Black holes' magnetic fields may be too weak to power jets

First precise measurement of a black hole's magnetic field leaves scientists puzzled.
By Laurel Kornfeld | Jan 03, 2018
Infrared observations of a nine-stellar mass black hole approximately 8,000 light years away surprised astronomers by revealing its magnetic field to be far too weak to generate the jets of material that all black holes eject into space.

Scientists have long attributed the high-speed jets emitted by black holes to their having powerful magnetic fields. This discovery may compel them to search for another mechanism to explain the jets.

Based at the University of Florida (UF), a team of astronomers used the Canarias InfraRed Camera Experiment (CIRCE) on the 10.4-meter Gran Telescopio Canarias (GTC) in the Canary Islands to study jets emitted by the black hole V404 Cygni in the constellation Cygnus over several weeks in 2015.

This particular black hole is a microquasar, a stellar-mass version of the active supermassive black holes at the centers of galaxies.

Galactic quasars are surrounded by accretion disks of material totaling several million solar masses. These accretion disks emit powerful radiation that makes quasars the brightest objects in the universe.

Parts of the light-years' long jets emitted by quasars experience changes that can be observed by scientists over just several months.

Such changes also occur in sections of the jets emitted by microquasars, and these can be studied over even shorter time periods, ranging from hours to weeks.

Microquasars' jets are dimmer than those of galactic quasars, and their accretion disks are smaller than those of their larger counterparts.

Outbursts of activity likely due to jet emissions have been observed coming from V404 Cygni in 1956, 1979, 1989, and 2015.

V404 Cygni is actually part of a binary system. Its companion star, from which the material in its accretion disk originates, is a K-giant of 0.7 solar masses. The two objects orbit each other in a period slightly under 6.5 Earth days.

The scientists, whose measurements were the first ever taken of a black hole's magnetic field, were surprised to find V404's magnetic field to be approximately 400 times weaker than expected.

"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 Stephen Eikenberry of the University of Florida.

"We weren't expecting this, so it changes much of what we thought we knew."

Research team members also observed V404 in X-ray, optical, and radio wavelengths.

A paper on the findings has been published in the journal Science.


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