Astronomers have detected the polarization structure of the fast radio burst

Polarization structure of the rapid radio burst
Fast Radio Bursts (FRB) - radio telescopes detect single radio pulses lasting several milliseconds of unknown nature.

Astronomers first conducted a detailed spectropolarimetric analysis of fast radio burst, that is, they simultaneously recorded all polarization components at several wavelengths with high temporal resolution.

It turned out that the burst of FRB 181112 consists of several individual pulses, which differ in many parameters. 

The data obtained say that these features are either directly related to the radiation mechanism, or appeared when moving through a relativistic plasma, but so far they still cannot make a final conclusion about the nature of these sources, the authors write in a preprint for publication in The Astrophysical Journal Letters website.

Fast Radio Bursts (FRB) are bright sources of radio frequency radiation that shine on the order of a few milliseconds. Separate and repeated bursts are distinguished: the former are known over a hundred, and the latter about two dozen. For some FRBs, redshift was measured - it turned out to be significant, approximately in the range from 0.2 to 0.6.

At the moment, the nature of this type of radiation is not clear, although dozens of hypotheses have been proposed in this regard. The determination of the radiation mechanism and the type of emitting objects is complicated by the uncertainty of many basic physical characteristics of the FRB. 

In particular, the minimum characteristic time of energy release during the burst, which allows one to estimate the physical size of the emitting region, is unknown. It is also still unclear what causes the short duration of the bursts and it is unclear whether such a fast process really lies in the basis, or if a narrow radiation cone is formed in it, which during rotation briefly crosses the line of sight.

Astronomers from South Korea, Australia and the United States presented the results of a spectropolarimetric analysis of radiation from a single burst of FRB 181112. It arose in the galaxy at redshift z = 0.4755, but also crossed the halo of another galaxy at z = 0.3674 along the way to Earth.

The high temporal resolution available on the Australian ASKAP radio interferometer made it possible to isolate four separate pulses inside the burst and to measure for the first time all polarization components in each of them completely.

The dynamics of the Stokes parameters describing the polarization
The dynamics of the Stokes parameters describing the polarization.

It turned out that individual pulses differ in parameters. In particular, their intensities are different: the first is the brightest, the third is significantly weaker, and the second and fourth are so weak that they do not allow to measure all parameters. The arrival times of the pulses do not correspond to any apparent periodicity.

The degree of impulse polarization was very high, up to 94 percent in the case of the former. 

However, the proportion of the circular polarization component varied significantly both between and within pulses. In addition, the authors measured the measure of dispersion, i.e. the difference in arrival time between different frequencies. 

According to this characteristic, the individual pulses were also different.

The researchers are considering four variants of the conditions that could have led to the formation of a spike with these parameters: the initial conditions of the source radiation, gravitational lensing, as well as passing through a cold or relativistic plasma. 

Astronomers conclude that the first and last signals are the most likely, but they also have problems. 

Thus, the differences in the degree of dispersion are difficult to explain in the first variant, and in the latter case the accompanying synchrotron radiation is also expected, which was not observed in this case. 

Nevertheless, the polarization properties of FRB 181112 are similar to radiation known from the Milky Way pulsars and magnetars, but for clear conclusions we will need data of similar quality for other bursts.

Last year, the first two separate rapid radio bursts were localized. Astronomers have also recently discovered for the first time the frequency of a second fast radio burst.