A possible imprint of quasi-periodic oscillations in the X-ray spectra of black hole binaries

¹ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
e-mail: varniere@apc.univ-paris7.fr
² Laboratoire AIM, CEA/IRFU-CNRS/INSU-Université Paris Diderot, CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France

Received: 7 December 2015
Accepted: 29 December 2015

Abstract

Context. While nobody would deny the presence of quasi-periodic oscillations in the power density spectrum of black hole binaries nor their importance in the understanding of the mechanisms powering the X-ray emissions, the possible impact on the time-averaged disk energy spectrum from the phenomenon responsible for quasi-periodic oscillations is largely ignored in models of sources emission.

Aims. Here we investigate the potential impact of such a structure on the resultant energy spectrum.

Methods. Using data from the well-documented outbursts of XTE J1550-564, we looked at possible hints that the presence of quasi-periodic oscillations actually impacts the energy spectrum emitted by the source. In particular, we look at the evolution of the relation between the inner disk radius and the inner disk temperature obtained from fits to the spectral data. We then test this further by developing a simple model to simulate the spectrum of a disk with a structure mimicking quasi-periodic oscillations that are increasing in strength simulated results to those obtained from real data.

Results. We detect a similar departure in the inner radius – inner temperature curve coming from the standard fit of our simulated observations as is seen in XTE J1550-564 data. We interpret our results as evidence that the structure at the origin of the quasi-periodic oscillations impacts the energy spectrum.

Conclusions. Furthermore, in states with significant disk emission the inaccuracy of the determination of the disk parameters increases with the strength of quasi-periodic oscillations, an increase that then renders the value given by the fit unreliable for strong quasi-periodic oscillations.