer accretion disc, which has an extent of 10 12 cm.…”

“…Detailed analysis of the cold (optically--emitting) phase of this wind places its formation region at the outer accretion disc, which has an extent of 10 12 cm. The minimum size scale of the wind launching zone is of order light--seconds 25 . These sizes are longer than the characteristic timescales associated with the sub--second flaring, arguing against reprocessing in this wind for their origin.…”