The Small Magellanic Cloud, Supersoft X‐Ray Binary 1E 0035.4−7230 (SMC 13)

Abstract

Details of simultaneous photometric and spectroscopic observations of the optical counterpart of the supersoft X-ray source in the Small Magellanic Cloud, 1E 0035.4-7230 (SMC 13), are presented. Although the spectrum is dominated by emission lines of He II, the Balmer series of hydrogen is also present in emission with a strong decrement, and Balmer lines are seen as broad absorptions. Several high-ionization emission features are also present, including O VI (at 3811, 3834, and 5290 A). Radial velocities and photometry confirm that the binary period is ~01719, and an improved value of the period is derived from 4 years of photometry and analysis of ROSAT HRI X-ray data. The orbital light variation is primarily due to an eclipse of the extensive accretion disk. X-ray and optical minima occur together. The UBV light curves are similar to each other, and no clear phase-related color variations are found. He II emission-line velocities show a semiamplitude of K ~ 100 km s-1, and maximum velocity occurs when the light curve indicates that the compact star would be moving away from the observer, suggesting that this emitting region may trace the orbital motion of the compact star. The range of possible masses implied for the X-ray source lies between 0.5 and 1.5 M☉ if the mass donor is a main-sequence star filling its Roche lobe. The light curve suggests values at the high end of this range. The broad H absorption lines appear to have a much larger velocity amplitude and a lower systemic velocity, making it difficult to understand their origin. We discuss possible models for the system.