Unusual radio variations suggest the existence of a new type of binary star system

In current models, binary star systems generally describe couples involving giant stars with white dwarfs or stars like our Sun. In these systems, the smaller companion emits electromagnetic bursts under the gravitational effect of the giant star. Recently, astrophysicists have detected, in the Milky Way, a binary system involving a giant star and an undetermined companion, the couple not responding to the classical properties of such systems. Given the data collected so far, it could be a whole new class of binary system.

After observing part of the southern hemisphere sky near the constellation Altar for about two months using MeerKAT, a radio telescope based in the Karoo Desert in South Africa, astrophysicists have noticed something strange. The radio emission of an object illuminated by a factor of three, about three weeks of observation. Intrigued, they continued to look at the object and followed the observations of other telescopes. They discovered that the unusual object came from a system of binary stars in the Milky Way.

The results of the study were published in the journal MNRAS. This is the first time that MeerKAT has discovered a "transient source" - an unstable object that is undergoing a significant change in brightness. In view of his name, MKT J170456.2-482100, he was found in the first observed field with the telescope, which means that it is likely to be the tip of an iceberg of transient sources waiting to discovery.

Luminosity variations detected thanks to the collaboration of several telescopes

The researchers began by matching the source to the position of a star, called TYC 8332-2529-1, about 1800 light-years from Earth. Because this star is brilliant, they had predicted that a number of different optical telescopes, detecting visible light rather than radio waves, would have already observed it in the past. And fortunately, this turned out to be the case, allowing them to use these data to learn more about this giant star (2.5 solar masses).

Graphic and images showing radio emission variations observed by astrophysicists. Credits: LN Driessen et al. 2019

Some of the optical telescopes, including ASAS, KELT and ASAS-SN, have provided more than 18 years of star observations. These revealed that the brightness of the star changed over a period of 21 days. Astrophysicists used the SALT telescope to obtain the optical spectra of the star. This can be used to determine the chemical elements present in the star, as well as the presence of a magnetic field.

In addition, they allow scientists to know if a star is moving, because the movement causes the shift of spectral lines (Doppler shift). Spectra revealed that the star had a magnetic field and gravitated around a companion star every 21 days.

A stellar companion of unknown nature

However, only a very weak signature of the companion star is visible. This suggests that the companion must be of less mass than the giant star, with however 1.5 solar masses. So, what could this companion be? A white dwarf may seem likely, as they are often part of binary star systems like this one. However, most have a lower mass than the companion observed.

The radio emission itself could be caused by the magnetic activity of the giant star, similar to a solar flare but much brighter and more energetic. However, these eruptions are usually observed on dwarf stars rather than on giant stars.

Many binary systems involve a giant star and a white dwarf, so the electromagnetic bursts result from the accretion of matter of the second by the first. But the different data collected show that this is not the case of the newly discovered system. Credits: Pearson Ed

Known star systems associating a giant star and a star similar to the Sun could explain the results obtained, the magnetic activity of the giant star giving rise to light flares. However, that does not suit, because nothing indicates in the spectrum that the binary companion is actually a star similar to that of the Sun.

A new class of binary system?

Ben Stappers, the principal investigator of MeerTRAP, one of the teams working on the project, explains that since the properties of the system do not fit easily with our current knowledge about binary stars, it could represent a source class entirely news. It could be a kind of exotic system never seen before and involving a giant star emitting radiation in orbit around a neutron star.

MeerKAT will continue to observe this source every week for the next four years as the ASAS-SN optical telescope continues to observe the giant star. This will inform the dynamics of this system, how the bursts occur, and ultimately help to understand how it was formed.


MKT J170456.2–482100: the first transient discovered by MeerKAT

L N Driessen, I McDonald, D A H Buckley, M Caleb, E J Kotze, S B Potter, K M Rajwade, A Rowlinson, B W Stappers, E Tremou ...

Monthly Notices of the Royal Astronomical Society,
Volume 491, Issue 1, January 2020, Pages 560–575,
Published: 30 October 2019

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