Discovery of Redshifted He-like Iron Absorption Line from Luminous Accreting Neutron Star SMC X-1

Abstract

We have analyzed X-ray data of SMC X-1 obtained with Suzaku and discovered the combination of an absorption line and an underlying broadened emission line centered at 6.4 keV in an observation performed on 2012 May 19. This absorption line is centered at {6.61}_-0.03^+0.02 {keV} with an absorption strength of {5.9}_-1.4^+1.7, naturally interpreted as an He _α resonance line of Fe at 6.7 keV that has a redshift of {4000}_-1300⁺¹⁴⁰⁰ {km} {{{s}}}^-1. Although Suzaku observed this system 10 times during 11 months in 2011-2012, the absorption feature has been seen only in a single observation when the neutron star (NS) was in a rising phase of the super-orbital modulation, which can be regarded as an egress from occultation by an extended accretion disk. We therefore attribute the line to a low density, highly ionized absorber in an accretion disk corona arising from the disk illuminated by the NS’s intense X-rays. This interpretation also agrees with a discussion on the photoionization degree and the line depth.