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

TL;DR

This study investigates the flux-dependent behavior of the '10 keV feature' in the X-ray spectra of 4U 0115+63, revealing its independence from other spectral components and suggesting a distinct formation mechanism.

Contribution

The paper provides flux-resolved analysis of the '10 keV feature' in 4U 0115+63, demonstrating its spectral independence and potential unique origin compared to other features.

Findings

The '10 keV feature' shows a change in spectral shape with flux, unlike harmonic CRSFs.

The centroid energy of the '10 keV feature' remains constant within observations.

An anticorrelation exists between continuum flux and the flux ratio of the '10 keV feature' to the continuum.

Abstract

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. The changes in count rate by a factor of two during the approximately 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. 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. 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. 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.