X-ray emission spectrum for axion-photon conversion in magnetospheres of strongly magnetized neutron stars

Abstract: Detecting axionic dark matter (DM) could be possible in an X-ray spectrum from strongly magnetized neutron stars (NSs). We examine the possibility of axion-photon conversion in the magnetospheres of strongly magnetized NSs. In the current work, we investigate how the modified Tolman Oppenheimer Volkoff (TOV) system of equations (in the presence of a magnetic field) affects the energy spectrum of axions and axions-converted-photon flux. We have considered the distance-dependent magnetic field in the modified TOV system of equations. We employ three different equations of states (EoSs), namely APR, FPS, and SLY, to solve these equations. We obtain the axions emission rate by including the Cooper-pair-breaking formation process and Bremsstrahlung process in the core of NSs using the NSCool code. We primarily focus on Magnificient seven (M7) star RXJ 1856.5-3754. We further investigate the impact of the magnetic field on the actual observables, such as axion energy spectrum and axion-converted-photon flux at an axion mass in meV range by assuming mass $M_{NS} \sim 1.4M_{\odot}$. We compare our calculated axion-converted-photon flux from all available archival data sets from PN+MOS+Chandra. We also study the variation of the energy spectrum at a fixed energy with varying central magnetic fields. Our predicted axion-converted-photon flux values as a function of axion energy closely follow the experimentally archival data, which allows us to put bounds on the axion mass for the three EoS.