Curvature from strong gravitational lensing: a spatially closed Universe
  or systematics?
  Model-independent constraints on the spatial curvature are not only closely
related to important problems such as the evolution of the Universe and
properties of dark energy, but also provide a test of the validity of the
fundamental Copernican principle. In this paper, with the distance sum rule in
the Friedmann-Lema\^{i}tre-Robertson-Walker metric, we achieve
model-independent measurements of the spatial curvature from the latest type Ia
supernovae and strong gravitational lensing (SGL) observations. We find that a
spatially closed Universe is preferred. Moreover, by considering different
kinds of velocity dispersion and subsample, we study possible factors which
might affect model-independent estimations for the spatial curvature from SGL
observations. It is suggested that the combination of observational data from
different surveys might cause a systematic bias and the tension between the
spatially flat Universe and SGL observations is alleviated when the subsample
only from the Sloan Lens ACS Survey is used or a more complex treatment for the
density profile of lenses is considered.
