The energetic phenomena known as Fast Radio Bursts (FRBs) are one of the greatest cosmic mysteries today. These mysterious flashes of light are visible in the radio wave part of the spectrum and usually last only a few milliseconds before fading away forever. Since the first FRB was observed in 2007, astronomers have looked forward to the day when instruments of sufficient sensitivity would be able to detect them regularly.
That day has arrived with the completion of the 500-Meter FAST Radio Telescope (aka. Tianyan, “Eye of Heaven”). Since it commenced operations, this observatory has vastly expanded the number of detected FRBs. In fact, according to research led by the National Astronomical Observatories of the Chinese Academy of Sciences (NAO/CAS), the observatory detected a total of 1,652 independent bursts from a single source in 47 days.
The research, which recently appeared in the journal Science, was conducted by researchers from the Commensal Radio Astronomy FAST Survey (CRAFTS) project. CRAFTS includes researchers from the Cornell Center for Astrophysics and Planetary Science, the Max-Planck-Institut für Radioastronomie, the Commonwealth Scientific and Industrial Research Organisation (CSIRO), and multiple universities in China, Australia, and the U.S.
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To break this phenomenon down, FBRs are highly energetic, producing a year’s worth of Solar output in the space of milliseconds. In rare cases, astronomers have found bursts that were repeating in nature, which has allowed them to conduct follow-up studies. While the origin of these bursts is still unknown, possible explanations range from hyper-magnetized neutron stars and black holes to cosmic strings left over from the Big Bang and even alien transmissions!
This exotic explanation is especially appealing where repeating FRBs are concerned, as repetition lends itself to artificial explanations. This includes the signal designated FRB 121102, which was originally detected in 2012 and is the first known repeater and the first well-localized FRB. Not only has this signal been traced to a dwarf galaxy 3 billion light-years away, but it is repeatedly bursting at pretty regular intervals.
Previous observations determined that it repeats on a 157-day cycle that consists of a 67-day inactive phase, followed by a 90-day period where it would repeatedly emit intense radio flares. In recent years, Pei Wang and the many institutions participating in the FAST telescope project have monitored FRB 121102’s and recorded several repeating bursts – one that consisted of 20 pulses in one day and another where 12 bursts were observed in two hours.
From these, Wang and his colleagues were able to refine estimates of FRB 121102’s cycle, which they now place at 156.1 days. But when they examined the backend data obtained by FAST during its commissioning phase, they noticed that FRB 121102 experienced a truly energetic period of activity. During the three months, running from Aug. 29th to Oct. 29th, 2019, FAST detected no less than 1,652 independent bursts in 59.5 hours spanning 47 days.
While the rate of radio pulses varied throughout this time, a record 122 bursts occurred during the peak hour – the highest event rate ever observed from an FRB. Based on the detected bursts, the researchers determined that they have a peak energy equivalence of 480 Nonillion (4.8 × 1037) ergs at 1.25 GHz, below which the detection of bursts is suppressed. As Dr. Wang said in a CAS Newsroom release:
“The total energy of this burst set already adds up to 3.8% of what is available from a magnetar and no periodicity was found between 1 ms and 1000 s, both of which severely constrains the possibility that FRB 121102 comes from an isolated compact object.”
They also determined that bursts’ energy distribution is bimodal in nature, meaning that they are distributed one of two ways, depending on the energy level. In other words, they found that weaker FRB pulses are more random while strong ones occur with greater consistency. Moreover, these latest results also allowed the team to investigate the range of theoretical causes and narrow them down.
For one, the lack of periodicity (or quasi-periodicity) of this repeating FRB challenges the notion that it results from a single rotating compact object (aka. a black hole or neutron star). Second, the high burst rate disfavors high-energy or contrived mechanisms, which casts doubt on theories of alien agency. But most of all, they found that the high-cadence of these bursts (where many happen in the course of hour-long spans) will facilitate future statistical studies.
Basically, they anticipate that astronomers will be able to conduct investigations into the periodic nature of these bursts, with searches lasting between 1 millisecond and 1000 seconds. What’s more, they anticipate that the FAST telescope will play a vital role. “As the world’s largest antenna, FAST’s sensitivity proves to be conducive to revealing intricacies of cosmic transients, including FRBs,” said Prof. Li.
In more recent news, the CRAFTS project has reported the discovery of six new FRBs, including a repeater that is similar to FRB 121102. These and other radio sources are cataloged on the CRAFTS website.