Linking jet emission and X-ray properties in the peculiar neutron star X-ray binary Circinus X-1

TL;DR

This study analyzes simultaneous X-ray and radio observations of the peculiar neutron star binary Cir X-1, revealing correlations between X-ray spectral states and radio jet activity, and comparing these to black hole systems.

Contribution

It provides new insights into the coupling between X-ray spectral states and radio jet emission in Cir X-1, a peculiar neutron star binary, and compares these behaviors to black hole candidates.

Findings

Radio jets are associated with X-ray spectral transitions.

X-ray power spectra vary with orbital phase, showing Z and atoll source characteristics.

Similar behaviors are observed in black hole systems like GX 339-4.

Abstract

We present the results of simultaneous X-ray and radio observations of the peculiar Z-type neutron star X-ray binary Cir X-1, observed with the Rossi X-ray timing explorer satellite and the Australia Telescope Compact Array in 2000 October and 2002 December. We identify typical Z source behaviour in the power density spectra as well as characteristic Z patterns drawn in an X-ray hardness-intensity diagram. Power spectra typical of bright atoll sources have also been identified at orbital phases after the periastron passage, while orbital phases before the periastron passage are characterized by power spectra that are typical neither of Z nor of atoll sources. We investigate the coupling between the X-ray and the radio properties, focusing on three orbital phases when an enhancement of the radio flux density has been detected, to test the link between the inflow (X-ray) and the outflow (radio jet) to/from the compact object. In two out of three cases we associate the presence of the radio jet to a spectral transition in the X-rays, although the transition does not precede the radio flare, as detected in other Z sources. An analogous behaviour has recently been found in the black hole candidate GX 339-4. In the third case, the radio light curve shows a similar shape to the X-ray light curve. We discuss our results in the context of jet models, considering also black hole candidates.