Optical spectroscopy of the quiescent counterpart to EXO 0748−676: a black widow scenario?

Abstract

We present phase-resolved optical spectroscopy of the counterpart to the neutron star low mass X-ray binary EXO 0748-676, almost one year after it turned into quiescence. The spectra display prominent Hbeta and Hgamma and weak Fe II lines in emission. An average of all the spectra (corrected for the orbital motion) also exhibits a very weak line from Mg I. Tomographic reconstructions show that the accretion disc is not contributing to the optical line emission, which is instead dominated by the irradiated hemisphere of the companion star facing the neutron star. We could not detect absorption features from the mass donor star in the spectra. The emission lines appear broad, with an intrinsic FWHM of 255+-22 km/s. Under the assumption that the width of the Fe II emission lines is dominated by rotational broadening, we obtain a lower limit on the compact object mass which is inconsistent with a NS accretor. We discuss this incongruity and conclude that either the lines are blends of unresolved features (although this requires some fine tuning) or they are broadened by additional effects such as bulk gas motion in an outflow. The fact that the Fe II lines slightly lag in phase with respect to the companion star can be understood as outflowing gas consistent with a Black-Widow like scenario. Nevertheless, we can not rule out the possibility that blends of various emission lines cause the apparent phase lag of the Fe II emission lines as well as their large width.