Quasi-periodic oscillations in GX 339-4 during the 2021 Outburst observed with Insight-HXMT

TL;DR

This study analyzes the 2021 outburst of GX 339-4 using Insight-HXMT data, revealing spectral state transitions, type-C QPOs with evolving frequencies, and first-time detection of QPOs above 50 keV, providing insights into black hole accretion physics.

Contribution

It presents the first detection of QPOs above 50 keV in GX 339-4 and details their evolution during spectral state transitions using Insight-HXMT data.

Findings

Type-C QPOs detected with frequencies from 0.1-0.6 Hz (LHS) and 1-3 Hz (HIMS).

QPO rms stable over time but decreases with energy above 10 keV.

Phase lag evolves from near zero to positive, with hard lag up to 1.2 rad at high energies.

Abstract

A new outburst of GX 339$-$4 in 2021 was monitored by the Hard X-ray Modulation Telescope (\textit{Insight}-HXMT). By using the data of \textit{Insight}-HXMT from February to March 2021, we make the X-ray timing analysis of this new outburst. Based on the results of count rates, hardness-intensity diagram (HID) and power density spectrum (PDS), we confirm that the source exhibits spectral transitions from the low-hard state (LHS) to the hard-intermediate state (HIMS). During the transition from the LHS to the HIMS, Low-frequency Quasi-periodic oscillations (LFQPOs) are detected in the PDS. We found that these QPOs are all type-C QPOs with centroid frequencies evolving from $0.1 -0.6$ Hz in the LHS and in the $1 -3$ Hz frequency range in HIMS. The QPO features above 50 keV are reported for the first time in this black hole by \textit{Insight}-HXMT. The QPO rms stays stable with time but decreases with energy at higher energy above $\sim 10$ keV. We also find that the phase lag of the type-C QPO is close to zero in the early outburst stage, but becomes positive as the outburst evolves, with a hard lag of $\sim$ 0.6-1.2 rad in $50 -100 $ keV. The implications of the phase lag in high energy bands and possible physical mechanisms to explain those observations are also discussed.