Spectral Inversion of 43 to 22 GHz During Small Flares in a Hard State of Cyg X-3 in February 2016

• Published in: Journal of the Korean Physical Society Vol. 77; no. 12; pp. 1274 - 1278
• Main Authors: Kim, Soon-Wook, Kim, Jeong Sook
• Format: Journal Article
• Language: English
• Published: Seoul The Korean Physical Society 01.12.2020; 한국물리학회
• Subjects: Mathematical and Computational Physics; Particle and Nuclear Physics; Physics; Physics and Astronomy; Theoretical; 물리학; X-ray binaries; Radio astronomy; Jets: galactic; microquasars; Stars: individual (Cygnus X-3)
• ISSN: 0374-4884; 1976-8524
• DOI: 10.3938/jkps.77.1274

We present simultaneous 22 and 43 GHz observations of the microquasar Cyg X-3 during a series of small-scale flaring activity in a hard state. In the end of January 2016, Cyg X-3 was in a state transition from a soft to hard state after a minimum hard X-ray flux at 15–50 keV. We have observed Cyg X-3 in 3–6 February immediately after Cyg X-3 went into the hard state. We found a series of episodic, low-flux flaring activity of < 500 mJy in the hard state at 22 and 43 GHz, simultaneously observed for the first time. The spectral slope was negative on 3 February, while there were the events of spectral inversion on 5 and 6 February, typical for a rise phase, indicating the optically thick emission, with an alternative possibility of a flat spectrum on 5 February. The previous observations of the small flares with the flux similar to our observation lasted < aday. The brightness temperature argument requires the time scale of such flares should be much less than a day for the flaring activity and associated jets to be non-thermal. Therefore, the flaring activity observed in 5 and 6 February would be more likely to be two separate flares of < 1 day in the rise, rather than a longer, single event.