2015A&A...577A..63I

The source X1822-371 is a low-mass X-ray binary system (LMXB) viewed at a high inclination angle. It hosts a neutron star with a spin period of ∼0.59s, and recently, the spin period derivative was estimated to be (-2.43±0.05)x10^–12s/s. Our aim is to address the origin of the large residuals below 0.8keV previously observed in the XMM/EPIC-pn spectrum of X1822-371. We analyse all available X-ray observations of X1822-371 made with XMM-Newton, Chandra, Suzaku and INTEGRAL satellites. The observations were not simultaneous. The Suzaku and INTEGRAL broad band energy coverage allows us to constrain the spectral shape of the continuum emission well. We use the model already proposed for this source, consisting of a Comptonised component absorbed by interstellar matter and partially absorbed by local neutral matter, and we added a Gaussian feature in absorption at ∼0.7keV. This addition significantly improves the fit and flattens the residuals between 0.6 and 0.8keV. We interpret the Gaussian feature in absorption as a cyclotron resonant scattering feature (CRSF) produced close to the neutron star surface and derive the magnetic field strength at the surface of the neutron star, (8.8±0.3)x10¹⁰G for a radius of 10km. We derive the pulse period in the EPIC-pn data to be 0.5928850(6) s and estimate that the spin period derivative of X1822-371 is (-2.55±0.03)x10^–12s/s using all available pulse period measurements. Assuming that the intrinsic luminosity of X1822-371 is at the Eddington limit and using the values of spin period and spin period derivative of the source, we constrain the neutron star and companion star masses. We find the neutron star and the companion star masses to be 1.69±0.13M_☉ and 0.46±0.02M_☉, respectively, for a neutron star radius of 10km. In a self-consistent scenario in which X1822-371 is spinning-up and accretes at the Eddington limit, we estimate that the magnetic field of the neutron star is (8.8±0.3)x10¹⁰G for a neutron star radius of 10km. If our interpretation is correct, the Gaussian absorption feature near 0.7keV is the very first detection of a CRSF below 1keV in a LMXB.