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

TL;DR

This paper investigates how cyclotron resonance scattering features (CRSFs) in X-ray spectra can distinguish between different types of accretion mounds on neutron stars, considering various physical and observational factors.

Contribution

It introduces a method to differentiate magnetic and thermocompositional accretion mounds by analyzing CRSF dependencies on multiple parameters using self-consistent models.

Findings

CRSF signatures vary with neutron star orientation and accreted mass.

Gravitational light bending influences CRSF profiles.

Future multimessenger observations could enhance understanding.

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.