A peculiar emission-line feature in the X-ray spectrum of the quasar PKS 0637-752

We report the results from an ASCA observation of the high-luminosity, radio-loud quasar PKS 0637-752 (redshift 0.654), covering the 0.8-15 keV band in the quasar frame. We find the source to have a luminosity of similar to 10(46) ergs s(-1) in the 2-10 keV band, which is a factor of similar to 3 lower than during a previous Ginga observation. The continuum appears to be well modeled by a simple power law with Gamma = 1.64 +/- 0.07, with no evidence for absorption by material intrinsic to the quasar or Fe-K emission (with an equivalent width less than or similar to 80 eV at 90% confidence). However, we do find evidence for a narrow emission line at an energy 1.60 +/- 0.07 keV and equivalent width 59(-34)(+38) eV (both in the quasar frame). Line emission at these energies has not been observed in any other active galaxy or quasar to date. We reject the possibility that this line is the result of instrumental artifacts and briefly explore possible identifications.