V404 Cygni

A repeating X-ray transient

V404 Cygni is a binary in which a black hole slowly draws gas from a Sun-like companion. Most of the time the system is faint, but every few decades it enters a dramatic outburst as material floods inward and the accretion disc becomes intensely luminous.

Historical outbursts were recorded in 1938 and 1956; the 1989 event confirmed the black-hole nature of the system. In 2015 it erupted again, becoming one of the brightest X-ray transients observed in decades.

A warped disc and wobbly jets

Radio observations during the 2015 outburst revealed jets that changed direction over a period of days. The best explanation is a misaligned, warped accretion disc: gas close to the black hole precesses, redirecting the inner disc and the jets it launches.

Such rapid changes are hard to see in most black holes, making V404 Cygni a valuable laboratory for the connection between accretion discs, jets and strong gravity.

Why it matters

The system shows that a quiet stellar-mass black hole can become a brilliant X-ray source and a jet factory in a matter of days. It also helps astronomers model how matter behaves when it is pulled into a black hole from a companion star.