Optical spectroscopy of the quiescent counterpart to EXO 0748-676: A black widow scenario?

E. M. Ratti, D. T H Steeghs, Peter G Jonker, Manuel A P Torres, C G Bassa, F. Verbunt

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Abstract

We present phase-resolved optical spectroscopy of the counterpart to the neutron star (NS) low-mass X-ray binary EXO 0748-676, almost one year after it turned into quiescence. The spectra display prominent Hβ and Hγ and weak Feii lines in emission. An average of all the spectra (corrected for the orbital motion) also exhibits a very weak line from Mgi. 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 NS. We could not detect absorption features from the mass donor star in the spectra. The emission lines appear broad, with an intrinsic full width at half-maximum (FWHM) of 255 ± 22 kms -1. Under the assumption that the width of the Feii emission lines is dominated by rotational broadening, we obtain a lower limit on the compact object mass which is inconsistent with an 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 Feii 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 cannot rule out the possibility that blends of various emission lines cause the apparent phase lag of the Feii emission lines as well as their large width.

Original languageEnglish
Pages (from-to)75-83
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Volume420
Issue number1
DOIs
Publication statusPublished - Feb 2012
Externally publishedYes

Keywords

  • Accretion, accretion discs
  • Binaries eclipsing
  • Stars: individual (EXO 0748-676)
  • X-rays: binaries
  • X-rays: individual: EXO 0748-676

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