Just over a decade ago, astronomers noticed bursts of radio waves coming from the cosmos, lasting barely milliseconds, now known as fast radio bursts (FRBs). Today, these bursts are still shrouded in mystery, as astronomers scramble to gather clues about their nature. This month (April 2021), an international team of astronomers announced that they have now broken an observational record for FRBs, by measuring radio bursts from one of the best-studied FRBs – known as the FRB name 20180916B – at lower frequencies (longer wavelengths) than ever before. before. They also found this very low frequency signal from FRB 20180916B is coming three days later higher frequency emission from the same object. This strange discovery provides new and important insights into the enigmatic origin of FRBs.
The research has been published in the Peer-reviewed Astrophysical Journal Letters April 9.
The main author of the journal Ziggy Pleunis, postdoctoral researcher at McGill University in Montreal, Canada, explained:
We detected fast radio bursts up to 110 MHz, where previously such bursts existed only up to 300 MHz. This tells us that the region around the source of the bursts must be transparent to low-frequency emissions, whereas some theories suggested 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 million light years. Because this 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, i.e. it sends a new burst every 16 days. This made it the first predictable radio burst.
Pleunis told EarthSky that there are two main explanations for the 16-day delay between bursts:
One possibility is that the FRB source is in a binary (double), and FRBs only become observable from Earth for a few days once per orbital rotation. The rest of the time the emission is pointed away from us or obscured. The other possibility is that the FRB source is processing [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 directed towards us.
These explanations could explain the 16-day delay between bursts. 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 arrived three days later than that of higher frequencies.
How is it possible ? 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 make the low frequency signal come so late? Pleunis explained to EarthSky these astronomers’ theory for the three-day delay:
In many models, FRBs are produced in the magnetic field surrounding a neutron star [a highly compact star], in a beam or cone emanating from the magnetic poles of the star. The emission produced at different altitudes in this magnetic field—nearer or farther from the body of the neutron star itself—is thought to have different characteristic frequencies due to changing magnetic field conditions. 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 from Earth, you would first face the regions closest to the star before “seeing” the regions of higher altitude. This means that you would first measure the emission with the higher frequencies and then, a few days later, you would observe the emission of the lower frequencies.
In other words, the delay in the arrival of the higher frequency emission could be a consequence of the orientation of the neutron star and its magnetic field (assuming the models are correct and that FRBs can be produced in the magnetic field of a neutron star). Pleunis continues:
If a similar FRB source is oriented differently with respect to Earth, it would be possible to see the low frequency radio waves before the high frequency radio waves in that system.
If you’re finding all of this hard to visualize, you’re not alone. The inherent motion of the FRB complicates matters, for one thing. To further complicate matters, magnetic fields are rarely uniform fields with two well-defined beams from each pole (textbook case). Instead, real magnetic fields in nature are much more disordered.
As Pleunis told EarthSky,
There are many unknowns regarding FRB progenitors and the emission mechanism…Emission need not be produced in the beams emanating from the [neutron star’s] magnetic poles, but the emission can also be produced in the magnetic field, when 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 from a companion star.
In other words, this is a very active area of research and there is still a lot to learn. Pleunis continues:
Why does the emission have a different characteristic frequency at different altitudes? It would also depend on the as yet unknown emission mechanism of FRBs.
Astronomers used two telescopes, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and the Dutch Low frequency network (LOFAR). LOFAR has stations spread across Europe to increase the detail of the data. For this project, the astronomers had tuned the telescope to observe in a range of 110-188 MHz (wavelength 2.7 to 1.6 meters).
Because the detections were found at the edge of this range, astronomers believe they may extend even lower and plan to observe at even lower frequencies to learn more.
The next video from JIVE and EVN describes repeating FRB 20180916B:
Note that electromagnetic emission waves – including light – are measured by both the length of the waves (wavelength) and their frequency (frequency). The longer the wavelength, the lower the frequency and vice versa; the shorter the wavelength, the higher the frequency. A good trick not to get confused 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 radio waves from a well-known repeating fast radio burst that are much longer than ever detected before. But not only that, the radio signal also arrived at the telescope three days after the most energetic part of the same radio burst.
