Pulsar or neutron star in nebula. 3D illustration
First discovered by a sheer stroke of luck while combing through data from Australia’s Parkes radio telescope, the West Virginia University team that discovered the first fast radio burst (or FRB) had few clues about its source.
They hypothesized that these high-energy, radio-luminous FRBs were likely produced by some type of compact object. That is, either a neutron star, a white dwarf, or some kind of energetically active black hole.
Almost two decades later, little is still known about the sources of FRBs—except that they appear by the thousands across the sky, mostly at great extragalactic distances. And they are likely produced by several different astrophysical phenomena. One of the more interesting ideas is that they are the result of accretion, the process by which a neutron star collapses into a black hole.
In a review paper recently published in Journal of Astrophysics and Space Sciencethe authors suggest that a fast millisecond radio burst represents the final signal of a spinning supermassive neutron star collapsing into a black hole.
Jumping neutron star magnetic field lines can turn a near-ordinary pulsar (a rapidly rotating, radio-emitting neutron star) into a bright radio “flash,” the authors note. This produces a massive radio burst observable 3 billion light-years away, they write.
The storm idea involves a neutron star being pushed past its mass limit by accreting material from some sort of companion star, Duncan Lorimer, the paper’s lead author and a professor of physics and astronomy at West Virginia University in Morgantown, told me via email.
The idea is that this neutron star would reach its theoretical limit in order to collapse into a stellar-mass black hole. This transition, in turn, results in a massive release of energy from the neutron star’s magnetic field.
In the case of a light, the energy contained within the neutron star’s magnetic field is released because the field can no longer anchor itself to its — now nonexistent — stellar surface, Lorimer says.
Oppenheimer Again Ahead of His Time
In late 1938, the famous American nuclear physicist Robert J. Oppenheimer collaborated with George Volkoff, on a paper entitled “On massive neutron nuclei”, painstakingly deriving their calculations from the slide rules, as noted in the book 2005, “American Prometheus: The Triumph and Tragedy of J. Robert Oppenheimer.” The two physicists suggested that there was an upper limit – now called the “Oppenheimer-Volkoff limit” – to the mass of these neutron stars; anything beyond this limit would become unstable.
Basically, neutron stars that exceed a limit of two to three solar masses would become a black hole, although at the time, Oppenheimer and colleagues did not yet use that name. However, it took decades before observing astronomers were able to detect black holes of such stellar mass.
What about the sources of other FRBs?
In 2007, we were part of a team that discovered the so-called ‘Lorimer 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 pulsations in the Magellanic Clouds that we thought might come from energetic pulsars,” says Lorimer.
An incredibly bright radio source
It turned out to be two degrees south of the Small Magellanic Cloud and was so bright that it saturated the electronics in the data acquisition system, Lorimer says. The brightness of this pulse was assumed to be about a trillion times brighter than the brightest seen from pulsars, he says.
A magnetar is a type of neutron star that is believed to have an extremely strong magnetic field. of … [+]
These explosions are incredibly fast
There are already some bursts being seen with features on time scales of 10s of nanoseconds, Lorimer says. So far, they’ve been seen from 100 MHz to about 8,000 MHz in the radio band where the most sensitive radio telescopes operate, he says.
FRBs as space probes
One of the amazing aspects of FRB studies is that they act as probes of intervening matter, even if you don’t fully understand their source populations, all that is needed is a sample of FRBs with well-defined redshifts, they write the authors. From this, it is possible to effectively count the number of electrons along different lines of sight in the Universe and directly measure the electron density, they note.
About ten percent of all FRBs recur.
This means that at least some FRBs come from ongoing sources (such as flares in neutron stars), rather than some cataclysmic one-time origin (such as neutron star mergers), Lorimer says.
In the end?
Understanding the sources of FRBs is essential to understanding stellar evolution and the final state of stars in general, Lorimer says. As we unravel their mysteries, it’s a good bet that compact objects (white dwarfs, neutron stars and black holes) will be included in all of them, he says.