X-ray observations of the very-faint X-ray transient XMMSL1 J171900.4--353217: A new candidate neutron star low-mass X-ray binary

XMMSL1 J171900.4-353217 is a very-faint X-ray transient that was discovered in 2010 March when it exhibited an outburst. We report on 7 observations, obtained with the X-Ray Telescope (XRT) aboard the Neil Gehrels {\it{Swift}} Observatory between 2010 May to October. By fitting a single absorbed power-law model to the XRT spectra, we infer power-law indices of $\mathrm{\Gamma }=1.8-2.7$ and an absorption column density of ${\mathrm{N}}_{\mathrm{H}}=(4.6-7.9)\times {10}^{22}{\mathrm{c}\mathrm{m}}^{-2}$. The inferred $0.5-10$~keV luminosity fluctuated irregularly and peaked at ${\mathrm{L}}_{\mathrm{X}}\simeq {10}^{35}-{10}^{36}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$ for a distance of $4-12$~kpc. Based on the evolution of the power-law index with varying luminosity, we propose that the source most likely is a transient neutron star low-mass X-ray binary located at several kpc. If true, it would be a good candidate to search for coherent millisecond pulsations when it enters a new accretion outburst.