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Russian astronomers from the Sternberg Astronomical Institute (SAI) have analyzed long-term observational data of a peculiar microquasar designated SS 433. Results of the new study, May 13 in the Â鶹ÒùÔºics-Uspekhi journal, deliver important insights into the nature of this object.

Quasars, or quasi-stellar objects (QSOs), are active galactic nuclei (AGN) in the centers of active galaxies, showcasing very high luminosity. Microquasars are their smaller versions—binary systems in which a compact object (such as a black hole or neutron star) draws matter from its companion star. Both quasars and microquasars are assumed to be powered by spinning supermassive black holes (SMBHs).

Discovered in 1978, SS 433 is the first identified microquasar. It is a massive X-ray binary system at an advanced stage of evolution, consisting of a stellar-mass black hole accreting matter from an A-type companion star. The orbital period of the system was measured to be approximately 13.08 days, and its distance to Earth is estimated to be some 18,000 light years.

SS 433 has been monitored for more than 45 years in almost all ranges of the electromagnetic spectrum: optical, infrared, radio, X-ray, and gamma-ray. Since 1994, the observations of SS 433 have also been carried out at SAI's Crimean Astronomical Station (CAS) and Caucasus Mountain Observatory (CMO). Now, a team of astronomers led by SAI's Anatol Cherepashchuk presents the results of this observational campaign.

The observations found a peculiar evolutionary increase of the orbital period of SS 433 at a rate of approximately 0.000114 milliseconds per second. The astronomers noted that both the orbital period and orbital separation in this system increase simultaneously. Given that the distance between the components of SS 433 increases with time, it evolves as a semi-detached binary, avoiding the common envelope phase.

Based on the observational data, the mass ratio of SS 433 is at least 0.8. Taking into account that the mass of the optical star in the system is estimated to be about 10 solar masses, the mass of the black hole should be at a level of 8 solar masses. This is close to the average mass of black holes in X-ray binaries.

The observations also detected the ellipticity of SS 433's orbit. This, according to the researchers, points to a slaved accretion disk tracking the precession of the rotation axis of the optical star.

"In the slaved disk model, the propagation of relativistic jets tracks the precessing rotation axis of the donor star. However, precessional changes in the direction of this axis are reflected in the direction of jets not instantaneously, but after some time, which is necessary for the matter with a new angular momentum vector to reach the formation site of relativistic jets, i.e., after about viscous time of the accretion disk," the scientists explained.

Summing up the results, the authors of the paper concluded that SS 433 is physically close to a new class of unique objects, the ultraluminous X-ray (ULX) sources discovered in recent years in many galaxies.

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