The NuSTAR view of the 2021 giant X-ray outburst of EXO 2030+375

In 2021, the BeXRB EXO 2030+375 underwent a giant X-ray outburst, the first since 2006, which reached a peak flux of ~600 mCrab. NuSTAR and NICER observed this outburst near the peak and once during the decline phase after a characteristic evolution of the pulse profile had been observed through extensive NICER monitoring. After this transition, the relative strength of two peaks is reversed in the pulse profile and hardness ratio. Our observations confirm the behavior seen in previous outbursts. Here, we present a joint NuSTAR and NICER analysis of the spectral transition associated with this pulse profile change. We also investigate the spectral variability with pulse phase of the two NuSTAR observations and compare the spectral evolution to geometrical models of the accretion column emission obtained from relativistic ray-tracing simulations. We further discuss putative CRSF detections reported in the literature, especially the possible 10-keV absorption feature in the context of the complex soft X-ray continuum.