Flux variability of the “10 keV feature” of 4U 0115+63

¹ Dr. Karl-Remeis-Observatory and Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
² University of Maryland, Department of Astronomy, College Park, MD 20742, USA
³ NASA Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD 20771, USA
⁴ CRESST and Center for Space Sciences and Technology, University of Maryland, Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA
⁵ Department of Astronomy, University of Geneva, Chemin d’Écogia, 16, 1290 Versoix, Switzerland
⁶ INAF, Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁷ Department of Astronomy and Astrophysics, University of California San Diego, La Jolla, CA 92093, USA
⁸ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692, Villanueva de la Cañada, Madrid, Spain
⁹ Howard University, Department of Physics and Astronomy, Washington, D.C., 20059, USA
¹⁰ NASA Goddard Space Flight Center, Astrophysics Science Division, Code 661, Greenbelt, MD 20771, USA
¹¹ Embry Riddle Aeronautical University, 3700 Willow Creek Road, Prescott, AZ 86301, USA
¹² Physics Dept., United States Naval Academy, Annapolis, MD 21402, USA
¹³ Dept. of Physics and Astronomy, George Mason University, Fairfax, VA 22030, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 June 2025
Accepted: 4 February 2026

Abstract

Context. X-ray spectra of accretion-powered X-ray pulsars can often be described using a power-law continuum with a high-energy cutoff, which might be further modified by additional spectral components. The Be X-ray binary system 4U 0115+63 is well known for having one of the highest numbers of detected harmonics of its cyclotron resonant scattering features (CRSFs), a pronounced spectral component known as the “10 keV feature”, and quasiperiodic oscillations (QPOs) with a period of about 500 s during outbursts.

Aims. The changes in count rate by a factor of two during the ∼500 s QPOs allow us to probe the variation in the spectral components with flux. We study the “10 keV feature” in emission, aiming to disentangle it from the broadband continuum and CRSFs and investigate its origin.

Methods. We focus on the flux-dependent behavior of the CRSF and its harmonics, and particularly the contribution of the “10 keV feature”, as seen in the flux-resolved analysis of two NuSTAR observations of the 2015 outburst.

Results. Comparing the flux-resolved spectra of a given observation with the respective total dataset revealed a distinct change in overall spectral shape at the position of the “10 keV feature” but no comparable deviation at the energies of the harmonic CRSFs. The change associated with the “10 keV feature” does not seem to involve its centroid energy, which remains constant within a given observation. We find indications for an anticorrelation between the continuum flux and the ratio of the “10 keV feature” flux to the continuum flux within each observation.

Conclusions. The analysis strengthens previous claims that the “10 keV feature” shows some independence from the remaining features. This result supports the interpretation that the “10 keV feature” has a different formation mechanism than the continuum emission, although its origin lies within the same physical environment.