Astronomers Think They Know Why This Black Hole Is Flashing Red Light

Last summer, a slumbering giant awoke as the black hole V404 Cygni roared to life following 26 years of silence. During its brightest outburst in recent years, astronomers observed intense, short bursts of red light coming from the back hole.

Astronomers had never seen anything like these mysterious red bursts, and were anxious to figure out what was causing them. Now, after further study, they believe they have figured out the explanation.

V404 Cygni, a tumultuous binary system composed of a black hole and a companion star, is located approximately 8,000 light-years from Earth and has the distinction of being the first black hole officially identified in our galaxy.

Being a black hole, V404 Cygni is feeding off of a companion star, slowly siphoning its material. The accumulated gas and dust forms a disc around the black hole. Astronomers around the world mobilized, training their trusty telescopes and instruments on the pair to witness its first activity since 1989. What they saw was a dramatic brightening that lasted about two weeks, but they also witnessed a dazzling display of red flashes.

In a paper published in the journal of Monthly Notices of the Royal Astronomical Society, a team of astronomers detailed their observations of enigmatic red flashes. The team observed the flashes emanating from very close to the black hole, and associate them with fast-moving jets spewing material back out into space. They are hoping the detection of these mysterious red flashes will help improve our understanding of how they form.

The flashes only lasted a few fractions of a second, but the lead author of the paper, Poshak Gandhi, suspects strong magnetic fields are responsible. "The very high speed indicates that the region where these flashes originate from has to be very compact," he said. "As such, we have determined that the red light is originating from the base of the black hole's jet. The jet's origin is still unknown, but we think strong magnetic fields are behind it."

According to the researchers, the flashes were most intense at the height of the black hole's feeding frenzy. Gandhi suspects that the black hole was essentially being force-fed by its companion. As gas accumulates, it can forms lumps within the disk, temporarily blocking the inward flow. Eventually one of these lumps (essentially cosmic dams) is overwhelmed, and massive amounts of gas rushes the black hole. Struggling to swallow it all, V404 Cygni violently spewed material back out in the form of a jet.

Within the jet, the team observed several intense flashes of red light, each shining as bright as 1,000 suns. These intense bursts of light were brief, with some lasting less than 1/40th of a second—that is, about ten times faster than the blink of an eye. Making observations like these require specialized technology, so astronomers used a fast-imaging camera known as ULTRACAM, which was mounted on the William Herschel Telescope in La Palma, on the Canary Islands at the time.

Gandhi and his team believe the duration of these flashing episodes could be related to the switching on and off of the jet, which we are seeing for the first time in detail. The team will continue to go through the data collected to more definitively determine what is causing the mysterious red flashes.