  Cyclotron resonance scattering features (CRSFs) in the X-ray spectrum of an
accreting neutron star are modified differently by accretion mounds sustained
by magnetic and thermocompositional gradients. It is shown that one can
discriminate, in principle, between mounds of different physical origins by
studying how the line energy, width, and depth of a CRSF depend on the
orientation of the neutron star, accreted mass, surface temperature
distribution, and equation of state. CRSF signatures including gravitational
light bending are computed for both phase-resolved and phase-averaged spectra
on the basis of self-consistent Grad-Shafranov mound equilibria satisfying a
global flux-freezing constraint. The prospects of multimessenger X-ray and
gravitational-wave observations with future instruments are canvassed briefly.
