The Optical Counterpart of the X-ray Transient RX J0117.6-7330: Spectroscopy and Photometry

Abstract

We conducted spectroscopic and photometric observations of the optical companion of the X-ray transient RX J0117.6-7330 in the Small Magellanic Cloud, during a quiescent state. The primary star is identified as a B0.5 IIIe with a mass M_* = (18 ± 2)M_sun and bolometric magnitude M_bol = -7.4 ± 0.2. The main spectral features are strong Halpha emission, Hbeta and Hgamma emission cores with absorption wings, and narrow HeI and OII absorption lines. Equivalent widths and full widths at half maximum of the main lines are listed. The average systemic velocity over our observing run is v_r = (184 ± 4) km s-1; measurements over a longer period of time are needed to determine the binary period and the K velocity of the primary. We determine a projected rotational velocity v sin i = (145 ± 10) km s-1for the Be star; if we assume a true rotational velocity at the equator v = (400 ± 50) km s-1, we deduce that the inclination angle of the system is i = (21 ± 3) deg.