Black-hole X-ray transient GS 2023+338 (V404 Cyg) wakes up after 26 years of slumber
On 15 June 2015, a long-time acquaintance made its comeback after a long period of
being quiet: GS 2023+338 (or V404 Cyg), a system comprising a black hole and a star
orbiting one another in the constellation Cygnus, the Swan.
In this type of binary system, material flows from the star towards the black hole and
gathers in a disc, where it is heated up, shining brightly at optical, ultraviolet and
X-ray wavelengths before spiralling into the black hole. V404 Cyg has an orbital period
of about 6.5 days. It is one of the best-established accreting black-hole binary systems,
with a black-hole mass of about 9.0 M_sun. The distance towards V404 Cyg is only 2.4 kpc,
i.e., one of the closest of its kind. Like most accreting black-hole X-ray binaries,
V404 Cyg is a transient system. Its last outburst occurred 26 years ago, in late May 1989,
when it was discovered by the Japanese X-ray satellite Ginga (hence "GS", see IAU Circ.
#
4782) and subsequently found in the optical (hence the "V", see IAU Circ.
#
4783). During its outbursts, the luminosity of the system increases by 6 orders of
magnitude, reaching the Eddington limit. An artist's impression of the system in outburst
is shown in the body of the image.
First signs of renewed X-ray activity in V404 Cygni were spotted by NASA's Swift/BAT
(GCN 17929),
detecting a sudden burst of gamma-rays, and then triggering observations with the XRT.
Soon after, MAXI onboard the ISS, observed an X-ray flare from the same patch of the sky
(ATel #7646).
These first detections triggered a massive campaign of observations from ground-based
telescopes and from space-based observatories, to monitor V404 Cygni at many different
wavelengths across the electromagnetic spectrum. As part of this worldwide effort,
INTEGRAL started monitoring the out-bursting black hole binary on 17 June. The source was
very clearly seen with IBIS/ISGRI (inset bottom left; ATels
#7662,
#7702,
#7726,
#7731),
as well as the other instruments, SPI (ATel
#7693),
JEM-X (ATels
#7662,
#7731)
and OMC (ATels
#7662,
#7717).
The first observations in
Revolution 1554
were made immediately public. Observations made during
Revolutions 1555 and 1556
were done under an AO-12 approved Target-of-Opportunity program (PI: Rodriguez).
Since the source is so bright, INTEGRAL is continuing to observe the source
until further notice
(see ATel #7695),
in the hope to detect 511 keV and polarization.
The behaviour of this source is extraordinary, with repeated bright flashes of light on
time scales shorter than an hour, something rarely seen in other black hole systems. In
these moments, it became the brightest object in the X-ray sky - in Revolution 1554 up
to twenty times brighter than the Crab Nebula (inset top right).
