V404 Cygni is a binary system comprising a black hole and a star orbiting one another. It lies in the constellation Cygnus at a distance of about 7,800 light-years from Earth. In this type of binary, material flows from the star towards the black hole and gathers in an accretion disk, where it is heated up, shining brightly at optical, UV and X-ray wavelengths before spiraling into the black hole. Powerful jets are one of the main ways in which such black holes provide feedback to their surroundings. In a paper published this week in the journal Nature, astronomers report a rapidly changing jet orientation — on a time scale of minutes to hours — in the V404 Cygni system, detected during the peak of its 2015 outburst.
An artist’s impression of the changing jet orientation in the V404 Cygni system; each segment (as separated by the clock hands) shows the jets at a different time, oriented in different directions as seen in our high angular resolution radio imaging. Image credit: ICRAR.
V404 Cygni first came to astronomers’ attention in 1938, when it experienced an outburst, and got its designation as a ‘variable star.’
Another outburst was observed in 1989, and follow-up studies revealed a previously-unnoticed outburst in 1956.
NASA’s Swift satellite detected a new outburst on June 15, 2015, triggering a worldwide observing effort.
“Everybody jumped on the outburst with whatever telescopes they could throw at it. So we have this amazing observational coverage,” said Dr. James Miller-Jones, an astronomer with the Curtin University node of the International Centre for Radio Astronomy Research.
Dr. Miller-Jones and colleagues used observations from the Very Long Baseline Array, a continent-sized radio telescope made up of 10 dishes across the United States, from the Virgin Islands in the Caribbean to Hawaii.
“This is one of the most extraordinary black hole systems I’ve ever come across,” Dr. Miller-Jones said.
“Like many black holes, it’s feeding on a nearby star, pulling gas away from the star and forming a disk of material that encircles the black hole and spirals towards it under gravity.”
When the astronomers studied the V404 Cygni black hole, they saw its jets behaving in a way never seen before.
Where jets are usually thought to shoot straight out from the poles of black holes, these jets were shooting out in different directions at different times. And they were changing direction very quickly.
The change in the movement of the jets was because of V404 Cygni’s accretion disk.
“The accretion disk is 6.2 million miles (10 million km) wide, and the inner few thousand miles was puffed up and wobbling during the bright outburst,” Dr. Miller-Jones said.
“The inner part of the accretion disk was precessing and effectively pulling the jets around with it.”
“You can think of it like the wobble of a spinning top as it slows down — only in this case, the wobble is caused by Einstein’s theory of general relativity.”
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James C. A. Miller-Jones et al. A rapidly changing jet orientation in the stellar-mass black-hole system V404 Cygni. Nature, published online April 29, 2019; doi: 10.1038/s41586-019-1152-0
