Discovery of Red-Skewed K_alpha iron line in Cyg X-2 with Suzaku

TL;DR

This paper reports the detection of a red-skewed iron K_alpha emission line in the neutron star binary Cyg X-2 using Suzaku, suggesting such features are common across different accreting compact objects.

Contribution

It presents the first observation of a red-skewed iron line in Cyg X-2, expanding the understanding of accretion phenomena in neutron star systems.

Findings

Red-skewed iron line detected in Cyg X-2

Both relativistic reflection and wind models fit the line profile

Wind model has advantages over disk reflection model

Abstract

We report on the Suzaku observation of neutron star low-mass X-ray binary Cygnus X-2 which reveals a presence of the iron K_alpha emission line. The line profile shows a significant red wing. This discovery increases the number of neutron star sources where red-skewed iron lines were observed and strongly suggests that this phenomenon is common not only in black holes but also in other types of accreting compact objects. We examine the line profile in terms of models which attribute its production to the relativistic effects due to reflection of X-ray radiation from a cold accretion disk and also as a result of the line formation in the extended wind/outflow configuration. Both models are able to adequately represent the observed line profile. We consider the results of line modeling in the context of subsecond variability. While we were unable to conclusively disqualify one of the models, we find that the wind paradigm has several advantages over the relativistic disk reflection model.