Likely detection of magnetic field related LFQPO in the soft X-ray rebrightening of GRS 1915+105

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls, Universität, Sand 1, 72076 Tübingen, Germany
e-mail: lingda.kong@mnf.uni-tuebingen.de
² School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai 519082, PR China
e-mail: jilong@mail.sysu.edu.cn
³ Key Laboratory for Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, PR China
⁴ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, PR China

Received: 6 November 2023
Accepted: 12 March 2024

Abstract

Utilizing NICER observations, we present an analysis of the soft X-ray rebrightening event of GRS 1915+105 observed in 2021. During this event, we observed the emergence of a stable long-lasting low-frequency quasi-periodic oscillation (LFQPO) with frequencies ranging from 0.17 to 0.21 Hz. Through a careful spectral analysis, we demonstrate that a low-temperature Compton-thick gas model characterizes the emitted radiation well. By examining the spectrum and identifying numerous absorption lines, we discerned a transition in the wind properties. This transition was marked by a shift from a state characterized by low speed, high column density, and high ionization degree to one featuring still low speed, but low column density and ionization degree. Intriguingly, the presence or absence of the QPO signal is perfectly correlated with these distinct wind characteristics. The low-speed wind observed could be indicative of a “failed wind”, while the observed shift implies a transition from a magnetically to a thermally driven wind. Notably, this QPO signal exclusively manifested itself during the magnetically driven phase, suggesting the possibility of a novel perturbation associated with magnetic effects.