Study of the reflection spectrum of the LMXB 4U 1702-429

¹ Dipartimento di Fisica e Chimica, Università di Palermo, via Archirafi 36, 90123 Palermo, Italy
e-mail: rosario.iaria@unipa.it
² Istituto Nazionale di Astrofisica, IASF Palermo, via U. La Malfa 153, 90146 Palermo, Italy
³ INAF-Istituto di Astrofisica Spaziale e Fisica Cosmica – Milano, via E. Bassini 15, 20133 Milano, Italy
⁴ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, KM 0.7, 09042 Monserrato, Italy
⁵ Institute of Space Sciences (ICE, CSIC-IEEC), Carrer de Can Magrans, S/N, 08193 Barcelona, Spain
⁶ INAF, Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone (Roma), Italy

Received: 28 January 2016
Accepted: 6 September 2016

Abstract

Context. The source 4U 1702-429 (Ara X-1) is a low-mass X-ray binary system hosting a neutron star. Albeit the source is quite bright (~10³⁷ erg s^-1) its broadband spectrum has never been studied. Neither dips nor eclipses have been observed in the light curve suggesting that its inclination angle is smaller than 60°.

Aims. We analysed the broadband spectrum of 4U 1702-429 in the 0.3–60 keV energy range, using XMM-Newton and INTEGRAL data, to constrain its Compton reflection component if it is present.

Methods. After excluding the three time intervals in which three type-I X-ray bursts occurred, we fitted the joint XMM-Newton and INTEGRAL spectra obtained from simultaneous observations.

Results. A broad emission line at 6.7 keV and two absorption edges at 0.87 and 8.82 keV were detected. We found that a self-consistent reflection model fits the 0.3–60 keV spectrum well. The broadband continuum is composed of an emission component originating from the inner region of the accretion disc, a Comptonised direct emission coming from a corona with an electron temperature of 2.63 ± 0.06 keV and an optical depth τ = 13.6 ± 0.2, and, finally, a reflection component. The best-fit indicates that the broad emission line and the absorption edge at 8.82 keV, both associated with the presence of Fe xxv ions, are produced by reflection in the region above the disc with a ionisation parameter of Log(ξ) ≃ 2.7. We have inferred that the inner radius, where the broad emission line originates, is 64⁺⁵²_-15 km, and the inner radius of the accretion disc is 39⁺⁶_-8 km. The emissivity of the reflection component and the inclination angle of the system are r^{-3.2+0.5_-5.1}and 44⁺³³_-6degrees, respectively. The absorption edge at 0.87 keV is associated to the presence of O viii ions and it is produced in a region above the disc with Log(ξ) ≃ 1.9.