Evolution of the disc atmosphere in the X-ray binary MXB 1659-298, during its 2015-17 outburst

We report on the evolution of the X-ray emission of the accreting neutron star (NS) low-mass X-ray binary (LMXB), MXB 1659-298, during its most recent outburst in 2015-17. We detected 60 absorption lines during the soft state (of which 21 at more than 3σ), that disappeared in the hard state (e.g. the Fe XXV and Fe XXVI lines). The absorbing plasma is at rest, likely part of the accretion disc atmosphere. The bulk of the absorption features can be reproduced by a high column density (log (N_H/cm^-2) ∼ 23.5) of highly ionized (log (ξ/erg cm s^-1) ∼ 3.8) plasma. Its disappearance during the hard state is likely the consequence of a thermal photo-ionization instability. MXB 1659-298's continuum emission can be described by the sum of an absorbed disc blackbody and its Comptonised emission, plus a blackbody component. The observed spectral evolution with state is in line with that typically observed in atoll and stellar mass black hole LMXB. The presence of a relativistic Fe K α disc-line is required during the soft state. We also tentatively detect the Fe XXII doublet, whose ratio suggests an electron density of the absorber of n_e > 10¹³ cm^-3, hence, the absorber is likely located at <7 × 10⁴ r_g from the illuminating source, well inside the Compton and outer disc radii. MXB 1659-298 is the third well monitored atoll LMXB showcasing intense Fe XXV and Fe XXVI absorption during the soft state that disappears during the hard state.