FRB 121102 is the most famous of all fast radio bursts, and it's just deepened the mystery of these cosmic explosions. FRBs are mysterious emissions of radio waves that release the equivalent energy of what the Sun produces over many months in a fraction of a second. But FRB 121102 is special. It was the first one discovered to repeat and the first that allowed us to locate its source. Now, it's the first caught releasing over 1,600 bursts in just a 47-day period, the largest set of FRBs ever released.
We don't know yet what causes FRBs, although magnetars (a type of neutron star) are a strong contender. Most FRBs just flare once, making it hard to trace them. Some have been caught repeating on a regular cycle, like FRB 121102, which makes it easier to predict when they will occur and observe them.
Since its discovery, radio astronomers have discovered that FRB 121102 has a period that spans about 157 days, where FRBs are emitted for 90 days and then are not for the remaining 67. New observations, published in Nature, caught 1,652 detections from the system over the course of 47 active days.
These observations allow for a deeper understanding of these mysterious events, particularly in investigating the timespan between each burst, but they also highlight that we don't understand them well at all and are probably a long way off from figuring them out.
When the system is in an active season, the bursts don’t follow a specific pattern – they lack periodicity – and the peak of the energy is at around 4.8x1030 Joules. That’s over twice the energy you would need to pull Mercury away from the Sun released in a fraction of a second. When all the bursts are combined, the energy release is of the order 6.4×1039 Joules. That’s the same energy released by the Sun in 530,000 years in just 47 days.
Without a defined periodicity and with such a huge energy budget, the authors of the new study consider it unlikely that these FRBs come from a single compact object, like a magnetar. The estimate means that almost 40 percent of the magnetar's energy would go into making these powerful events.
"The total energy of this burst set already adds up to 38% of what is available from a magnetar and no periodicity was found between 1 ms and 1,000 s, both of which severely constrains the possibility that FRB 121102 comes from an isolated compact object," co-author Dr Pei Wang said in a statement.
The observations, caught between August 29 and October 29, 2019, were conducted using FAST, the Five-hundred-meter Aperture Spherical radio Telescope. This is the largest single-dish telescope in the world and it has already discovered plenty of FRBs.
"As the world's largest antenna, FAST's sensitivity proves to be conducive to revealing intricacies of cosmic transients, including FRBs," said the paper’s lead author Professor Di LI.
FRB 121102 has been a crucial discovery into the mysterious field of fast-radio burst and it is still telling us how little we understand of these events.