Speaker
Description
The basic picture of what forms the pulsed X-ray emission in strongly magnetized accreting pulsars is understood since decades, but the wealth of current data greatly exceeds our ability to model them.
HMXB are variable on many timescales, from spin, to orbit, arriving to super-orbital modulations. Variability on time scales above the spin period is instrumental to explore the accretion environment and constrain properties of the binary environment, such as clumpiness of the wind and large-scale structures as accretion wakes, but it might also allow us to disentangle emission from the different magnetic poles.
To investigate the geometry of the accretion stream and the optical properties of matter in a strong magnetic field, we need to fold the light curves at the spin period to accumulate energy-dependent pulse profiles. Their modeling is particularly challenging, owing to pronounced asymmetries, and sharp features emerging just at some energies. They suggest that the thin accretion column forming in strong magnetic fields might be amplified by gravitational lensing at some phases or self-obscured at others. However, there is not an established way of disentangling single-column contributions.
Each source shows different phenomenology and requires dedicated study, while general patterns have not been clearly highlighted, despite the deepness of archive mining. Modeling is very challenging, due to the difficulty of coupling matter and radiation in the vicinity of a neutron star. A push forward might come by federating intellectual resources for modeling and exploiting new missions with higher collecting area to explore shorter time scales.