X-ray spectral and flux variability of the microquasar GRS 1758−258 on timescales from weeks to years

¹ Dr. Karl Remeis-Observatory & ECAP, Universität Erlangen-Nürnberg, Sternwartstr. 7, 96049 Bamberg, Germany
e-mail: maria.hirsch@fau.de
² CRESST and NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
³ Department of Physics and Center for Space Science and Technology, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁴ Department of Physics and Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA
⁵ Department of Chemistry, Physics and Astronomy, Georgia College and State University, Milledgeville, GA 31061, USA
⁶ Max Planck Computing and Data Facility, Gießenbachstr. 2, 85748 Garching, Germany
⁷ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁸ Lawrence Livermore National Laboratory, 7000 East Ave., Livermore, CA 94550, USA
⁹ GRAPPA & Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 Amsterdam, The Netherlands
¹⁰ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
¹¹ Department of Physics, Montana State University, Bozeman, MT 59717-3840, USA
¹² Department of Physics, Washington University, CB 1058, One Brookings Drive, St. Louis, MO 63130-4899, USA
¹³ Instituto Nacional de Pesquisas Espaciais, Avenida dos Astronautas, 1.758 São José dos Campos, Brazil
¹⁴ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
¹⁵ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, Sydney, NSW 2006, Australia
¹⁶ Space Sciences Laboratory, 7 Gauss Way, University of California, Berkeley, CA 94720-7450, USA

Received: 15 November 2018
Accepted: 19 December 2019

Abstract

We present the spectral and timing evolution of the persistent black hole X-ray binary GRS 1758−258 based on almost 12 years of observations using the Rossi X-ray Timing Explorer Proportional Counter Array. While the source was predominantly found in the hard state during this time, it entered the thermally dominated soft state seven times. In the soft state GRS 1758−258 shows a strong decline in flux above 3 keV rather than the pivoting flux around 10 keV more commonly shown by black hole transients. In its 3–20 keV hardness intensity diagram, GRS 1758−258 shows a hysteresis of hard and soft state fluxes typical for transient sources in outburst. The RXTE-PCA and RXTE-ASM long-term light curves do not show any orbital modulations in the range of 2–30 d. However, in the dynamic power spectra significant peaks drift between 18.47 and 18.04 d for the PCA data, while less significant signatures between 19 d and 20 d are seen for the ASM data as well as for the Swift/BAT data. We discuss different models for the hysteresis behavior during state transitions as well as possibilities for the origin of the long term variation in the context of a warped accretion disk.