Fast radio bursts: the last sign of supramassive neutron stars

Abstract: Several fast radio bursts have been discovered recently, showing a bright, highly dispersed millisecond radio pulse. The pulses do not repeat and are not associated with a known pulsar or gamma-ray burst. The high dispersion suggests sources at cosmological distances, hence implying an extremely high radio luminosity, far larger than the power of single pulses from a pulsar. We suggest that a fast radio burst represents the final signal of a supramassive rotating neutron star that collapses to a black hole due to magnetic braking. The neutron star is initially above the critical mass for non-rotating models and is supported by rapid rotation. As magnetic braking constantly reduces the spin, the neutron star will suddenly collapse to a black hole several thousand to million years after its birth. We discuss several formation scenarios for supramassive neutron stars and estimate the possible observational signatures {making use of the results of recent numerical general-relativistic calculations. While the collapse will hide the stellar surface behind an event horizon, the magnetic-field lines will snap violently. This can turn an almost ordinary pulsar into a bright radio "blitzar": Accelerated electrons from the travelling magnetic shock dissipate a significant fraction of the magnetosphere and produce a massive radio burst that is observable out to z>0.7. Only a few percent of the neutron stars needs to be supramassive in order to explain the observed rate. We suggest that fast radio bursts might trace the solitary formation of stellar mass black holes at high redshifts. These bursts could be an electromagnetic complement to gravitational-wave emission and reveal a new formation and evolutionary channel for black holes that are not seen as gamma-ray bursts. Radio observations of these bursts could trace the core-collapse supernova rate throughout the universe.

• The paper introduces a hypothesis that FRBs are produced when supramassive neutron stars lose rotational support and collapse into black holes.
• It employs numerical general-relativistic simulations to demonstrate how magnetic braking triggers violent reconnection events that emit powerful radio bursts.
• The study implies that only a few percent of neutron stars need to be supramassive, linking FRB occurrences with stellar-mass black hole formation and core-collapse supernova rates.

Fast Radio Bursts: The Last Sign of Supramassive Neutron Stars

This paper introduces a compelling hypothesis for the origin of fast radio bursts (FRBs), proposing that they are the last observable signals from supramassive neutron stars (SMNS) as they collapse into black holes. Discovered relatively recently, FRBs are characterized by bright, millisecond-duration radio pulses that do not correspond with any known pulsars or gamma-ray bursts. Their high dispersion measures suggest that they originate from cosmological distances, necessitating extremely high energy outputs far surpassing those of typical pulsar signals.

The authors posited that an FRB signifies the final catastrophic event in the life of an SMNS, which collapses into a black hole due to magnetic braking. Initially, the SMNS is supported against gravitational collapse by rapid rotation, as it is above the critical mass for a non-rotating neutron star. However, as magnetic braking reduces the star's spin, it eventually reaches a threshold where centrifugal forces can no longer prevent the collapse, resulting in the formation of a black hole. This process can occur over a timescale ranging from thousands to a few million years after the neutron star's birth, depending on its initial spin and magnetic field strength.

The study explores multiple formation scenarios for SMNS and calculates potential observational signatures using recent advancements in numerical general-relativistic calculations. During the collapse, the neutron star's surface is rapidly concealed behind an event horizon, but the magnetic field lines experience a violent reconnection event. This generates a powerful electromagnetic shock that travels through the magnetosphere, triggering a massive radio burst, or "blitzar," visible from significant cosmological distances, potentially exceeding redshift values of 0.7.

A notable implication of this model is that only a small fraction, a few percent, of neutron stars need be supramassive to account for the observed rate of FRBs, which aligns with current estimates. The authors propose that FRBs could represent an electromagnetic counterpart to gravitational wave emissions, offering insights into a previously undefined evolutionary pathway for neutron stars and black holes that do not create the energetic signatures typically associated with gamma-ray bursts.

From an observational perspective, these findings suggest that FRB detections might trace the formation rate of stellar-mass black holes and their high redshift environments. Moreover, if SMNS originate from births rather than accretion processes, FRB observations could potentially map the core-collapse supernova rate across the universe.

This hypothesis underscores the value of FRBs for understanding compact object evolution and offers a unique perspective on high-energy astrophysical phenomena. Future research should focus on corroborating the link between SMNS and FRBs through continued radio observations and the search for potential signatures of magnetic reconnection in such events. Expansion in high-sensitivity and rapid-response observational capabilities could further elucidate the physical mechanisms at play, contributing to a broader understanding of the life cycle of neutron stars and the formation of black holes.