Evidence of a Broad Relativistic Iron Line from the Neutron Star Low-Mass X-ray Binary Serpens X-1

TL;DR

This paper presents the first strong evidence of a relativistic broad iron line in a neutron star low-mass X-ray binary, indicating the inner accretion disk as its origin and opening new avenues for probing neutron star properties.

Contribution

It provides the first clear detection of a relativistic iron line in a neutron star LMXB, supporting the inner disk origin of such lines and their use in studying neutron star parameters.

Findings

Relativistic iron line detected in Ser X-1

Line shape consistent with accretion disk models

Inner disk origin of the line supported

Abstract

We report on an analysis of XMM-Newton data from the neutron star low mass X-ray binary (LMXB) Serpens X-1 (Ser X-1). Spectral analysis of EPIC PN data indicates that the previously known broad iron K$\alpha$ emission line from this source has a significantly skewed structure with a moderately extended red wing. The asymmetric shape of the line is well described with the laor and diskline models in XSPEC and strongly supports an inner accretion disk origin of the line. To our knowledge this is the first strong evidence of a relativistic line in a neutron star LMXB. This finding suggests that the broad lines seen in other neutron star LMXBs likely originate from the inner disk as well. Detailed study of such lines opens up a new way to probe neutron star parameters and their strong gravitational fields. The red wing of the iron line from Ser X-1 is not as broad as that observed from some black hole systems. This is not unreasonable for a neutron star system, as the accretion disk has to terminate at or before the hard stellar surface. Finally, the inferred source inclination angle in the approximate range 40 degree to 60 degree is consistent with the lack of dips and eclipses from Ser X-1.