Ask Astro: Do black holes have stronger magnetic fields than neutron stars?

By | Published: August 19, 2022
With a magnetic field over a thousand trillion times greater than Earth’s, magnetars are the reigning magnetic kings of the cosmos.
ESO/L. Calçada

I’ve read that the strength of a neutron star’s magnetic field is greater than any other found in the universe. Wouldn’t a supermassive black hole have a stronger one?

Patrick Clough 

Wichita, Kansas 

The answer to this question is quite complicated. There is a so-called no-hair theorem, which basically states that only three observable parameters can be determined for each black hole: its mass, electric charge, and rotation. The hair here is a metaphor for all other possible parameters, including magnetic fields, which disappear inside the black hole and become inaccessible to scientists. So, a black hole by itself does not have any measurable magnetic field.

However, any matter that accretes onto a black hole could be magnetized. In this case, the magnetic field will become stronger as the matter approaches the black hole and is compressed. So, magnetic fields do exist around supermassive black holes, but their source is the accretion disk, not the black hole itself.

For example, when the Event Horizon Telescope collaboration imaged the supermassive black hole in M87, they observed radio waves that were polarized by the magnetic field in the surrounding accretion disk. The team estimated the magnetic field strength to be between two to 50 times stronger than Earth’s magnetic field. But that is a tiny magnetic field compared with the magnetic fields around pulsars and magnetars. In particular, magnetars retain the strongest magnetic fields in the universe, at a thousand trillion times stronger than Earth’s field.

Andrei Igoshev 

Astronomy Research Fellow, University of Leeds, Leeds, United Kingdom