Fast radio bursts are extragalactic transient phenomena that shine in radio wavelengths for short durations lasting only 1-10 milliseconds. Some sources of fast radio bursts are known to produce many bursts repeatedly. Repeaters are thought to be neutron stars, but the causes of bursts and the radiation mechanism are not well understood.
Totani & Tsuzuki found remarkable similarities between the statistical properties of fast radio bursts and earthquakes, especially the laws on aftershock occurrence. Image credit: NASA’s Goddard Space Flight Center.
Fast radio bursts (FRBs) are enigmatic and rarely detected bursts of energy that come from far beyond the Milky Way Galaxy.
These events have durations of milliseconds and exhibit the characteristic dispersion sweep of radio pulsars.
They emit as much energy in one millisecond as the Sun emits in 10,000 years, but the physical phenomenon that causes them is unknown.
The first FRB was discovered in 2007, although it was actually observed some six years earlier, in archival data from a pulsar survey of the Magellanic Clouds.
To date, more than one hundred FRBs have been detected, yet only some of these have so far been observed to repeat.
“The cause of FRBs is unknown, but some ideas have been put forward, including that they might even be alien in origin,” said University of Tokyo astronomers Tomonori Totani and Yuya Tsuzuki.
“However, the current prevailing theory is that at least some FRBs are emitted by neutron stars.”
“These stars form when a supergiant star collapses, going from eight times the mass of our Sun (on average) to a superdense core only 20-40 km across.”
“Magnetars are neutron stars with extremely strong magnetic fields, and these have been observed to emit FRBs.”
“It was theoretically considered that the surface of a magnetar could be experiencing a starquake, an energy release similar to earthquakes on Earth,” Professor Totani added.
“Recent observational advances have led to the detection of thousands more FRBs, so we took the opportunity to compare the now large statistical data sets available for FRBs with data from earthquakes and solar flares, to explore possible similarities.”
“So far, statistical analysis of FRBs has focused on the distribution of wait times between two successive bursts.”
“However, calculating only the wait-time distribution does not take into account correlations that might exist across other bursts.”
So the astronomers decided to calculate correlation across two-dimensional space, analyzing the time and emission energy of nearly 7,000 bursts from three different repeater FRB sources.
They then applied the same method to examine the time-energy correlation of earthquakes (using data from Japan) and of solar flares (using records from the Hinode mission), and compared the results of all three phenomena.
They were surprised that, in contrast to other studies, their analysis showed a striking similarity between FRBs and earthquake data, but a distinct difference between FRBs and solar flares.
“The results show notable similarities between FRBs and earthquakes in the following ways,” Professor Totani said.
“First, the probability of an aftershock occurring for a single event is 10-50%.”
“Second, the aftershock occurrence rate decreases with time, as a power of time.”
“Third, the aftershock rate is always constant even if the FRB-earthquake activity (mean rate) changes significantly.”
“And fourth, there is no correlation between the energies of the main shock and its aftershock.”
This strongly suggests the existence of a solid crust on the surface of neutron stars, and that starquakes suddenly occurring on these crusts releases huge amounts of energy which we see as FRBs.
“By studying starquakes on distant ultradense stars, which are completely different environments from Earth, we may gain new insights into earthquakes,” Professor Totani said.
“The interior of a neutron star is the densest place in the Universe, comparable to that of the interior of an atomic nucleus.”
“Starquakes in neutron stars have opened up the possibility of gaining new insights into very high-density matter and the fundamental laws of nuclear physics.”
The team’s work was published in the Monthly Notices of the Royal Astronomical Society.
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Tomonori Totani & Yuya Tsuzuki. 2023. Fast radio bursts trigger aftershocks resembling earthquakes, but not solar flares. MNRAS 526 (2): 2795-2811; doi: 10.1093/mnras/stad2532
