V404 Cyg (or GS 2023+338) is a transient X-ray binary system where a
black hole is feeding material from its low-mass companion star. In June
2015 it had a spectacular feast, which resulted in X-ray fireworks that
was also observed by INTEGRAL. The in-falling gas heated up to hundreds
of millions of degrees as it approached the black hole causing V404 Cyg
to shine brighter than a million Suns, while at the same time its black
hole spewed out matter in form of winds (see INTEGRAL
POM July 2016) and
rapidly moving jets (see INTEGRAL
POM June 2019). The X-ray brightness
of V404 Cyg varies much more erratically compared to other similar
feeding black holes: its bright X-ray flares are sandwiched by much
dimmer periods in its X-ray light curve (see INTEGRAL
POM July 2015).
A new study - utilizing INTEGRAL's instruments, i.e., the optical
monitor camera (OMC), the X-ray monitor JEM-X and the imager IBIS -
focused on how V404 Cyg started to brighten up from one of these dim
periods. It was found that the short nap in between the brighter flares
ended badly out of sync. The optical and soft X-ray brightness rose
relatively gradually at first, and a few minutes later the hard
X-ray brightness jumped up in a matter of about 15 seconds. The rapid
brightness variation was accompanied with a spectral transition that
occurred much faster than seen in most of black-hole binary X-ray
transients. The only comparable event observed is the one from the
black-hole binary X-ray transient V4641 Sgr: it also shows strong winds
that occasionally obscure the black hole, like in V404 Cyg.
This suggests, that the transition seen could be related to a change in
the wind structure, rather than the manner in which the black hole was
being fed.
