Astronomers have discovered a fast radio burst (FRB) activity cycle that repeats in a rather steady and predictable manner. This could mean that the unrestrained unpredictability of puzzling deep-space FRBs could, in fact, be an issue with our detection abilities.
FRB 121102 is already renowned for beings the most active FRB spotted yet, generating repeated bursts several times since it was identified in 2012. It was initially believed to have no rhyme or reason, but new observations of these explosions have unveiled a pattern.
Unusually-Behaving Radio Bursts
In thorough research of new and older data, astronomers found that FRB 121102 produces repeated burst activity for approximately 90 days, before going quiet for about 67 days. This 157-day cycle keeps repeating, with the latest observations stating that the source should have entered a new cycle around June 2nd.
The new discovery is unexpected and could help experts narrow down proposed causes for these puzzling signals. However, at the same time, it is a very clear demonstration of how peculiar and difficult to detect these signals actually are.
“This is an exciting result as it is only the second system where we believe we see this modulation in burst activity,” explained astronomer Kaustubh Rajwade of The University of Manchester. “Detecting a periodicity provides an important constraint on the origin of the bursts, and the activity cycles could argue against a precessing neutron star.”
FRBs are extremely energetic flares of radiation in the radio spectrum with a lifespan of just a few milliseconds at most. In that period, they can generate as much power as hundreds of millions of Suns. The majority ignite only once, randomly, and they never get detected again. This makes then impossible to foresee, even though experts are improving at tracing these bursts to their origins.
What Origins do FRBs Have, Though?
A smaller amount of sources show repeat activity, which was initially believed to be random until a source called FRB 180916 was discovered to be repeating on a cycle. For four days, it would ignite once or twice an hour, before getting quiet for 12 days, which would make its cycle 16.35 days.
This is ten times shorter than FRB 121102’s cycle, but if we were to say the two sources are similar and that the periodicity is caused by orbital motion, that extent can be compared to known cosmic bodies to narrow down what could be producing them.
“If we consider now also orbital motion to be the cause of the observed periodicity in FRB 121102, the large range in the observed periods (16-160 days) can constrain the possible binary systems,” the researchers wrote in their paper. “High-mass X-ray binaries are systems with a neutron star in an orbit with a massive O/B star. HMXBs in our Galaxy and the Small Magellanic Cloud have a large range of orbital periods, ranging from few tens to hundreds of days… On the other hand, binaries where the donor star fills the Roche lobe of the system have much shorter periods (< 10 days) and are unlikely to be possible progenitors.”
Until now, possible sources could be neutron stars, black holes, pulsars with companion stars, imploding pulsars, a theoretical type of star called a blitzar, a link with gamma-ray bursts, magnetars discharging giant flares, and aliens, which is rather not probable.
The finding of a magnetar inside the Milky Way galaxy emitting an incredibly FRB-like outburst back in April strongly indicates that magnetars are the origins of some FRBs. However, there are numerous variations within the category of FRBs, such as repeaters and non-repeaters, so it is possible that there are other sources as well.
“This exciting discovery highlights how little we know about the origin of FRBs,” said physicist and astronomer Duncan Lorimer of West Virginia University. “Further observations of a larger number of FRBs will be needed in order to obtain a clearer picture of these periodic sources and elucidate their origin.”
The study has been published in the Monthly Notices of the Royal Astronomical Society.