On the Non-relativistic Origin of Red-skewed Iron Lines in CV, Neutron Star and Black Hole Sources

TL;DR

This study shows that red-skewed iron lines in CVs, neutron stars, and black holes are likely caused by outflowing winds rather than relativistic effects, challenging previous interpretations of these spectral features.

Contribution

The paper demonstrates that wind outflow models can explain red-skewed iron lines across different compact sources, questioning the relativistic origin hypothesis.

Findings

Red-skewed iron lines are observed in CVs, NSs, and BHs.

Wind outflow models fit the asymmetric line profiles well.

Relativistic effects are unlikely the cause of line skewness.

Abstract

We perform the analysis of the iron K_alpha lines detected in three sources representing of three types of accreting compact sources: cataclysmic variable (CV) GK Per, neutron star (NS) Serpens X-1 and black hole (BH) GX 339-4. We find, using data from Epic-PN Camera on-board XMM-Newton observatory,that the iron K_alpha emission line in GK Per has a noticeable red-skewed profile. We compare the GK Per asymmetric line with the red-skewed lines observed by XMM-Newton in Serpens X-1 and GX 339-4. The observation of the K_alpha emission with red-skewed features in CV GK Per cannot be related to the redshift effects of General Relativity (GR). Therefore, if the mechanism of the K_alpha-line formation is the same in CVs, NSs and BHs then it is evident that the GR effects would be ruled out as a cause of red skewness of K_alpha line. The line reprocessing in an outflowing wind has been recently suggested an alternative model for a broad red-shifted iron line formation. In the framework of the outflow scenario the red-skewed iron line is formed in the strong extended wind due to its illumination by the radiation emanating from the innermost part of the accreting material. In this Paper we demonstrate that the asymmetric shapes of the lines detected from these CV, NS and BH sources are well described with the wind (outflow) model. While this fact is hard to reconcile with the relativistic models, it is consistent with the outflowing gas washing out high frequency modulations of the radiation presumably originated in the innermost part of the source.