Raman scattering as a diagnostic tool in the massive X-ray binary 4U1700 – 37

Abstract

HD153919, an extreme Of star (spectral type O6.5Iaf+; ref. 1) has been identified as the optical counterpart of the 3.411-day eclipsing massive X-ray binary 4U1700 – 37 (ref. 2) by spectroscopic3–5 and photometric6 studies which revealed periodic optical variations in phase with the X-ray period. Here we report that the ultraviolet spectra taken with the International Ultraviolet Explorer satellite also vary in phase with the X-ray emission. In an interval of ∼200 A centred on the subordinate He II line at 1,640 A, emission lines 2–3 A wide appear and disappear, with maximum flux at binary phase 0.5 (when the neutron star is in front of the O supergiant) and minimum at phase 0.8. We suggest that these variable lines are due to Raman scattering of extreme ultraviolet photons from the X-ray source by He II ions. These Raman lines can thus be used to reconstruct a part of the unobserved extreme ultraviolet spectrum in the He II scattering zone.