Something in a galaxy located billions of light years away is blasting out pulses of light on a bizarre periodic schedule that lasts 157 days, according to a new study. The discovery may help solve the mystery of fast radio bursts (FRBs), which are sudden and intense bursts of light that last only milliseconds. Though there are many theories about their origins and causes, FRBs are still unexplained at this time.
When the first FRB was identified in 2007, scientists thought it might be a cataclysmic event such as the explosion of a massive star. Over the next decade, though, it became clear that some of these bursts are “repeaters,” meaning that they emit multiple pulses of light that cannot be explained by a one-time cosmic cataclysm like a supernova. The first known repeater signal was dubbed FRB 121102, and for years a pattern in its signals eluded scientists, until now.
A team led by Kaustubh Rajwade, an astronomer at the University of Manchester, carefully examined five years of observations of FRB 121102, which is located about three billion light years from Earth. Using this long-term approach, Rajwade and his colleagues were able to identify a periodic cycle to its bursts, marking only the second time that periodicity has been spotted in an FRB, according to a recent study published in Monthly Notices of the Royal Astronomical Society.
“FRB 121102 is the first FRB that was found to repeat in 2016,” Rajwade said in an email. “Since then, it has been extensively monitored by radio telescopes around the globe and so naturally, a lot more bursts have been recorded from this source.”
“We have only just started to discover new repeating FRBs (in the last year or so) since telescopes like CHIME (Canadian Hydrogen Intensity Mapping Experiment) and ASKAP (Australian Square Kilometre Array Pathfinder) have started surveying the sky for new FRBs,” he added.
In addition to pulling together observations of FRB 121102 from multiple telescopes, Rajwade and his colleagues monitored the pulses using the Lovell Telescope at Jodrell Bank Observatory. The results enabled the team to pick out an intriguing signal in the noise: The FRB appears to flash for approximately 90 days, then goes dark and dormant for 67 days, before beginning the 157-day cycle again.
In fact, FRB 121102 should “turn on” for its active phase this week, according to the study, though unfortunately Rajwade’s team cannot check in on it due to the ongoing coronavirus pandemic.
“I would have loved to put our predictions to the test but due to Covid-19, Lovell Telescope is shut down at the moment and will only be available in a month or so,” Rajwade said. “I do hope that other radio telescopes around the world are monitoring the source to see if our predictions are correct.”
Regardless, this tentative 157-day cycle of FRB 121102 harkens back to another recent study, which identified the first example of periodicity in an FRB. That research revealed that FRB 180916, which is located about 450 million light years from Earth, has an active phase with clustered flashes that last for four days, followed by a quiet period that lasts for 12 days, for a total cycle of 16 days.
Obviously, FRB 121102 experiences a much longer cycle than FRB 180916, which hints at “a large range in the periodicities of repeating FRBs,” according to Rajwade’s study. Moreover, the fact that both repeaters display an “on” and “off” phase has big implications for constraining hypotheses about the sources of these cryptic bursts.
For example, one explanation proposed to explain repeaters is that their pulses are emitted by a magnetar, which is a type of dead star with an extraordinarily intense magnetic field. Flares unleashed by a magnetar could fall into a periodic pattern related to its precession, which is a type of wobble around a rotational axis. However, magnetars are predicted to spin once every second, or even faster, so this hypothesis isn’t a neat fit for the 16-day pattern of FRB 180916, let alone the nearly half-year cycle of FRB 121102.
Scientists have also proposed that these repeating bursts could be the result of sparks flying between two extreme objects in close orbits around each other. Binary systems containing a massive living star and a compact dead star, such as a black holes or a neutron star, could potentially produce the periodic effect. In this scenario, the dormant period observed in these two FRBs might be explained by a massive star blocking out the light bursts from our perspective on Earth.
To find out for sure, scientists will need to keep searching for patterns from these enigmatic sources. The new discovery of a cycle that lasts more than five months shows that long-term monitoring of these radiant phenomena is necessary to unlock their individual rhythms.
“We plan to confirm our observations with more monitoring of FRB 121102,” Rajwade said. “Along with that, we also plan to monitor other known repeating FRBs to see if they also show any signs of periodic bursting activity and refine our understanding of the timescales and nature of the variations in FRB 121102 and other repeating FRBs.”
“This will take us closer to understanding the true nature of FRBs,” he concluded.