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

S. Piraino^1,2, A. Santangelo², P. Kaaret³, B. Mück², A. D’Aì⁴, T. Di Salvo⁴, R. Iaria⁴, N. Robba⁴, L. Burderi⁵ and E. Egron⁵

¹ INAF-IASF di Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
e-mail: santina.piraino@ifc.inaf.it
² Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Sand 1, 72076 Tübingen, Germany
³ Department of Physics and Astronomy University of Iowa, Iowa City, IA 52242, USA
⁴ Dipartimento di Fisica, Università degli Studi di Palermo, via Archirafi, 90100 Palermo, Italy
⁵ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, Monserrato, Italy

Received: 26 March 2012
Accepted: 8 May 2012

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°.