The Hardness-Intensity Diagram of Cygnus X-3: Revisiting the Radio/X-Ray States

TL;DR

This study constructs a hardness-intensity diagram for Cygnus X-3, revealing six distinct X-ray states, including a newly identified hypersoft state, and links these states to radio and gamma-ray activity to better understand its complex behavior.

Contribution

The paper introduces a new classification of six X-ray states for Cygnus X-3, including a novel hypersoft state, based on combined X-ray and radio observations, enhancing understanding of its emission mechanisms.

Findings

Cygnus X-3's HID resembles other black hole binaries but with differences.

Identification of a new hypersoft X-ray state associated with quenched radio emission.

Correlation between the hypersoft state and high-energy gamma-ray flux.

Abstract

Cygnus X-3 is one of the brightest X-ray and radio sources in the Galaxy, and is well known for its erratic behaviour in X-rays as well as in the radio, occasionally producing major radio flares associated with relativistic ejections. However, even after many years of observations in various wavelength bands Cyg X-3 still eludes clear physical understanding. Studying different emission bands simultaneously in microquasars has proved to be a fruitful approach towards understanding these systems, especially by shedding light on the accretion disc/jet connection. We continue this legacy by constructing a hardness-intensity diagram (HID) from archival Rossi X-ray Timing Explorer data and linking simultaneous radio observations to it. We find that surprisingly Cyg X-3 sketches a similar shape in the HID to that seen in other transient black hole X-ray binaries during outburst but with distinct differences. Together with the results of this analysis and previous studies of Cyg X-3 we conclude that the X-ray states can be assigned to six distinct states. This categorization relies heavily on the simultaneous radio observations and we identify one new X-ray state, the hypersoft state, similar to the ultrasoft state, which is associated to the quenched radio state during which there is no or very faint radio emission. Recent observations of GeV flux observed from Cyg X-3 (Tavani et al. 2009; Fermi LAT Collaboration et al. 2009) during a soft X-ray and/or radio quenched state at the onset of a major radio flare hint that a very energetic process is at work during this time, which is also when the hypersoft X-ray state is observed. In addition, Cyg X-3 shows flaring with a wide range of hardness.