Περίληψη :
Accretion-powered X-ray Pulsars (XRPs) are strongly magnetized neutron stars (NSs), that accrete matter from a donor companion star, often exhibiting a cyclotron resonant scattering feature (CRSF) in their X-ray spectra. Accretion onto their magnetic poles is responsible for the emergence of X-rays, while the quantization of electrons motion perpendicular to the NS's magnetic field results in the emergence of absorption/emission lines (i.e., CRSFs) via matter-radiation resonant scattering. The CRSF encodes important information about the magnetic field in the line-forming region. Nevertheless, the site of the CRSF formation is
still an open puzzle. For low-/sub-critical luminosity sources, the CRSF is believed to form above the magnetic pole's hotspot, while for the high-luminosity (L_X >~ 1e37 erg/s) ones, two places are suspect for the formation of a CRSF: the surface of the neutron star and the radiative shock in the accretion column arising above the magnetic pole.
In this talk, I'll give an overview of the proposed cyclotron-line formation scenarios, introduce the fundamental Physics involved, and briefly discuss the observational signatures/correlations used to constrain the current viable models. Subsequently, I'll focus on the high-luminosity sources in which the existence of a radiative shock in the accretion column is feasible. I'll demonstrate that the observed properties of CRSFs in these sources cannot be explained by the scenario where the site of the CRSF formation is the surface of the NS. Next, I'll describe our relativistic Monte Carlo Radiative Transfer (MCRT) code developed to study the spectral formation in the remaining candidate site of cyclotron-line formation, i.e., the radiative shock in the accretion column. I'll present the derived emergent spectra, show that a power law, hard X-ray continuum, and a prominent CRSF are naturally produced by the first-order Fermi energization as the photons criss-cross the shock, and thus provide concrete evidence that the radiative shock is the site of CRSF formation. Finally, I'll conclude that the combination of bulk-, thermal-Comptonization, and resonant Compton scattering of photons by electrons in a radiative shock is efficient in producing spectra with prominent CRSFs similar to the ones observed in high-luminosity XRPs.
