What? A few milliseconds are the same as decades? Unveiling the mystery of fast radio bursts

Produced by: Science Popularization China

Author: Yun Chaoang and Wen Zhigang (Xinjiang Astronomical Observatory, Chinese Academy of Sciences)

Producer: China Science Expo

In 2007, astronomers discovered an overlooked signal in long-archived pulsar data from the Parkes radio telescope - a short, bright radio pulse lasting a few milliseconds.

The energy of this instantaneous radio pulse in the radio band is equivalent to tens of billions of times the world's current annual power generation, that is, the total energy emitted by the sun in the radio band over the past few decades!

This signal has been traveling in space for billions of years, gradually slowing down after flying through interstellar gas and dust. After effects such as dispersion, scintillation, and scattering, its pulse profile has changed significantly. So what exactly is this signal?

Parkes radio telescope (Image credit: Flickr/Amanda Slater, CC BY-SA)

Mysterious signal from microwave oven

Let's turn the clock back to 1998, when the Parkes radio telescope in Australia discovered a mysterious signal - it was high-frequency at first, then the frequency gradually decreased, and finally returned to calm.

Astronomers have different opinions about this signal. Some think it is a pulse caused by lightning, while others think it is a signal from aliens.

In the following days, this familiar signal appeared intermittently, every time during the day and at mealtime. Therefore, the staff of the Parkes telescope has developed a habit of waiting for the signal with meals in hand during the 17 years of observation, from 1998 to 2015, rain or shine for 17 years.

Astronomers have written hundreds of papers on this signal and published them in authoritative journals. They even gave the signal a name - peryton , a half-deer, half-bird monster with a human-shaped shadow. It is very similar to the radio signal that was ignored in 2007.

Peryton image (Image source: Wiki)

It was not until 2015 that a new colleague discovered that the frequency of the signal was around 2.4 GHz, consistent with the frequency of microwave ovens. From this, they finally confirmed that the signal came from a microwave oven in a research laboratory in the astronomical station, and the scientists' 17 years of hard work were instantly brought to nothing.

This incident became a famous blunder in the history of scientific research, revealing the cruelty of astronomical research.

Fast radio bursts: astrophysical phenomena of unknown origin

As for the signal that was ignored in 2007, scientists from all over the world have repeatedly confirmed that it was neither a malfunction of the telescope nor noise, and they have ruled out the possibility of a signal radiated by a microwave oven. They call it "fast radio bursts," or FRBs (fast radio bursts), an astrophysical phenomenon of unknown origin that manifests as transient radio pulses lasting an average of a few milliseconds.

Since the first discovery of fast radio bursts by the Parkes 64-meter radio telescope in 2007, scientists have included a total of about 700 sources emitting such signals in the FRB catalog, of which 63 sources have emitted at least 2 FRB signals (data as of May 2023).

Spatial distribution of the Parkes telescope's observation area in the galactic coordinate system. The orange stars show the distribution of 81 fast radio burst candidates, and the gray shaded area shows the telescope's observation area from 1997 to 2001. The five purple stars are the fast radio bursts detected by the Parkes telescope in 2001 that were previously published (including the first fast radio burst discovered by Lorimer et al.). (Image source: Monthly Notices of the Royal Astronomical Society)

Such repeated FRBs are called "repeater bursts". A very famous one is FRB180916.J0158+65. The number indicates the time when the FRB was detected.

This fast radio burst was first detected on September 16, 2018. It is the only fast radio burst with a fixed period detected so far , with a period of 16.35 days. FRBs that occur only once are called "non-repeating bursts."

What is the significance of detecting fast radio bursts?

Scientists have been studying FRB for more than a decade, but people's understanding of it is still relatively vague. For example, what is its origin star? Where is it located? What is the cause and mechanism of its generation?

These questions require more observational data to provide answers. At this stage, scientists can already infer some physical results from the detected FRBs.

When a fast radio burst reaches the Earth, the signal it carries has a different wavelength, a different propagation speed, and takes a different time to arrive at the Earth.

By recording the arrival times of different wavelengths, scientists can know how much interstellar matter the fast radio burst has passed through during its long journey, and use them to detect the density distribution of free electrons in the interstellar medium.

FRB121102 (Image credit: Gemini Observatory/AURA/NRC/NSF/NRAO)

Fast radio bursts can also test the weak equivalence principle in Einstein's general theory of relativity (the weak equivalence principle, which states that when neutral particles with no rest mass (such as photons) or negligible rest mass (such as neutrinos) propagate in a gravitational field, the time it takes to travel is different from that in the absence of a gravitational field).

Since the light curve of a fast radio burst generally presents a simple single pulse feature, the time difference between photons of different frequencies in the fast radio burst arriving at the earth can be used to accurately verify the weak equivalence principle hypothesis.

Using dispersion measurements (DM values) of fast radio bursts, scientists can detect the "missing" baryons in the low-redshift universe and provide one possible theoretical explanation for the "missing" baryons.

In addition, fast radio bursts can also be used as a "standard candle" to observe the expansion of the universe at high redshift (z>5). If there is a significant dark energy effect on the high redshift scale, fast radio bursts will be the best means of observation in this redshift range.

Artistic conception of a fast radio burst and its host galaxy

The telescopes in the picture are China's Sky Eye (FAST) and the Keck Telescope, a single-aperture optical telescope with the highest spatial resolution (illustration: Yu Jingchuan, Fu Hai) (Image source: Science Academy WeChat account)

Fast Radio Burst Chronicle

Astronomers speculate that although the number of fast radio bursts we have currently detected is limited, it should be a common cosmic phenomenon . They believe that fast radio bursts occur in the universe at a frequency of about once per second (about 700 have been detected so far).

Although scientists have not yet fully understood the scientific mysteries of fast radio bursts, decades of observation and research have enabled us to achieve a lot of results.

Conclusion

The FRB field is a research field full of mystery and challenges. With the help of the high-sensitivity observation capability of China's Sky Eye FAST and its observation advantages in the low-frequency band, and with the completion of the 110-meter omnidirectional movable radio telescope at the Qitai Observatory of the Xinjiang Astronomical Observatory of the Chinese Academy of Sciences, we can observe more FRB signals in a higher frequency range and a larger sky area. I believe that in the future mankind will make a qualitative leap in the study of fast radio bursts.

References:

[1] Macquart JP, Keane E, Grainge K, et al. Fast Transients at Cosmological Distances with the SKA[J]. Advancing Astrophysics with the Square Kilometre Array, 2015:055.DOI:10.22323/1.215.0055.

[2] Fender R , Stewart A , Macquart JP ,et al.Transient Astrophysics with the Square Kilometre Array[J].Physics, 2015.DOI:10.48550/arXiv.1507.00729.