Pulsar or neutron star in a nebula. 3D illustration
When a team from West Virginia University first discovered fast radio bursts (FRBs) by sheer luck while examining data from the Parkes radio telescope in Australia, they had few clues about their source.
They speculated that these high-energy, radio-emitting FRBs were probably produced by some kind of compact object: a neutron star, a white dwarf, or perhaps some kind of active black hole.
Nearly 20 years later, we still know very little about the source of FRBs, except that they appear by the thousands throughout the sky, most of which are found at vast distances outside our galaxy. They are also thought to be the result of a number of different astrophysical phenomena. One intriguing theory is that FRBs are the result of blitzer events, a process in which a neutron star collapses into a black hole.
In a recently published review article, Journal of Astrophysics and Space SciencesThe authors suggest that the fast millisecond radio bursts represent the final signal of a supermassive, spinning neutron star collapsing into a black hole.
When the neutron star’s magnetic field lines break, the authors note, it could transform a nearly ordinary pulsar — a rapidly rotating, radio-emitting neutron star — into a bright radio “blitz.” This would produce a massive radio burst that could be observed up to 3 billion light years away, the authors write.
Blitzer’s theory suggests that a neutron star exceeds its mass limit by absorbing material from some kind of stellar companion, Duncan Lorimer, lead author of the paper and professor of physics and astronomy at West Virginia University in Morgantown, said in an email.
The neutron star is believed to reach its theoretical limit and collapse into a stellar-mass black hole, a transition that releases a huge amount of energy from the neutron star’s magnetic field.
In Blitzer’s case, Lorimer said, the energy contained in the neutron star’s magnetic field is released because the field is no longer pinned to the (now defunct) surface of the star.
Oppenheimer was ahead of his time again
In late 1938, renowned American atomic physicist Robert J. Oppenheimer collaborated with George Volkoff on a paper titled “On Massive Neutron Nuclei,” and laboriously derived calculations from a slide rule, as described in the 2005 book “American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer.” The two physicists suggested that there was an upper limit to the mass of these neutron stars (now known as the “Oppenheimer-Volkoff limit”), beyond which they would become unstable.
Essentially, any neutron star that exceeds the two-to-three solar mass limit becomes a black hole, though Oppenheimer and his colleagues had not yet adopted that moniker at the time. However, it would be several decades before observational astronomers were able to detect such stellar-mass black holes.
What about other sources of FRBs?
In 2007, we were part of the team that discovered the so-called “Lorimar burst,” FRB 20010724, the first example of a new general class of objects now known as fast radio bursts, the authors note.
“We were looking for individual radio pulses in the Magellanic Clouds that we thought might be coming from active pulsars,” Lorimer said.
Incredibly bright radio source
The pulse, located two degrees south of the Small Magellanic Cloud, was so bright that it saturated the electronics in the data-gathering system, Lorimer said, and was estimated to be about a trillion times brighter than the most luminous pulse ever observed from a pulsar.
Magnetars are a type of neutron star that are thought to have an extremely powerful magnetic field. … [+]
These bursts are super fast
Some bursts with characteristics on timescales of tens of nanoseconds have now been seen, Lorimer said, and so far they’ve been seen in the radio bands operating at the most sensitive radio telescopes, from 100 MHz to about 8,000 MHz.
The FRB as a space probe
One surprising aspect of studying FRBs is that they act as a probe of intervening matter: Even without a full understanding of their source, all we need is a sample of FRBs with a well-defined redshift, the authors write. This would allow us to effectively count the number of electrons along different lines of sight in space and directly measure the electron density, the authors note.
About 10% of all FRBs are repeated.
This means that at least some FRBs come from a persistent source (such as a neutron star flare) rather than a one-off cataclysmic origin (such as a neutron star merger), Lorimer said.
The conclusion?
Understanding the source of FRBs is crucial to understanding stellar evolution and the final state of stars in general, Lorimer said. As we unravel the mystery, he said it seems likely that compact objects (white dwarfs, neutron stars, black holes) are all involved.
