Infrared and X-ray Variability of Cyg X-3

Abstract

CYGNUS X-3 is a candidate for the radio source which in September 1972 experienced a series of exceptional radio outbursts¹. The X-ray emission is chiefly distinguishable as showing periodic intensity variations with a period of 4.8 h (ref. 2); if these variations are caused by an eclipse, the orbital period is the shortest observed up to this time from an X-ray source. The definite identification of the radio source with the X-ray source has remained unconfirmed up to now because of the poor positional accuracy of the X-ray source; the error box in the 3U catalogue is about 1 x 2 arc min (ref. 3). Although there is no visual candidate for Cyg X-3 to a limit of ∼2 X 10⁻³² W m⁻² Hz⁻¹ (V∼23 mag) (ref. 4), at infrared wavelengths Becklin et al.⁵ found a source coincident to ±2 arc s with the radio position. At 2.2 μm, the flux density is approximately 2 X 10⁻²⁸ W m⁻² Hz⁻¹, whereas the 1.5 to 2.2 µm colour of the infrared source is consistent with a Rayleigh-Jeans spectrum which is reddened by 1.5 mag at 2.2 µm by the interstellar medium. There is no reason to suppose that the source is intrinsically more luminous in the infrared than at visible wavelengths.