The Giant Out Burst of EXO 2030+375 II: Broadband Spectroscopy and Evolution

In 2021, the High-mass X-ray Binary EXO 2030+375 underwent a giant X-ray outburst, the first since 2006, which reached a peak flux of ∼600 mCrab (3–50 keV). The goal of this work is to study the spectral evolution over the course of the outburst, search for possible cyclotron resonance scattering features (CRSFs), and to associate spectral components with the emission pattern of the accretion column. We use broadband spectra taken with NuSTAR, NICER, and Chandra near the peak and during the decline phase of the outburst. We describe the data with established empirical continuum models and perform pulse-phase-resolved spectroscopy. We compare the spectral evolution with pulse phase with a proposed geometrical emission model. We find a significant spectral hardening toward lower luminosity, a behavior that is expected for super-critical sources. The continuum shape and evolution cannot be described by a simple power law model with exponential cutoff but requires additional absorption or emission components. We can confirm the presence of a narrow absorption feature at ∼10 keV in both NuSTAR observations. The absence of harmonics question the interpretation of this feature as a CRSF. The empirical spectral components cannot be directly associated with identified emission components from the accretion column.