E. M. Ratti, D. T. H. Steeghs, P. G. Jonker, M. A. P. Torres, C. G. Bassa, F. Verbunt
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.
View original:
http://arxiv.org/abs/1110.3963
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