Spectral analysis of the dipping LMXB system XB 1916-053

¹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, Via Archirafi 36, 90123 Palermo, Italy
e-mail: angelofrancesco.gambino@unipa.it
² Università degli Studi di Cagliari, Dipartimento di Fisica, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
³ INAF-Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, Italy
⁴ INAF/IASF Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy
⁵ IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France

Received: 8 March 2018
Accepted: 3 April 2019

Abstract

Context. XB 1916-053 is a low mass X-ray binary system (LMXB) hosting a neutron star (NS) and showing periodic dips. The spectrum of the persistent emission was modeled with a blackbody component having a temperature between 1.31 and 1.67 keV and with a Comptonization component with an electron temperature of 9.4 keV and a photon index Γ between 2.5 and 2.9. The presence of absorption features associated with highly ionized elements suggested the presence of partially ionized plasma in the system.

Aims. In this work we performed a study of the spectrum of XB 1916-053, which aims to shed light on the nature of the seed photons that contribute to the Comptonization component.

Methods. We analyzed three Suzaku observations of XB 1916-053: the first was performed in November 2006 and the others were carried out in October 2014. We extracted the persistent spectra from each observation and combined the spectra of the most recent observations, obtaining a single spectrum with a higher statistic. We also extracted and combined the spectra of the dips observed during the same observations.

Results. On the basis of the available data statistics, we infer that the scenario in which the corona Comptonizes photons emitted both by the innermost region of the accretion disk and the NS surface is not statistically relevant with respect to the case in which only photons emitted by the NS surface are Comptonized. We find that the source is in a soft spectral state in all the analyzed observations. We detect the Kα absorption lines of Fe XXV and Fe XXVI, which have already been reported in literature, and for the first time the Kβ absorption lines of the same ions. We also detect an edge at 0.876 keV, which is consistent with a O VIII K absorption edge. The dip spectrum is well described by a model that considers material in different ionization states covering the persistent spectrum and absorbing part of the rear radiation. From this model we rescale the distance of the absorber to a distance that is lower than 1 × 10¹⁰ cm.