On June 15th a quiet black hole, V404 Cygni, suddenly flared for the first time since 1989.
When V404 Cygni woke up, it set off over seventy flares in just one week. Each time there was a flare, the Gamma-ray Burst Monitor (GBM) on Fermi sent an email to those scientists monitoring the system. As one scientist, David Yu (Max Planck Institute of Extraterrestrial Physics), described it on social media, “Achievement Unlocked: Mailbox spammed by a blackhole."
V404 Cygni is part of a binary system with a star similar to our Sun. As they orbit each other, the black hole’s immense mass yanks gas away from its partner that then gathers around the black hole in a disk. Due to angular momentum, some of the gas gets locked into a stable orbit around the black hole, but when the gas builds up it becomes unstable, overflows, and hurtles toward the black hole’s center. When this happens the disk spews out radiation all along the spectrum, including X-rays, from the innermost circles of the accretion disk. This is what happened to V404 Cygni on June 15th.
The team of researchers monitoring the black hole, led by Andrew Beardmore (University of Leicester, UK), used the X-ray Telescope on NASA’s Swift satellite to collect images of V404 Cygni from June 30th to July 4th. They then created a short animation of what this active system looks like.
The rings around the black hole are actually illusory light echoes. The X-rays from the flare bounced off dust layers between the black hole and the Swift satellite’s telescope, which skewed their trajectories. Scattered X-rays arrive at the telescope at different times, and the time delay creates the illusion of a set of rings around the source that expand over time.
These rings are similar to the recent observations of light echoes around Circinus X-1. However, V404’s shorter distance (about 8,000 light years away, as opposed to 30,700 for Circinus X-1) allowed the team to observe the rings expanding more easily – they are larger and brighter from a closer source. In addition, since the rings’ expansion rate decreases over time, the team’s quick observations allowed them to catch it at the height of its action. Both of these factors enabled them to create an animation of the expanding rings.
The size of the X-ray rings is about half the apparent diameter of the full Moon.