Intermittent properties of the quasi-periodic oscillations of MAXI J1820+070 revealed by Insight-HXMT

¹ College of Science, China Three Gorges University, Yichang 443002, PR China
e-mail: zhangpeng@ctgu.edu.cn
² Center for Astronomy and Space Sciences, China Three Gorges University, Yichang 443002, PR China
³ College of Astronomy and Space Sciences, University of the Chinese Academy of Sciences, Beijing 100049, PR China
e-mail: rsoria@nao.cas.cn
⁴ INAF-Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
⁵ Sydney Institute for Astronomy, School of Physics A28, The University of Sydney, Sydney, NSW 2006, Australia
⁶ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, 19B Yuquan Road, Beijing 100049, PR China
e-mail: szhang@ihep.ac.cn
⁷ School of Physics and Astronomy, Sun Yat-Sen University, Zhuhai 519082, PR China
⁸ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Eberhard Karls, Universität, Sand 1, 72076 Tübingen, Germany
⁹ University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, PR China
¹⁰ CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei 230026, PR China
¹¹ School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, PR China
¹² Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
¹³ School of Sciences, Hangzhou Dianzi University, Hangzhou 310018, PR China

Received: 3 March 2023
Accepted: 11 July 2023

Abstract

We investigate the dynamical properties of low-frequency quasi-periodic oscillations (QPOs) observed from the black hole X-ray binary MAXI J1820+070 during the early part of its 2018 outburst, when the system was in a bright hard state. To do this, we used a series of observations from the Hard X-ray Modulation Telescope Insight-HXMT and applied a wavelet decomposition (weighted wavelet Z-transforms) to the X-ray light curve. We find that the QPO phenomenon is intermittent within each individual observation, with some subintervals in which the oscillation is strongly detected (high root-mean-square amplitude) and others in which it is weak or absent. The average lifetime of individual QPO segments is about 5 oscillation cycles, with a 3σ tail of up to about 20 cycles. There is no substantial difference between the energy spectra during intervals with strong and weak or absent QPOs. Alternatively, we suggest that the changes in oscillation amplitude can be caused by changes in the jet speed. We discuss a possible reason for the intermittent QPO strength within the precessing-jet model previously proposed for MAXI J1820+070: we suggest that the changes in oscillation amplitude are caused by changes in the jet speed. We argue that a misaligned precessing-jet scenario is also consistent with other recent observational findings that suggest that the oscillation of the Compton reflection component is in phase with the QPOs.