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

We have analyzed X-ray data of SMC X-1 obtained with $\mathit{\text{Suzaku}}$, and discovered a combination of an absorption line and an underlying broadened emission line centered at 6.4 keV in an observation performed on May 19, 2012. 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${}_{\alpha }$ resonance line of Fe at 6.7~keV that has a redshift of $4000_{-1300}^{+1400}\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}$. Although $\mathit{\text{Suzaku}}$ observed this system ten times during 11 months in 2011–2012, the absorption feature has been seen only in a single observation when the 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.