The evolution of the accretion disk wind during the violent outburst of the black-hole X-ray transient V404 Cygni
V404 Cygni is a transient X-ray binary system, where a black hole of around
10 times the mass of the Sun is swallowing material from a nearby companion
star. During this process material falls towards the black hole and forms an
accretion disc, whose hotter, innermost zones emit in X and gamma-rays. In
June 2015, V404 Cygni went into outburst after a quiescence of over 25 years
(see INTEGRAL POM
July and
November 2015).
During this period its brightness increased a million fold in a few days,
becoming temporarily the brightest X-ray source in the sky. The evolution of
the outburst was extensively monitored by INTEGRAL.
Optical spectroscopic observations carried out with the GTC 10.4m telescope
discovered the presence of a wind of cold material, which is formed in the
outer layers of the accretion disc, regulating the accretion of material onto
the black hole. This wind has a very high velocity (up to 3,000 km/s) so that
it can escape from the gravitational field around the black hole.
The bottom panel of the figure shows a trailed spectrum of the He I-5876 Å
emission line, covering 75 minutes with 55 optical spectra taken on 19 June 2015.
The normalized intensity scale is such that absorptions, caused by the presence
of the wind, are represented in blue colours, while emissions are represented
in red. The simultaneous INTEGRAL/IBIS/ISGRI (25-200 keV) normalized light-curve
is shown in the top panel. The outflowing wind is detected along the observation,
but its properties quickly change in response to the X-ray flaring, which
modifies the ionization state of the accretion disc. The strongest features
become evident at low X-ray fluxes, right after INTEGRAL detected a sharp flare
(dashed line). During this event the wind almost disappeared, and higher
excitation emission lines (not shown) appear in the optical spectrum.
