Supermassive black holes that lurk at the center of most galaxies are surrounded by coronas — mysterious features that are sources of highly energetic particles. Like the Sun, black hole coronas are theoretically believed to be heated by their magnetic activity. Now, astrophysicists using the Atacama Large Millimeter/submillimeter Array (ALMA) have measured the magnetic fields surrounding central supermassive black holes in two active galaxies. Surprisingly, the strength of these magnetic fields does not appear to be high enough to power the black hole coronas.
An artist’s rendering of the corona around a black hole. Image credit: RIKEN.
In a 2014 study, Japanese astrophysicists Yoshiyuki Inoue and Akihiro Doi predicted that electrons in the plasma surrounding black holes would emit a special kind of light — called synchrotron radiation — as they exist together with the magnetic forces in the coronas.
Specifically, this radiation would be in the radio band, meaning light with a very long wavelength and low frequency. So the researchers decided to measure the coronal magnetic fields.
They looked at data from two active galaxies: IC 4329A, which is about 200 million light-years away, and NGC 985, which is approximately 580 million light-years away.
“We began by collecting measurements with ALMA, and then compared them to observations from two other radio telescopes: the VLA observatory in the United States, and the ATCA observatory in Australia, which measure slightly different bands,” the researchers said.
“We found indeed that there was an excess of radio emission originating from synchrotron radiation, in addition to emissions from the jets case out by the black holes.”
The coronal magnetic field of both objects is estimated to be 10 gauss on scales of 40 Schwarzschild radii (the radius of a black hole from which not even light can escape) from the central black holes. This magnetic field strength is weaker than the prediction from the magnetically heated corona scenario.
“The surprise is that although we confirmed the emission of radio synchrotron radiation from the coronas in both objects, it turns out that the field of the magnetic field we measured is much too weak to be able to drive the intense heating of the coronas around these black holes,” Dr. Inoue said.
“The same phenomenon was observed in both galaxies, implying that it could be a general phenomenon.”
The findings were published in the Astrophysical Journal (arXiv.org preprint).
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Yoshiyuki Inoue & Akihiro Doi. 2018. Detection of Coronal Magnetic Activity in nearby Active Supermassive Black Holes. ApJ 869, 114; doi: 10.3847/1538-4357/aaeb95
