Relativistic Iron Line Emission from the Neutron Star Low-mass X-ray Binary 4U 1636-536

TL;DR

This paper analyzes X-ray data from the neutron star binary 4U 1636-536, revealing a broad, relativistically broadened iron emission line indicating high disk inclination and multiple ionization states.

Contribution

It provides the first detailed analysis of relativistic iron line emission in 4U 1636-536, highlighting the line's broadness and ionization complexity.

Findings

Detection of a broad, asymmetric iron emission line

Line profile consistent with relativistic broadening from the inner accretion disk

Evidence of multiple iron ionization states contributing to the line

Abstract

We present an analysis of XMM-Newton and RXTE data from three observations of the neutron star LMXB 4U 1636-536. The X-ray spectra show clear evidence of a broad, asymmetric iron emission line extending over the energy range 4-9 keV. The line profile is consistent with relativistically broadened Fe K-alpha emission from the inner accretion disk. The Fe K-alpha line in 4U 1636-536 is considerably broader than the asymmetric iron lines recently found in other neutron star LMXBs, which indicates a high disk inclination. We find evidence that the broad iron line feature is a combination of several K-alpha lines from iron in different ionization states.