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

Abstract

We present the results of a spectroscopic study of the Fe Kα 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 L_X ~ 10³⁷ erg s^-1. When fitted with a two-component model, the X-ray spectrum shows broad residuals at ~6-7 keV that can be ascribed to an iron K_α fluorescence line. In addition, lower energy features are observed at ~3.3 keV, ~3.9 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 ~25R_g and a disk inclination of 35° < i < 44°.