Trushkin S.A., Bursov N.N., Shevchenko A.V., Tsybulev P.G., Nizhelsky N.A. (SAO RAS)

    In the multi-frequency (2-30 GHz) studies of an X-ray binary star with a black hole (BH), the prototype microquasar GRS 1915+105 with the RATAN-600 radio telescope, an extremely bright burst after the source transition to an extremely low and hard X-ray state was detected. The brightness of the burst reached 5.5 Jy at the frequency 2.3 GHz (that is, the radio luminosity increased up to 10³⁵ erg/s) which is higher than the values received over 30 years from the date of discovery of the object. Then, for the first time, we traced the evolution of the flare at two frequencies 4.7 and 8.2 GHz in the new multi-azimuth (MA) observation mode of the telescope, when measurements were carried out every 8.6 minutes 31 times daily. The radio flare was accompanied by a gradual increase of the soft X-ray flux (MAXI, 2-20 keV). In the optically thin radio spectrum of the synchrotron electron radiation, the spectral index smoothly changed from +0.15 to −0.95 over the first eight days. The detected flux variations at times from 30 to 300 minutes correspond to the physical size of the radiation region from 4 to 40 astronomical units probably inside the jet emission. In eight MA measurements, for the first time quasi-periodic oscillations with an amplitude of up to 10% of the average flux were discovered on time intervals from 30 to 100 minutes. This giant flare in GRS 1915+105 could be associated with formation of relativistic jet emission during the decreasing accretion, when, as has recently become known, a strong disk wind simultaneously obscures the main X-ray flux and reduces the rate of accretion. The obvious similarity of the burst event in GRS 1915+105 with giant bursts in microquasars Cyg X-3, SS433, and Swift J1727.8-1613 implies similar physical processes of accretion onto black holes of stellar masses.

    Over the 30-year history of studying the GRS 1915+105, for the first time we have encountered an extremely unusual burst phenomenon in the X-ray binary system, that sheds light on the physics of accretion onto black holes of stellar masses and the formation of powerful collimated jets.

Fig. 1. Light curves at five frequencies from measurements with the Northern sector of RATAN-600 (the crosses) combined with the multi-azimuth measurements at the Southern sector with the Flat reflector (the dots).

Fig. 2. Evolution of the radio spectrum during the first ten days of the burst. The spectrum gradually became steeper, or in the X-ray terms, softer.

The work was carried out within the framework of the state assignment of the Special Astrophysical Observatory of the Russian Academy of Sciences.

Published:
ATel #15974, #16008; #16168, HEA-2023.