Ultrastrong magnetic fields could prevent neutron stars from forming black holes

 

Biggish bang Artist's impression of neutron stars merging, producing gravitational waves and resulting in a kilonova explosion. (Courtesy: University of Warwick/Mark Garlick/CC BY 4.0)

The merger of two neutron stars might produce a large and exotic sort of neutron star that would avoid becoming a black hole — at least temporarily. Despite being above the theoretical mass limit for black hole creation, Arthur Suvorov of Manly Astrophysics in Australia and Kostas Glampedakis of the University of Tübingen in Germany have estimated that magnetically supramassive neutron stars could stave off gravitational collapse.

The LIGO–Virgo team discovered the first gravitational waves emitted by two neutron stars spiralling into each other and merging in 2017. Astronomers were able to analyse the aftermath of the merger using a variety of telescopes as a result of this occurrence, but fundamental questions regarding the object that was generated remain unanswered.

Because neutron stars are projected to have masses of at least 1.1 times that of the Sun, a merger of two neutron stars can result in a mass bigger than the Tolman–Oppenheimer–Volkoff limit. A neutron star generated in a merger that exceeds this limit, estimated to be between 2–3 solar masses, is likely to collapse directly into a black hole.

Magnetic effect

The Tolman–Oppenheimer–Volkoff limit only applies to non-rotating neutron stars, however astrophysicists believe that if neutron stars are spinning, they can prevent collapse briefly. Suvorov and Glampedakis have proven in their latest theoretical research that a neutron star over the Tolman–Oppenheimer–Volkoff limit may prevent collapse if it was generated with a magnetic field 1017 times greater than the Sun.

As the magnetic field weakens, the duo predicts that these "magnetically supramassive" neutron stars will be able to avoid collapsing for 1–10 years. The exact timescale is determined by the star's core temperature, internal field strength, birth mass, and mass-density relationship.

Short gamma-ray bursts would accompany the birth of a magnetically supramassive neutron star, according to Suvorov and Glampedakis, followed by a strong but short-lived X-ray afterglow. The ensuing magnetic shocks would accelerate surrounding electrons to relativistic speeds, culminating in a flash of fast radio bursts if it collapsed to a black hole.

If their estimates are true, the team believes that existing telescopes may detect these distinct signals. If that's the case, they might be detected alongside future observations of gravitational waves emitted by merging neutron stars. According to them, such multimessenger observations would yield a "smoking gun" signature, proving that this rare type of neutron star exists.

Reference:

Physical Review D.