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Detection of Five Giant Flares from Microquasar Cyg X-3 in 2024 |
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The physical processes occurring in the vicinity of stellar-mass black holes (BH) are studied in X-ray binary systems (XRBS) including BHs and ordinary main-sequence stars. In conditions of intense matter outflow from these stars, either due to a powerful stellar wind, or due to the star filling its Roche lobe, a hot accretion disk is formed around the BH, which intensively emitted the X-rays. The main part of the matter falling on the BH goes beyond its event horizon, but the smaller part is ejected from the system with relativistic speed, resulting in the formation of collimated outflows of the matter on both sides from the disk, they are massive ejections or jets. Thermal electrons are accelerated to ultra-relativistic speeds in them (99% of the speed of light), and these electrons emit bright synchrotron radiation (SR) in the radio range from millimeter to meter waves. Moreover, this SR is highly variable due to various non-stationary phenomena associated with the conditions of accretion on the BH. XRBS with jets are called microquasars by analogy with powerful extragalactic sources with jets, quasars. One of the most amazing microquasars is the Cyg X-3 system consisting of a BH and a massive Wolf-Rayet star. Rarely and irregularly, this microquasar becomes the brightest radio source in the sky with the flux exceeding that of the most distant quasars. Powerful flare radio emission from jet emissions is the main indicator of Cyg X-3 activity.
In 2024, during multi-frequency (1-30 GHz) monitoring with the RATAN-600 radio telescope, we detected five giant (with a flux of above 10 Jy) flares from the XRBS Cyg X-3. Such activity of the object is quite rare, if not unique. For example, no any flares were detected in 2021-2023. The onset of all five flares was clearly associated with the detection of significant gamma-ray radiation at 0.1-300 GeV (the data from the Fermi space observatory). On the other hand, all the flares occurred during transition of the system from the hyper-soft X-ray state to the hard state, i.e., when the flux of hard X-ray radiation (15-50 keV) strongly increased from almost zero, and a flux of soft radiation of 4-10 keV (the data from the Japanese MAXI X-ray monitor on the board the ISS) decreased. We explain this behavior in the framework of a physical picture with a changing and (as was recently found by Veledina et al., 2024) supercritical rate of matter accretion onto the black hole, variations in the state of the corona and the accretion disk, and changes in the collimation process of jet emissions. Although, the radio flares evolved on different time scales from 10 to 60 days, they had similar properties: at first, the flux grew linearly with time, and the radio emission was optically thick at frequencies below 2 GHz, then after the maximum of the flare, there was an exponential attenuation with a gradual “softening” of the spectrum (the spectral index varied from -0.1 to -0.9 at high frequencies). In Fig. 1, from the Gamma-ray and radio light curves of Cyg X-3, it can be clearly seen that the gamma radiation correlates with the onset of radio flares, and the delay is less than one day. Figure 2 shows the spectra of the flare onset in March-April with a characteristic dip at frequencies below 3 GHz, probably due to either synchrotron self-absorption or thermal electrons in the jets. All flares were studied in a new multi-azimuthal mode at the “Southern sector with a flat reflector” antenna system at RATAN-600 of SAO RAS at two frequencies of 4.7 and 8.2 GHz. For the first time, it was possible to detect and study the spectral and temporal variability of Cyg X-3 at times from 9 to 300 minutes.
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