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

Abstract

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.