Isolated Millimeter Flares of Cyg X-3

TL;DR

This study reports on isolated millimeter flares of Cyg X-3 during its quiescent phase, analyzing their properties and spectra to understand the physical conditions of the emitting plasma.

Contribution

First detailed observation of isolated millimeter flares in Cyg X-3, providing insights into their temporal and spectral evolution and plasma characteristics.

Findings

Flares have short durations of 1-2 hours and small amplitudes.

The spectrum during flares is flat or inverted, then steepens over time.

The plasma exhibits properties consistent with adiabatic expansion.

Abstract

Cygnus X-3 (Cyg X-3) is a well-known microquasar with relativistic jets. Cyg X-3 is especially famous for its giant radio outbursts, which have been observed once every few years since their first discovery. Each giant outburst presumably consists of a series of short-duration flares. The physical parameters of the flares in the giant outbursts are difficult to derive because the successive flares overlap. Here, we report isolated flares in the quiescent phase of Cyg X-3, as observed at 23, 43, and 86 GHz with the 45-m radio telescope at Nobeyama Radio Observatory. The observed flares have small amplitude (0.5--2 Jy) and short duration (1--2 h). The millimeter fluxes rapidly increase and then exponentially decay. The lifetime of the decay is shorter at higher frequency. The radio spectrum of Cyg X-3 during the flares is flat or inverted around the peak flux density. After that, the spectrum gradually becomes steeper. The observed characteristics are consistent with those of adiabatic expanding plasma. The brightness temperature of the plasma at the peak is estimated to be $T_B\gtrsim 1 \times 10^{11}$ K. The magnetic field in the plasma is calculated to be $0.2 \lesssim H \lesssim 30$ G.