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Variability of the accretion disk of V926 Sco inferred from tomographic analysis

We present phase-resolved spectroscopic observations of the low-mass X-ray binary V926 Sco (4U 1735-44), covering the orbital period of 0.23d, obtained with the Walter Baade 6.5m Magellan Telescope at the Las Campanas Observatory in June 2010 and June 2011. We use H$α$ radial velocities to derive a systemic velocity of -109 $\pm$ 4 km s$^{-1}$. The FWHM of the lines observed in common with previous authors are significantly lower during our observations suggesting much reduced velocities in the system. The equivalent width of the Bowen fluorescence lines with respect to He II $λ$4686 are factors of two or more lower during our observations in comparison to those previously reported for the system, suggesting reduced irradiation of the secondary. Doppler and modulation tomography of H$α$ and He II $λ$4686 show assymmetric emission that can be attributed to a bulge in the accretion disk, as inferred from He II observations by previous authors. The X-ray fluxes from the source at times concurrent with the optical observations are significantly lower during our observations than during optical observations taken in 2003. We suggest that the system is in a lower accretion state compared to earlier observations; this explains both the lower velocities observed from the disk and the reduction of emission due to Bowen fluorescence detected from the secondary.