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

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 km, and the inner radius of the accretion disc is km. The emissivity of the reflection component and the inclination angle of the system are and degrees, 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.