Cyclotron line signatures of thermal and magnetic mountains from accreting neutron stars

Abstract

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.