Cygnus X-3 revealed as a Galactic ultraluminous X-ray source by IXPE

Radio and sub-mm light curves of Cyg X-3. The light curves of the source around the dates of Main (panels A-C) and ToO (panels D and E) observations, as obtained with various telescopes. IXPE dates are marked with blue stripes. Note high intraday variations of the radio flux caused by the orbital variability. Data are given as the mean values with error bars corresponding to their variance.

Veledina Alexandra; Muleri Fabio; Nizhelsky Nikolaj; Shevchenko Anton; Trushkin Sergei; et al.

The accretion of matter by compact objects can be inhibited by radiation pressure if the luminosity exceeds a critical value known as the Eddington limit. The discovery of ultraluminous X-ray sources has shown that accretion can proceed even when the apparent luminosity considerably exceeds this limit. A high apparent luminosity might be produced due to the geometric beaming of radiation by an outflow. The outflow half-opening angle, which determines the amplification due to beaming, has never been robustly constrained. Using the Imaging X-ray Polarimetry Explorer, we measured the X-ray polarization in the Galactic X-ray binary Cygnus X-3 (Cyg X-3). We found high, >20%, nearly energy-independent linear polarization orthogonal to the direction of the radio ejections. These properties unambiguously indicate the presence of a collimating outflow from the X-ray binary Cyg X-3 and constrain its half-opening angle to ≲15°. Thus, the source can be used as a laboratory for studying the supercritical accretion regime. This finding underscores the importance of X-ray polarimetry in advancing our understanding of accreting sources.