2013MNRAS.429.2986M

In this paper we present X-Shooter time-resolved spectroscopy and RXTE PCA light curves of the ultra-compact X-ray binary candidate 4U 0614+091. The X-Shooter data are compared to the GMOS data analysed previously by Nelemans et al. We confirm the presence of Ciii and Oii emission features at ~4650 Å and ~5000 Å. The emission lines do not show evident Doppler shifts that could be attributed to the motion of the donor star/hotspot around the centre of mass of the binary. We note a weak periodic signal in the red-wing/blue-wing flux ratio of the emission feature at ~4650 Å. The signal occurs at P = 30.23 ± 0.03 min in the X-Shooter and at P = 30.468±0.006 min in the GMOS spectra when the source was in the low/hard state. Due to aliasing effects the period in the GMOS and X-Shooter data could well be the same. We deem it likely that the orbital period is thus close to 30min; however, as several photometric periods have been reported for this source in the literature already, further confirmation of the 30 min period is warranted. We compare the surface area of the donor star and the disc of 4U 0614+091 with the surface area of the donor star and the disc in typical hydrogen-rich low-mass X-ray binaries and the class of AM Canum Venaticorum stars and argue that the optical emission in 4U 0614+091 is likely dominated by the disc emission. Assuming that the accretion disc in 4U 0614+091 is optically thick and the opening angle of the disc is similar to the one derived for the hydrogen-rich low-mass X-ray binaries we conclude that the donor star may in fact stay in the disc shadow. Additionally, we search for periodic signals in all the publicly available RXTE PCA light curves of 4U 0614+091 which could be associated with the orbital period of this source. A modulation at the orbital period with an amplitude of ~10 per cent such as those that have been found in other ultra-compact X-ray binaries (4U 0513-40, 4U 1820-30) is not present in 4U 0614+091. We propose that the X-ray modulation found in 4U 0513-40 and 4U 1820-30 could be a signature of an outflow launched during the high/soft state near the stream impact region.