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Earlier this year, astronomers announced a dazzling discovery. A rapid radio burst called FRB 121102 not only repeated itself – it repeated in a discernible cycle.
For about 67 days, the source is silent. Then, for about 90 days, it wakes up again, spewing out radio flares repeated in milliseconds before falling silent, and the entire 157-day cycle repeats.
However, the rapid radio bursts are extremely mysterious, and there was no guarantee the cycle would continue. So it’s pretty exciting that the source woke up, just at the right time – in line with its business cycle forecast.
This suggests that there is significant value in monitoring known sources of rapid radio bursts – but also in continuing to watch FRB 121102 to try to understand what could be causing the phenomenon.
A quick reminder: fast radio bursts are, as the name suggests, very fast radio wave bursts of just a few milliseconds from galaxies millions to billions of light years away. But they are also extremely powerful; in those milliseconds, they can discharge as much energy as hundreds of millions of suns.
Most of the time they erupt once and we haven’t heard from since, making them impossible to predict and very difficult to trace. And we don’t know what causes them, although recent evidence quite strongly points to a type of neutron star called magnetars.
But a handful of sources of the rapid radio bursts have been detected repeating themselves, and these could be one of the keys that at least partially help solve the mystery.
Before its cycle was discovered by University of Manchester astronomer Kaustubh Rajwade and his team, FRB 121102 was already famous for being the most active rapid radio burst discovered to date, spitting several times since its discovery in 2012.
Because it repeats itself, astronomers could monitor the activity and trace it back to a source galaxy. It was the first rapid radio burst to be located, in a region of star formation in a dwarf galaxy 3 billion light years away.
The discovery of the periodicity of its activity – based on five years of data – could impose significant constraints on what it could be.
For example, high-mass x-ray binaries in the Milky Way – those that contain neutron stars – can have orbital periods of up to hundreds of days. But there are certain types of binary systems with much shorter periods – these could be excluded for FRB 121102.
And now the periodicity is supported by new sets of observations – although the timing may require revision.
A team led by Marilyn Cruces of the Max Planck Institute for Radio Astronomy detected 36 bursts of FRB 121102 using the Effelsberg 100m radio telescope between September 2017 and June 2020. Combined with data from Rajwade’s research, tThe team deduced a periodicity of 161 days, in a new pre-printed paper uploaded to arXiv.
This article gives dates between July 9 and October 14, 2020 for the active period of the source.
But Cruces and his team aren’t the only ones looking. A team led by Pei Wang of the National Astronomical Observatory of China used the five-hundred-meter-aperture spherical radio telescope to monitor the location of FRB 121102 on several dates between March and August 2020.
Between mid-March and the end of July, they did not detect any gusts. But on August 17, FAST detected at least 12 bursts of FRB 121102 – suggesting that the source is in the active phase again – although the team calculated a different periodicity for both Rajwade’s team and the Cruces team.
“We combine the gusts collected in Rajwade et al. (2020) and Cruces et al. (2020) with these newly detected by FAST in 2019 and 2020, and achieve a new best-fit period of ~ 156.1 days, ”they wrote in a notice posted on The Astronomer’s Telegram.
According to the calculations of Wang’s team, the active phase should end between August 31 and September 9, 2020. If the FRB 121102 continues to show activity beyond these dates, it could suggest that the periodicity n is not real or that it has evolved in some way, they noted in their post.
Of course, it is also possible that the periodicity calculations need to be refined. Which means we need to continue to keep an eye on FRB 121102.
“We are encouraging more monitoring efforts from other radio observatories,” the researchers wrote.
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