Speaker
Description
Accreting neutron stars in high-mass X-ray binaries are endowed by a strong magnetic field that has a major impact on their electromagnetic signal. Modulated by orbital motion and stellar rotation, the light curve is variable on different time scales from millisecond to years. The signal is also dramatically different as function of the photon energy. While the phenomenology is known in broad terms, it is still challenging to link theoretical models to the actual data, which have increasingly high quality.
In observations of different sources at variable luminosity, the spin-dependent average emission is encoded into a matrix of count rates at different spin phases and energy ranges. We can extract reduced information in several ways; in this work, we explore the energy-dependent pulsed fractional amplitude of periodically modulated signal, dubbed pulsed-fraction spectra. We show how characteristic signatures in correspondence of cyclotron resonant scattering features, iron lines, and emergence of different spectral components can be efficiently coupled with spectral model to constrain the source geometry and emission mechanisms. Our group has deeply analyzed hard X-ray observations of eight sources using NuSTAR and we are now capable of assessing the full potential and limits of this method.