Observations of the X-ray Millihertz Quasiperiodic Oscillations in Hercules X-1

TL;DR

This study systematically analyzes X-ray observations of Hercules X-1, confirming millihertz quasi-periodic oscillations (QPOs), identifying transient behaviors, and exploring their potential physical origins and relation to luminosity.

Contribution

First application of wavelet analysis to identify coexisting and transient mHz QPOs in Hercules X-1, revealing different physical mechanisms for distinct QPO types.

Findings

Confirmed presence of ~0.01 Hz QPOs in Hercules X-1

Identified coexisting ~0.005-0.009 Hz QPOs with transient behavior

Found positive correlation between QPO frequency and luminosity

Abstract

With a systematic timing investigation of the persistent X-ray binary pulsar Her X-1 based on a large number of Insight-HXMT observations between 2017 to 2019, we confirm the presence of X-ray millihertz quasi-periodic oscillations (mHz QPOs) at $\sim 0.01$ Hz. By applying wavelet analysis in our data analysis procedures, we firstly identified $\sim 0.005-0.009$ Hz QPOs coexisting with the $\sim 0.01$ Hz QPOs. Wavelet analysis suggests that these QPO features show transient behaviors, frequencies of mHz QPOs evolved in short time scales. There exists a positive relation between QPO centroid frequency (from $\sim 0.005-0.009$ Hz) and the X-ray luminosity, while the 10 mHz QPO frequencies keep nearly constant for different luminosities, which suggests different physical mechanisms for two types of mHz QPOs. The 10 mHz QPOs in both X-ray and UV bands would have the same origin related to the beat frequency where the Alfv$\acute{e}$n radius is close to the corotation radius, and the 5 mHz QPOs may originate from magnetic disk precession.