On June 21, the Nature Astronomy journal published a paper by Alexandra Veledina with a large number of co-authors including employees of the Institute of Space Research of the Russian Academy of Sciences and radio astronomers of the Special Astrophysical Observatory of the Russian Academy of Sciences. The paper "Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE" talks about the famous microquasar Cygnus X-3, whose giant flares have been studied for many years in observations with the RATAN-600 radio telescope.

The originality and, to some extent, the polemic of the research with the new European X-ray satellite IXPE (Imaging X-ray Polarimetry Explorer) lies in the fact that measurements in 2019 have shown that if the object was oriented along the line of sight, then in terms of a key property - the X-ray luminosity - it should belong to the relatively recently identified in external galaxies group of ultraluminous X-ray sources (the so-called ULX objects), the nature of which still causes much debate. Let us only note that these can be either active neutron stars or accreting black holes as relativistic components of close binary star systems. It is also worth noting that Prof. S.N. Fabrika and his colleagues have been successfully researching ULX objects for many years in our observatory.

In the abstract of the paper, the authors noted that a major discovery was made based on the original X-ray polarimetry, which adds much to our understanding of accreting sources.

It is known that radiation pressure can prevent compact objects from accreting matter if their luminosity exceeds a critical value known as the Eddington limit. The discovery of ultraluminous X-ray sources in space has shown that accretion can continue even when apparent luminosity is well above this limit. Then high apparent brightness can be achieved due to geometric redistribution of radiation owing to a strong outflow.

With the new IXPE instrument, high (>20%) linear polarization of X-ray emission was measured in the binary star Cygnus X-3 for the first time; the polarization was actually energy-independent and the polarization vector was orthogonal to the direction of the elongated structure of radio emission measured with radio interferometry methods. These properties clearly indicate the presence of a collimated outflow of matter (jet) from the central regions of the microquasar Cyg X-3, and its half-opening angle does not exceed 15 degrees.

It is emphasized that the source can become a space laboratory for studying the regime of supercritical accretion in distant objects. It is worth noting that in March of this year S. A. Trushkin and his colleagues detected a powerful (15 Jy) radio flare from Cyg X-3 that correlated with high-energy gamma-ray emissions measured from the object by the Fermi space telescope, indicating a common source of radiation for such widely separated frequency bands of the electromagnetic spectrum.

Fig. 1. Time changes of the flux and polarization for the IXPE main observation. (A) The total intensity in the 2-8 keV energy range binned in time intervals of 500 s. Parameter PD (B) and Parameter PA (C) are averaged over one orbit according to the binary ephemerides determined by Cherepashchuk et al. The dashed horizontal lines represent the average values. (D) The hardness ratio defined as the ratio of the difference in the IXPE count rates in the 4-8 and 2-4 keV energy bands to their sum over 1000 seconds. Alternating vertical bands indicate different orbits.

Fig. 2. Spectral distribution of the radiation energy over the entire range

Fig. 3. States of the source (blue and orange stars) on the "X-ray flux-Radio flux" diagra

Published:
Veledina A. et al., Nature Astronomy, Advanced Online Publication, 2024 (https://www.nature.com/articles/s41550-024-02294-9)