Multi-wavelength observations of the unusual soft X-ray transient ASASSN-16oh

Abstract

We present our results from the analysis of optical and X-ray observations of the unusual X-ray transient ASASSN-16oh. The optical light curve revealed that at quiescence the source was fainter ($I$ = 20.3) and brightened up to $I$ = 15 during the outburst. The outburst light curve looks remarkably symmetric, with a total duration of $\sim$300 days. The optical spectra from SALT show a strong emission line of He II $\lambda$4686, He I and some weak Balmer lines. The X-ray and near ultraviolet fluxes correlate with the optical brightness. Such behavior is unusual amongst the known supersoft sources (hereafter SSS), and is difficult to assimilate into the standard model, but whatever the cause, a period of significantly enhanced mass transfer from the donor is required.