A little over a decade ago, astronomers noticed bursts of radio waves originating from the cosmos lasting a few milliseconds, now known as rapid radio bursts (FRBs). Today, these bursts are still shrouded in mystery, as astronomers scramble to gather clues as to their nature. This month (April 2021), an international team of astronomers announced that they had broken an observation record for FRBs, measuring radio bursts from one of the best-studied FRBs – known as FRB 20180916B – at lower frequencies (longer wavelengths) than ever. before. They also discovered that this very low frequency signal from FRB 20180916B is coming in three days later higher frequency emission of the same object. This strange discovery provides new and important information about the enigmatic origin of FRBs.
The research has been published in the peer-reviewed journal Letters from astrophysical journals April 9.
Lead author of the article, Ziggy Pleunis, postdoctoral researcher at McGill University in Montreal, Canada, explained:
We have detected rapid radio bursts up to 110 MHz, whereas previously these bursts only existed up to 300 MHz. This tells us that the region around the source of the bursts must be transparent to low-frequency emission, while some theories suggest that all low-frequency emissions would be absorbed immediately and could never be detected.
The team studied a repeating FRB, known as FRB 20180916B, which was discovered in 2018. It is located on the outskirts of a galaxy similar to our Milky Way galaxy, at a distance of about 500 millions of light years. Because it is considered close in astronomical measurements and because the burst repeats itself, the FRB has been the subject of several studies, revealing, for example, that it has a periodicity of 16.3 days in its activity, which means it sends out a new burst every 16 days. This made it the first predictable radio burst.
Pleunis told EarthSky that there are two dominant explanations for the 16-day delay between gusts:
One possibility is that the FRB source is in a binary (dual) system and the FRBs only become observable from Earth a few days after each orbital rotation. The rest of the time the show is pointed away from us or obscured. The other possibility is that the FRB source precedes [its magnetic pole is changing direction], and FRBs only become observable from Earth for a few days once per precession period when the emission is pointed at us.
These explanations could explain the 16-day delay between the gusts. But the new research also revealed that the FRB show is coming. at different times, as a function of frequency (i.e. in a way directly related to the wavelength of the signal). The team found that the newly observed low-frequency radio emission consistently came three days later than that of the higher frequencies.
How can that be? All electromagnetic emissions travel at the same speed, the speed of light (186,000 miles per second, or 300,000 km per second). What would cause the low frequency signal to come in so late? Pleunis explained these astronomers’ theory for the three-day timeframe to EarthSky:
In many models, FRBs are produced in the magnetic field surrounding a neutron star [a highly compact star], in a beam or a cone emanating from the magnetic poles of the star. The emission produced at different altitudes in this magnetic field – closer or further from the body of the neutron star itself – is believed to have different characteristic frequencies due to the changing conditions of the magnetic field. High frequency radio waves would be produced at lower altitudes [closer to the neutron star] than low frequency radio waves.
If there is indeed this type of relationship between the distance from the star where the burst is produced and the frequency of the burst, Pleunis explained, then, due to the movement of the FRB in the two 16-day burst scenarios, in looking down from Earth, you would first face the regions closest to the star before “seeing” the higher elevation regions. This means that you would first measure the emission with the higher frequencies, and then a few days later you would observe the emission with the lower frequencies.
In other words, the delay in the arrival of the longer frequency emission could be a consequence of the orientation of the neutron star and its magnetic field (assuming the models are correct that the FRBs can be produced in the magnetic field of a neutron star). Pleunis continued:
If a similar FRB source is oriented differently with respect to Earth, it would be possible to see low frequency radio waves before high frequency radio waves in that system.
If you find all of this difficult to visualize, you are not alone. The inherent movement of the FRB complicates matters, on the one hand. To make matters even more difficult, magnetic fields are rarely uniform fields with two well-defined beams from each pole (the textbook case). Instead, true magnetic fields in nature are much messier.
As Pleunis told EarthSky,
There are many unknowns concerning the FRB progenitors and the emission mechanism… It is not necessary that the emission be produced in the beams emanating from the [neutron star’s] magnetic poles, but the emission can also be produced in the magnetic field, as it sizzles and cracks, or it can be produced further away by the interaction of the neutron star’s magnetic field with, for example, the wind of a companion star.
In other words, this is a very active area of research and there is still a lot to learn. Pleunis continued:
Why does the emission have a different characteristic frequency at different altitudes? It would also depend on the still unknown issuance mechanism of FRBs.
Astronomers used two telescopes, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Dutch Low Frequency Array (LOFAR). LOFAR has stations distributed throughout Europe to increase the detail of the data. For this project, astronomers had set the telescope to observe in a range of 110-188 MHz (2.7 to 1.6 meters in wavelength).
Since the detections were found at the edge of this range, astronomers believe they could spread even lower and plan to observe at even lower frequencies to find out more.
The following video from JIVE and the EVN describes repeated FRB 20180916B:
Note that electromagnetic emission waves – including light – are measured by both wavelength (wavelength) and frequency (frequency). The longer the wavelength, the lower the frequency and vice versa; the shorter the wavelength, the higher the frequency. A good tip to avoid making mistakes is to remember the letter L for for the Llow frequency /Llong wavelength region, which are the waves we are discussing in this article.
Conclusion: Astronomers have measured the radio waves of a well-known repetitive rapid radio burst which are much longer than ever detected before. But not only that, the radio signal also arrived at the telescope a surprising three days after the most energetic part of the same radio burst.
Source: LOFAR Detection of 110-188 MHz emissions and frequency dependent activity from FRB 20180916B
Via McGill University