The discovery and study of the optical counterparts of the transient X-ray pulsars RXJ0052.1-7319 and XTEJ0111.2-7317 in the SMC

Abstract

We report on the discovery and confirmation of the optical counterparts of the two transient X-ray pulsars, RXJ0052.1-7319 and XTEJ0111.2-7317. In the narrow (~3" radius) X-ray error circle of RXJ0052.1-7319 we found a V=14.6 (B-V=0.1) 09.5IIIe (a classification as a B0Ve star is also possible, since the luminosity class depends on the uncertainty on the adopted reddening). Medium resolution spectra for this object show Balmer lines in emission with an equivalent width of Halpha=-12/-16A. In the X-ray error box of XTEJ0111.2-7317 we found a relatively bright object (V=15.4, B-V=0.06) which has been classified as a B0.5-B1Ve star and that was later confirmed by Coe et al. (2000) as the most plausible counterpart for XTEJ0111.2-7317. Also in this case we easily detect Balmer emission lines with an Halpha equivalent width of about -21A. There is also evidence for the presence of a surrounding nebula, possibly a supernova remnant. A further bright B0Ve star was found just outside the X-ray error circle of XTEJ0111.2-7317. We discuss the implication of these results in the light of the current knowledge of Be/X-ray binary systems in the Magellanic Clouds and within our Galaxy.