Year: 2024
Type: article
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 ∼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 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 ∼10 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.
Source: Astronomy and Astrophysics
Institutions: Goddard Space Flight Center, University of Maryland, College Park, Friedrich-Alexander-Universität Erlangen-Nürnberg, Massachusetts Institute of Technology, Embry–Riddle Aeronautical University, University of Maryland, Baltimore County, Howard University, European Space Astronomy Centre, George Mason University, United States Naval Academy, International Space Science Institute, United States Naval Research Laboratory, Naval Research Laboratory Space Science Division, University of California San Diego, University of Tübingen
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $1,732
OpenAlex ID: W4401766255