V404 Cygni

V404 Cygni is a microquasar and a binary system consisting of a black hole with a mass of about 12±3 solar masses and a late G or early K companion star of mass slightly smaller than the Sun in the constellation of Cygnus. The star and the black hole orbit each other every 6.47129 d at fairly close range. Due to their proximity the main sequence star would be distorted into egg shape by the black hole's gravity and lose mass to the black hole. The "V" in the name indicates that it is a variable star, which repeatedly gets brighter and fainter over time. It is also considered a nova, because at least three times in the 20th century it produced a bright outburst of energy. Finally, it is a soft X-ray transient because it periodically emits short bursts of X-rays.

In 2009, the black hole in the V404 Cygni system became the first black hole to have an accurate parallax measurement for its distance from the Solar System. Measured by very-long-baseline interferometry using the High Sensitivity Array, the distance is 2.39±0.14 kiloparsecs, or (7.80±0.46)×10³ light-years.

Discovery

On May 22, 1989 the Japanese Ginga Team discovered a new X-ray source that was cataloged as GS 2023+338. This source was subsequently identified as coincident in position with a previously known nova cataloged as V404 Cygni.

2015 Outburst

On 15 June 2015 NASA's Swift satellite detected the first signs of renewed activity. A worldwide observing campaign was commenced and on 17 June ESA's INTEGRAL Gamma-ray observatory started monitoring the outburst. INTEGRAL has been detecting "repeated bright flashes of light time scales shorter than an hour, something rarely seen in other black hole systems", and during these flashes V404 Cygni is the brightest object in the X-ray sky - up to fifty times brighter than the Crab Nebula. This outburst is the first since 1989. Other outbursts occurred in 1938 and 1956, and the outbursts are probably caused by material piling up in a disk around the black hole until a tipping point is reached. The outburst was unusual in that physical processes in the inner accretion disk were detectable in optical photometry from small telescopes; previously, these variations were thought to be detectable with space-based X-ray telescopes only. A detailed analysis of the INTEGRAL data revealed the existence of so-called pair plasma near the black hole. This plasma consists of electrons and their antimatter counterparts, positrons.