On the Binary Nature of 1RXS J162848.1-415241

We present spectroscopy of the optical counterpart to 1RXS J162848.1-41524, also known as the microquasar candidate MCQC J162847-4152. All the data indicate that this X-ray source is not a microquasar and that it is a single-lined chromospherically active binary system with a likely orbital period of 4.9 days. Our analysis supports a K3 IV spectral classification for the star, which is dominant at optical wavelengths. The unseen binary component is most likely a late-type (K7-M) dwarf or a white dwarf. Using the high-resolution spectra, we have measured the K3 star's rotational broadening to be vsini=43+/-3 km s^-1 and determined a lower limit to the binary mass ratio of q(=M₂/M₁)>2.0. The high rotational broadening together with the strong Ca II H and K/Hα emission and high-amplitude photometric variations indicate that the evolved star is very chromospherically active and responsible for the X-ray/radio emission.