Neutron stars that are so massive that they should collapse into a black hole can still exist for several years thanks to an extremely strong magnetic field.

A star more than ten times the mass of our sun explodes as a supernova when it runs out of fuel. After such a supernova explosion, there are two possibilities. A neutron star may remain: an extremely compact object in which a mass greater than the sun has been compressed into a ball with a diameter of about 10 kilometers. But if the supernova remnant is heavier than about 2.1 times the mass of the Sun, a neutron star is not an option. Then the gravity is so great that a black hole is formed.

What happens when two neutron stars merge, as was observed in 2017? Then you can get a new neutron star that is so massive that it should immediately collapse into a black hole. But, so write astronomers Arthur Suvorov and Kostas Glampedakis in the scientific journal Physical Review D: a strong magnetic field can mean a postponement of execution.

Not more than a decade

Now such a brand new, slightly too heavy neutron star must have a very strong magnetic field: more than 100 quadrillion gauss. (By comparison, the magnetic field of a refrigerator magnet has a strength of about 100 gauss, the magnetic field of the Earth only half a gauss.) “It is unclear whether such strong magnetic fields occur in nature,” write Suvorov and Glampedakis . However, they note later in the article: there have been observations that may indicate such strong fields.

Such an extreme magnetic field can stabilize the neutron star, preventing it from collapsing into a black hole. At least for a while. The magnetic field gradually disappears, and then at a certain moment gravity takes over after all. As a result, according to the researchers, it is unlikely that such a neutron star “will last longer than a decade”.

Delayed Death

The next question then is: can we somehow determine whether this is indeed happening? Suvorov and Glampedakis suggest looking at short gamma-ray bursts. These are gamma-ray bursts that can occur when two neutron stars converge. Such a gamma-ray burst is followed by an afterglow, and how it behaves exactly, you could tell whether you are dealing with a neutron star that immediately becomes a black hole, or one that can survive a little longer thanks to a strong magnetic field.

The delayed death of the neutron star would then have to be accompanied by a one-time burst of radio emission. The combination of the right afterglow, followed a few years later by such a radio burst, is, according to the researchers, a smoking gun‘ for a neutron star that is too heavy and has been able to survive for a while thanks to a magnetic field.

Whole challenge

Anna Watts, an astronomer at the University of Amsterdam, calls the described scenario “certainly plausible” and the lifetime of this type of neutron star “surprisingly long”. According to her, it is important to indeed observe a gamma-ray burst associated with the merger of two neutron stars. “If you only have to rely on gravitational waves, you don’t know exactly where that merger took place. And then it can prove quite challenging to observationally link such a merger to a collapse that occurs a few years later.”