The spectral-timing analysis of Cygnus X-1 with Insight-HXMT

¹ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: menglei.zhou@astro.uni-tuebingen.de
² European Space Agency (ESA), European Space Research and Technology Centre (ESTEC), Keplerlaan 1, 2201 AZ Noordwijk, The Netherlands
³ Laboratoire de Physique Nucléaire et des Hautes Énergies, Sorbonne Université, 4 Place Jussieu, 75005 Paris, France
⁴ Dr. Karl Remeis-Sternwarte and ECAP, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
⁵ School of Physics and Astronomy, Sun Yat-Sen University, 519082 Zhuhai, PR China
⁶ Physics Department, Washington University CB 1105, St Louis, MO 63130, USA
⁷ CRESST and Astroparticle Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Department of Physics and Center for Space Sciences and Technology, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁹ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM de Paris-Saclay, 91191 Gif sur Yvette, France
¹⁰ Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 100049 Beijing, PR China
¹¹ University of Chinese Academy of Sciences, Chinese Academy of Sciences, 100049 Beijing, PR China

Received: 10 June 2022
Accepted: 12 September 2022

Abstract

Cygnus X-1, as the first discovered black hole binary, is a key source for understanding the mechanisms of state transitions and the scenarios of accretion in extreme gravity fields. We present a spectral-timing analysis of observations taken with the Insight–Hard X-ray Modulation Telescope (HXMT) mission, focusing on the spectral-state-dependent timing properties in the broad energy range of 1−150 keV, thus extending previous studies based on Rossi X-ray Timing Explorer (RXTE) to both lower and higher energies. Our main results are the following: (a) We successfully use a simple empirical model to fit all spectra, confirming that the reflection component is stronger in the soft state than in the hard state. (b) The evolution of the total fractional root mean square (rms) depends on the selected energy band and the spectral shape, which is a direct result of the evolution of the power spectral densities (PSDs). (c) In the hard/intermediate state, we see clear short-term variability features and a positive correlation between the central frequencies of the variability components and the soft photon index Γ₁, which we also see at energies above 15 keV. In the soft state, the power spectrum is instead dominated by red noise. These behaviors can be traced to at least 90 keV. (d) Finally, the coherence and the phase-lag spectra show different behaviors, depending on the different spectral shapes.