The giant outburst of EXO 2030+375 II: Broadband spectroscopy and evolution

TL;DR

This study analyzes the spectral evolution of the 2021 giant outburst of the high-mass X-ray binary EXO 2030+375 using broadband observations, revealing spectral hardening at lower luminosities and questioning the nature of a 10 keV absorption feature.

Contribution

It provides the first detailed broadband spectral analysis of the 2021 outburst, including pulse-phase-resolved spectroscopy and insights into the emission geometry and spectral features.

Findings

Spectral hardening observed at lower luminosities.

Presence of a 10 keV absorption feature without harmonic confirmation.

Complex spectral shape requiring additional components beyond simple models.

Abstract

In 2021, the high-mass X-ray binary EXO 2030+375 underwent a giant X-ray outburst, the first since 2006, that reached a peak flux of ${\sim}600\,\mathrm{mCrab}$ (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 used broadband spectra taken with the Nuclear Spectroscopic Telescope Array (NuSTAR), the Neutron Star Interior Composition Explorer (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 using 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; it requires additional absorption or emission components. We can confirm the presence of a narrow absorption feature at ${\sim}10\,\mathrm{keV}$ in both NuSTAR observations. The absence of harmonics puts into 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.