A sudden outburst of jets of particles from a black hole 7,800 light years from Earth has offered astronomers a rare opportunity to measure the surrounding magnetic field, only to find it weaker than expected.
According to a study, the results appear to go against our current understanding of black holes which are famous for their intense gravitational pulls that can gobble up entire stars and launch streams of matter into space at nearly the speed of light. The paper titled “A precise measurement of the magnetic field in the corona of the black hole binary V404 Cygni” was published on December 8, 2017 in the international journal Science.
“We weren’t expecting this, so it changes much of what we thought we knew,” said study co-author Stephen Eikenberry, a professor of astronomy in University of Florida’s College of Liberal Arts and Sciences, in a statement.
On June 15, 2015, astronomers detected a sudden flare from a binary system called V404 Cygni, which consists of a red giant star orbiting a 40-mile-wide (64-kilometer-wide) black hole. The flare was generated by charged particles — electrons and protons — embedded in the black hole’s magnetic field.
The astronomers quickly mobilized radio, infrared, optical, and X-ray telescopes to collect data during the two week-long outburst. They compared how fast the flux decayed at each wavelength, which allowed them to constrain the size of the emitting region, determine that the plasma within it cooled through synchrotron radiation, and measure the magnetic field around the black hole.
By measuring how quickly the flare dimmed, the team determined that the magnetic field was only about 460 Gauss, just a few times stronger than a typical bar magnet, that is, orders of magnitude lower than previous estimates for similar systems.
These are the first precise measurements of a black hole’s magnetic field. It turned out the magnetic energy around the black hole is about 400 times lower than previous estimates.
“Our surprisingly low measurements will force new constraints on theoretical models that previously focused on strong magnetic fields accelerating and directing the jet flows,” said Eikenberry.
So far, astronomers are baffled on how “jets” of particles traveling at nearly the speed of light shoot out of black holes’ magnetic fields, while everything else is sucked into their abysses, he said.
Outbursts of small black holes, like the one observed for the study, occur suddenly and are short-lived, said the study.
The 2015 outbursts of V404 Cygni lasted only a couple of weeks and caused the surrounding plasma to brighten suddenly, briefly becoming the brightest x-ray source in the sky. The previous time the same black hole had a similar episode was in 1989.
“To observe it was something that happens once or twice in one’s career,” said study lead author Yigit Dalilar, a doctoral student in University of Florida’s astronomy department. “This discovery puts us one step closer to understanding how the universe works.”