The black hole Cygni V404 has a magnetic field much weaker than predicted

Cygni V404 during its outburst (Image Andrew Beardmore (Univ. of Leicester) and NASA/Swift)
Cygni V404 during its outburst (Image Andrew Beardmore (Univ. of Leicester) and NASA/Swift)

An article published in the journal “Science” describes a precise measurement of the magnetic field of the corona of the black hole V404 Cygni. A team of researchers used the data collected in 2015 during a violent outburst of energy connected to the emission of jets from the black hole detected at many wavelengths using various space and ground-based telescopes. The result of this measurement was very surprising, being about 400 times lower than previous estimates.

Nearly 8,000 light years away from Earth, the black hole Cygni V404 has an estimated diameter of almost 65 kilometers (about 40 miles) and a mass estimated at almost 10 times the Sun’s. It was discovered in 1989 by the Japanese astronomical satellite Ginga, which detected an outburst of X-rays. Actually it wasn’t the first time that something anomalous was detected in that position but the instruments available in previous decades allowed to notice only a generic nova.

Subsequent studies revealed that the black hole V404 Cygni is part of a binary system together with a class K companion, a normal star a little smaller than the Sun. As often happens in these cases, the black hole is stealing materials from its companion and this enriches an accretion disk around it. These materials heat up in what is called a corona to such an extent that they emit very strong electromagnetic radiation, even X-rays and gamma rays. A part of these materials is ejected in two jets at the black hole’s poles.

In this situation, violent outbursts occur periodically when the black hole swallows up materials in huge amounts and during the 2015 one, which lasted about a couple of weeks, it was possible to examine V404 Cygni with much more sophisticated instruments than previous events. NASA’s NuSTAR space telescope observed it at X-rays, the William Herschel Telescope (WHT) at visible light, the Arcminute Microkelvin Imager (AMI) at radio waves and the Gran Telescopio Canarias (GTC) at infrareds through its CIRCE (Canarias InfraRed Camera Experiment) instrument.

All these observations allowed to examine what happens near the black hole V404 Cygni in a detailed way much better than it was ever possible on such occasions. During that event, Cygni fired energy-charged electrons into the corona’s magnetic field, where they emitted electromagnetic radiation at all wavelengths. The observation of the electrons’ cooling consequent to their energy emission allowed to measure the magnetic field, with the surprise for the researchers.

The theoretical models developed so far predicted that black holes had a powerful magnetic field, of the order of 400 times greater than the one measured in V404 Cygni’s corona. The surprising result of the measurement will help to better understand the conditions of the materials in the accretion disk and in the jets at the poles, where they’re accelerated until they reach speeds close to the light’s.

Black holes are extreme objects so the materials that orbit them are in extreme conditions, especially when violent outbursts such as the V404 Cygni one observed in 2015 happen. Very rarely they can be detected and studied in such details and it was an event that may have provided important information for future advances in physics and astrophysics.

Artist's concept of V404 Cygni and its companion (Image NASA / Goddard Space Flight Center / Conceptual Image Lab)
Artist’s concept of V404 Cygni and its companion (Image NASA / Goddard Space Flight Center / Conceptual Image Lab)

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