Speaker
Description
Be X-ray binaries (BeXRBs) are highly variable systems that host the majority of X-ray pulsars (XRPs). In these systems, accreting material is transferred via the magnetic field lines of the Neutron Star (NS) and concentrated into an accretion column (AC) near its magnetic poles. The AC dynamics suggest a presence of sub-critical and super-critical regimes in which the luminosity and the spectral distribution are highly affected. Moreover, the most luminous outbursts of BeXRBs are known to break through the Eddington limit, and offer our nearest window onto Super-Eddington (SE) accretion. The Becker & Wolff (2007) model (BW07) has successfully reproduced XRP spectra in the super-critical regime, particularly for luminosities in the range of 10$^{37}$−10$^{38}$erg s$^{-1}$. However, its application to SE sources remains limited.
In this work, we apply the BW07 model to BeXRB spectra with luminosities exceeding 10$^{38}$ erg s$^{-1}$. Our analysis begins with a parametric study of the BW07 radiative model to identify self-consistent solutions that conserve energy. We then present best-fit results for the observed X-ray spectra of these systems during their major outbursts. Importantly, all these sources exhibit cyclotron resonant scattering features (CRSF), which provide a direct measure of the NS surface magnetic field strength. In our study, we take into account the characteristic height of CRSF formation and examine degeneracy induced by model assumptions.
For systems with multiple observations across varying luminosity states, we explore the evolutionary track of model parameters, offering insight into the transitions between sub-critical and super-critical regimes, with application to Swift/XRT energy bands.
