Nasa has detected what could be a spontaneous flip of a black hole 236 million light-years away.
The magnetic field of the cosmic body seemingly reversed, leading the team to detect a “rare and enigmatic outburst” from the distant galaxy.
“Rapid changes in visible and ultraviolet light have been seen in a few dozen galaxies similar to this one,” said Sibasish Laha, a research scientist at the University of Maryland, Baltimore County and Nasa’s Goddard Space Flight Center.
“But this event marks the first time we’ve seen X-rays dropping out completely while the other wavelengths brighten.”
In March 2018, astronomers were alerted that a galaxy called 1ES 1927+654 had brightened by nearly 100 times – with further research showing that the eruption had begun at the end of 2017. Nasa’s Neil Gehrels Swift Observatory, a three-telescope space observatory, detected that the galaxy’s emission was elevated by 12 times but steadily declining, indicating an earlier unobserved peak. In June, the high-energy emission vanished.
“It was very exciting to delve into this galaxy’s strange explosive episode and try to understand the possible physical processes at work,” said José Acosta-Pulido, a co-author at the Canary Islands Institute of Astrophysics (IAC)
Most big galaxies have a supermassive black hole millions or billions of times the Sun’s mass at their centre. When matter falls in it becomes flattened into an accretion disk. The material heats up and emits visible UV and x-ray light that can be detected by scientists.
Near the black hole, a cloud of extremely hot particles – called the corona – produces higher-energy x-rays, with the brightness of these emissions dependant on how much material streams toward it.
“An earlier interpretation of the eruption suggested that it was triggered by a star that passed so close to the black hole it was torn apart, disrupting the flow of gas,” said co-author Josefa Becerra González, also at the IAC. “We show that such an event would fade out more rapidly than this outburst.”
Astronomers believe the black hole’s magnetic field creates and sustains the corona, so any magnetic change could impact its X-ray properties.
“A magnetic reversal, where the north pole becomes south and vice versa, seems to best fit the observations,” said co-author Mitchell Begelman, a professor in the department of astrophysical and planetary sciences at the University of Colorado Boulder. “The field initially weakens at the outskirts of the accretion disk, leading to greater heating and brightening in visible and UV light,” he explained.
As the flip goes on, the magnetic field becomes too weak to support the corona before gradually strengthening in a new orientation. After four months the x-rays reappeared, suggesting that the flip had been completed.
A paper describing the findings, led by Ms Laha, is accepted for publication in The Astrophysical Journal.
