The giant outburst of EXO 2030+375

II. Broadband spectroscopy and evolution

¹ University of Maryland College Park, Department of Astronomy, College Park, MD 20742, USA
e-mail: ballhaus@umd.edu
² CRESST and NASA Goddard Space Flight Center, Astrophysics Science Division, 8800 Greenbelt Road, Greenbelt, MD 20771, USA
³ Dr. Karl Remeis-Observatory & ECAP, Universität Erlangen-Nürnberg, 96049 Bamberg, Germany
⁴ Department of Physics and Astronomy, Embry-Riddle Aeronautical University, 3700 Willow Creek Road, Prescott, AZ 86301, USA
⁵ Massachusetts Institute of Technology, Kavli Institute for Astrophysics and Space Research, 70 Vassar St., Cambridge, MA 02139, USA
⁶ University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁷ Department of Physics and Astronomy, Howard University, Washington, DC 20059, USA
⁸ ESA/ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ George Mason University, Fairfax, VA 22030, USA
¹⁰ Department of Physics, United States Naval Academy, Annapolis, MD 21402, USA
¹¹ International Space Science Institute, Hallerstrasse 6, 3012 Bern, Switzerland
¹² Space Science Division, U.S. Naval Research Laboratory, Washington, DC 20375−5352, USA
¹³ Department of Astronomy and Astrophysics, University of California San Diego, 9500 Gilman Dr., La Jolla, CA 92093, USA
¹⁴ Institut für Astronomie und Astrophysik, Eberhard-Karls-Universität Tübingen, Sand 1, 72076 Tübingen, Germany

Received: 13 November 2023
Accepted: 15 February 2024

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.