Sun and other stars in the Milky Way emit radiation that can be detected by human eyes. Astronomers, however, know many celestial objects that can radiate their energy in the form of X-rays. An international team of scientists, which includes astronomers from the Astronomical Observatory of the University of Warsaw, reported in the “Nature Astronomy” journal the discovery of a new class of X-ray sources.

Astronomers reported the study of ASASSN-16oh, a transient that was detected by the All Sky Automated Survey for Supernovae in December 2016. The object is located in the nearby galaxy called the Small Magellanic Cloud. This galaxy has been also regularly monitored by the OGLE sky survey, led by astronomers from the University of Warsaw. Their observations showed that ASASSN-16oh brightened a few dozen times compared to the pre-outburst level.


Additional observations from the NASA’s Swift Observatory and the Chandra X-ray Observatory helped to verify the finding. Scientists found that the object emitted soft X-ray radiation, indicating the temperature of almost one million degrees.


Supersoft X-ray emission – a very strong level of the weakest X-rays – has long been considered a result of nuclear fusion on the surface of a white dwarf, a small, very dense star. As a white dwarf captures material from a companion star, the material piles up on the surface and becomes hot, and, eventually nuclear fusion takes place, much like in a hydrogen bomb.


But observations of ASASN-16oh show that the emission is coming from a region smaller than the surface of the white dwarf, and there are strong arguments against any kind of explosion having taken place on the white dwarf.


The source of these emissions, then, is thought to be accretion – the process of accumulating matter – not fusion. The scientists believe the system consists of a highly evolved red giant star and a white dwarf with an extremely large disk of emission around it. The rate of inflow of matter through the disk is unstable, and when the material starts flowing more quickly, the brightness of the system shoots upward.


So what this finding shows is that there are two ways by which supersoft emission can be made: nuclear fusion and accretion.


As exciting as this finding is on its own, perhaps the most important part is that it may change how astrophysicists measure the expansion of the universe. Super-soft X-ray sources were thought to be one of the main ways by which white dwarfs grow in mass and eventually explode as Type Ia supernovae.


This finding – that there is a new way to make supersoft sources – will cause the astronomers to re-think their approach to matching up the populations of these objects with the rates of the supernovae.


The paper entitled „Unconventional origin of supersoft X-ray emission from a white dwarf binary” has been published in the “Nature Astronomy” journal.

Scientists from the Astronomical Observatory of the University of Warsaw, Prof. Andrzej Udalski, Prof. Michał Szymański, Prof. Igor Soszyński, Prof. Szymon Kozłowski, Prof. Paweł Pietrukowicz, Dr. Krzysztof Ulaczyk, Przemysław Mróz and Radosław Poleski, are among the authors of the article.