Correlation between X-ray intensity and radio outbursts of Cygnus X-3

Abstract

The relationship between the X-ray flux (1-20 keV) and the radio flux density (S and X bands) of Cygnus X-3 is investigated during a 4.5 yr interval, spanning 1987 March-1991 September. This period included seven strong radio outbursts with flux densities greater than 5 Jy at S band. All radio flares with maximum flux density greater than 0.5 Jy, including the seven strong radio outbursts, occurred when the 1-6 keV X-ray flux, averaged within 5 days before and after the peak of the radio flare, was greater than 0.37 counts/s/sq cm, which was corrected for the 4.8 hr intensity modulation. These X-ray fluxes correspond to a high state with a luminosity of roughly 10<SUP>38</SUP> ergs/s at 8.5 kpc, implying that a very high accretion rate induces the radio flares. However, no simultaneous increase of X-rays during the radio outburst was detected. This fact suggests that the direct cause of the radio outbursts is some other process, during the high accretion rate, such as a gradual storage and a sudden release of energy by a twisting magnetic field in the accretion process. Population distributions of the mean X-ray flux (1-6 keV) and the X-ray softness ratio (1-6 keV/6-20 keV) was also studied, and they indicated the existence of two distinct states, i.e., high (soft) and low (hard) states. This fact suggests a bimodal behavior of the accretion disk.