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

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.

Accepted for publication in A&A, 15 pages, 11 figures

References in corpus (21)

• The Gaia mission
• Gaia Data Release 3: Summary of the content and survey properties
• Estimating distances from parallaxes. V: Geometric and photogeometric distances to 1.47 billion stars in Gaia Early Data Release 3
• Thermal and Bulk Comptonization in Accretion-Powered X-Ray Pulsars
• A model for cyclotron resonance scattering features
• Giant Outbursts in Be/X-ray Binaries
• No Time for Dead Time: Timing analysis of bright black hole binaries with NuSTAR
• A Further Study of the Luminosity-Dependent Cyclotron Resonance Energies of the Binary X-ray Pulsar 4U0115+63 with RXTE
• Luminosity-dependent changes of the cyclotron line energy and spectral hardness in Cep X-4
• Cyclotron resonant scattering feature simulations. I. Thermally averaged cyclotron scattering cross sections, mean free photon-path tables, and electron momentum sampling
• A New Two-Fluid Radiation-Hydrodynamical Model for X-ray Pulsar Accretion Columns
• Dynamical and Radiative Properties of X-ray Pulsar Accretion Columns: Phase-Averaged Spectra
• Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1
• LAXPC / AstroSat Study of ~ 1 and ~ 2 mHz Quasi-periodic Oscillations in the Be/X-ray Binary 4U 0115+63 During its 2015 Outburst
• Fitting strategies of accretion column models and application to the broadband spectrum of Cen X-3
• The giant outburst of 4U 0115+634 in 2011 with Suzaku and RXTE
• Monitoring clumpy wind accretion in supergiant fast X-ray transients with XMM-Newto
• Vacuum polarization alters the spectra of accreting X-ray pulsars
• Cyclotron line formation in the radiative shock of an accreting magnetized neutron star
• Luminosity dependence of the multiple cyclotron lines in 4U 0115$+$63
• Don't torque like that. Measuring compact object magnetic fields with analytic torque models