A radio signal from the center of the Milky Way

This is unheard of … A radio signal from the center of the Milky Way leaves researchers puzzled, according to a study conducted by the University of Sydney and published on October 12, 2021 in the Astrophysical Journal. It was first captured by Australia’s ASKAP radio telescope. On six occasions, between January and September 2020, astronomers recorded a very irregular signal, its intensity ranging from 1 to 100 before disappearing, only to reappear a few hours or days later, completely randomly. And it’s just a radio signal. Scientists have tried to find its traces in other areas of the electromagnetic spectrum: X, infrared, visible. In vain. For now “ASKAP J173608.2-321635”, its current name for lack of a better one, therefore only a radio source … But what type?

“The strangest feature of this signal is its very large polarization”

In November 2020, the team requested the more sensitive MeerKAT radio telescope in South Africa to complete the study. In the beginning, radio silence, it is the case to say it … Then, on February 7, 2021, the source reappears, intensely, before disappearing after only one day. Not enough to give a lot of new items to the team. On the other hand, the examination of the signal confirms another peculiarity already registered by ASKAP. “The strangest feature of this signal is its very high polarization. This means that its light oscillates in one direction, but this direction rotates over time.“, Explains Zinteng Wang’s page, co-author of the publication. The light, regardless of the field (radio, visible, UV, X …), is actually an electromagnetic wave consisting of an electric field (E) and a magnetic field (B), perpendicular to each other. They are located in a plane which is itself perpendicular to the direction of propagation of the light beam. When the he-wave is not polarized, the pair (E, B) will rotate around this axis in a random manner. On the other hand, if the field E always points in the same direction, the wave is polarized in a rectilinear manner. If E describes a circle, the polarization is circular. Finally, if it rotates about its axis and sees its intensity vary, the polarization is elliptical. In the case of ASKAP J173608.2-321635, the polarization is rectilinear and circular … Several phenomena may be the cause of a polarization of light. In astronomy, the most common is the scattering of light through against clouds of dust or gas. Or the presence of a strong magnetic field. The sources of polarization are therefore numerous, it is impossible to know whether the light is polarized at the level of the star, or at the intersection of interstellar matter between the source and us.

A new category of stars?

The polarization study therefore hardly informs the researchers about the nature of the source. The team of astronomers has therefore reviewed various candidates who more or less meet the extraordinary characteristics of ASKAP J173608.2-321635. In fact, objects with variable brightness are legion: pulsars, supernovae, gamma-ray bursts, variable stars … “At first we thought it might be a pulsar – a dead, very dense, spinning star – or some type of star experiencing huge solar flares. But the signals from this new source do not match what we expect from these types of celestial bodies. “, notes Zinteng Wang. For lack of something better, ASKAP J173608.2-321635 has therefore been classified among those galactic center radio transients (GCRT), or “Galactic Center Transient Signals”. Three objects of this type have been discovered since 2002, without their character being identified. Is ASKAP J173608.2-321635 family? This would prove that a still unidentified but significant category of celestial bodies actually populate the center of the galaxy. But there again, the strange signal confuses the tracks. “Although ASKAP J173608.2-321635 shares some properties with GCRTs, there are also differences. And since we still do not really understand these sources, it adds to the mystery … “, summarizes David Kaplan, of the University of Wisconsin-Milwaukee.

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