A relativistic iron emission line from the neutron star low-mass X-ray binary GX 3+1

TL;DR

This study detects a relativistically broadened iron emission line in the X-ray spectrum of the neutron star binary GX 3+1, revealing details about the inner accretion disk's structure and inclination.

Contribution

First detection of a relativistic iron Kα line in GX 3+1, providing insights into the accretion disk's inner radius and inclination in this neutron star system.

Findings

Relativistically smeared iron line at 6-7 keV detected.

Inner disk radius estimated at approximately 25 gravitational radii.

Disk inclination constrained between 35° and 44°.

Abstract

We present the results of a spectroscopic study of the Fe K{\alpha} emission of the persistent neutron-star atoll low-mass X-ray binary and type I X-ray burster GX 3+1 with the EPIC-PN on board XMM-Newton. The source shows a flux modulation over several years and we observed it during its fainter phase, which corresponds to an X-ray luminosity of Lx~10^37 ergs/s. When fitted with a two-component model, the X-ray spectrum shows broad residuals at \sim6-7 keV that can be ascribed to an iron K{\alpha} fluorescence line. In addition, lower energy features are observed at \sim3.3 keV, \sim3.9 keV and might originate from Ar XVIII and Ca XIX. The broad iron line feature is well fitted with a relativistically smeared profile. This result is robust against possible systematics caused by instrumental pile-up effects. Assuming that the line is produced by reflection from the inner accretion disk, we infer an inner disk radius of \sim25 Rg and a disk inclination of 35{\deg} < i < 44{\deg}.