The hypersoft state of Cygnus X–3

Abstract

Context. Cygnus X–3 is a unique microquasar in the Galaxy hosting a Wolf-Rayet companion orbiting a compact object that most likely is a low-mass black hole. The unique source properties are likely due to the interaction of the compact object with the heavy stellar wind of the companion. Aim. In this paper, we concentrate on a very specific period of time prior to the massive outbursts observed from the source. During this period, Cygnus X–3 is in a so-called hypersoft state, in which the radio and hard X-ray fluxes are found to be at their lowest values (or non-detected), the soft X-ray flux is at its highest values, and sporadic γ-ray emission is observed. We use multiwavelength observations to study the nature of the hypersoft state. Methods. We observed Cygnus X–3 during the hypersoft state with Swift and NuSTAR in X-rays and SMA, AMI-LA, and RATAN-600 in the radio. We also considered X-ray monitoring data from MAXI and γ-ray monitoring data from AGILE and Fermi. Results. We found that the spectra and timing properties of the multiwavelength observations can be explained by a scenario in which the jet production is turned off or highly diminished in the hypersoft state and the missing jet pressure allows the wind to refill the region close to the black hole. The results provide proof of actual jet quenching in soft states of X-ray binaries.

The obscured X-ray binaries V404 Cyg, Cyg X–3, V4641 Sgr, and GRS 1915+105

“…The interstellar absorption for Cyg X-3 is relatively high as a result of its location in the plane of the Galaxy, and likely because it is located behind two spiral arms and the Cygnus X star-forming region (McCollough et al 2016). We fixed the lower limit of the hydrogen column to 3.5 × 10 22 atoms cm −2 , which is approximately the value found in studies where instruments with softer X-ray response were used (Koljonen et al 2018;Kallman et al 2019). Because the reprocessing matter in the model is neutral, we added a smeared edge in model B to account for the absorption of highly ionized iron.…”

“…Because the companion is a Wolf-Rayet star exhibiting a heavy stellar wind that extends much farther than the binary orbit, Cyg X-3 is constantly embedded in a highdensity environment that affects its X-ray spectra in all accretion states (Szostek & Zdziarski 2008;Zdziarski et al 2010;Koljonen et al 2018). This material is optically thick in X-rays as a result of the absorption of metals and Compton scattering, causing iron absorption edges (Koljonen et al 2018) and Compton downscattering (Zdziarski et al 2010) and/or Compton scattering out of the line of sight to the intrinsic X-ray continuum. Highly ionized iron lines are resolved with Chandra and reveal a distinct component of gas at much higher ionization, in addition to a component from fluorescence by neutral or near-neutral material.…”

Strong low-frequency radio flaring from Cygnus X-3 observed with LOFAR

“…For example, Gregory & Seaquist (1974) considered four explanations for spectral turnover as part of their modelling study of the 1972 outburst from Cygnus X-3: (i) synchrotron self-absorption, (ii) freefree absorption from thermal plasma mixed with the synchrotronemitting plasma, (iii) free-free absorption from a foreground screen, and (iv) the Razin-Tsytovich effect (also see e.g. Miller-Jones et al 2004 andKoljonen et al 2018 for similar investigations of other outbursts). The relevant models are then as follows:…”

On the Apparent Absence of Wolf–Rayet+Neutron Star Systems: The Curious Case of WR124

“…Only one of them, Cyg X-3, is in the Galaxy, (van Kerkwijk et al 1992). Being X-ray bright (L X ≈10 38 erg s −1 ), it most likely harbors a low-mass BH (e.g., Koljonen et al 2018).…”