  Neutron stars (NSs) in scalar-tensor (ST) theories of gravitation can acquire
scalar charges and generate distinct spacetimes from those in General
Relativity (GR) through the celebrated phenomenon of spontaneous scalarization.
Taking on an ST theory with the mass term of the scalar field, we determine the
theory parameter space for spontaneous scalarization by investigating the
linearized scalar field equation. Then the full numerical solutions for slowly
rotating NSs are obtained and studied in great detail. The resulted spacetime
is used to calculate test-particle geodesics. The lightlike geodesics are used
to construct the profile of X-ray radiation from a pair of hot spots on the
surface of scalarized NSs, which potentially can be compared with the data from
the Neutron star Interior Composition Explorer (NICER) mission for testing the
ST theory.
