The hardness-intensity diagram of Cygnus X-3: revisiting the radio/X-ray states

Abstract

Cygnus X-3 is one of the brightest X-ray and radio sources in the Galaxy, and\nis well known for its erratic behaviour in X-rays as well as in the radio,\noccasionally producing major radio flares associated with relativistic\nejections. However, even after many years of observations in various wavelength\nbands Cyg X-3 still eludes clear physical understanding. Studying different\nemission bands simultaneously in microquasars has proved to be a fruitful\napproach towards understanding these systems, especially by shedding light on\nthe accretion disc/jet connection. We continue this legacy by constructing a\nhardness-intensity diagram (HID) from archival Rossi X-ray Timing Explorer data\nand linking simultaneous radio observations to it. We find that surprisingly\nCyg X-3 sketches a similar shape in the HID to that seen in other transient\nblack hole X-ray binaries during outburst but with distinct differences.\nTogether with the results of this analysis and previous studies of Cyg X-3 we\nconclude that the X-ray states can be assigned to six distinct states. This\ncategorization relies heavily on the simultaneous radio observations and we\nidentify one new X-ray state, the hypersoft state, similar to the ultrasoft\nstate, which is associated to the quenched radio state during which there is no\nor very faint radio emission. Recent observations of GeV flux observed from Cyg\nX-3 (Tavani et al. 2009; Fermi LAT Collaboration et al. 2009) during a soft\nX-ray and/or radio quenched state at the onset of a major radio flare hint that\na very energetic process is at work during this time, which is also when the\nhypersoft X-ray state is observed. In addition, Cyg X-3 shows flaring with a\nwide range of hardness.\n