Optical and X-Ray Observations of MAXI J1820+070 During the Early Outburst Phase in 2018: Zooming in the Low Frequency QPOs*

Abstract

Abstract We report a further investigation of the optical low frequency quasi periodic oscillations (LF QPOs) detected in the black hole transient MAXI J1820+070 in the 2018 observations with the YFOSC mounted on Lijiang 2.4 m telescope (LJT). In addition, we make use of the Insight-HXMT/HE observations to measure the properties of the quasi-simultaneous X-ray LF QPOs of MAXI J1820+070 on the same day. We compared the centroid frequency, the full width at half maximum (FWHM) and the fractional rms of the LF QPOs in both wavelength ranges. We found that the centroid frequency of the optical QPO is at a frequency of 51.58 mHz, which is consistent with that of the X-ray LF QPO detected on the same day within 1 mHz. We also found that the FWHM of the optical LF QPO is significantly smaller than that of the X-ray LF QPO, indicating that the optical QPO has a higher coherence. The quasi-simultaneous optical and the X-ray LF QPO at a centroid frequency of about 52 mHz suggests that the actual mechanisms of these LF QPOs as the Lense–Thirring precession should work in the vicinity of a radius of about 80–117 gravitational radii (R g = GM/c 2, M is the mass of the black hole) from the black hole if the QPO frequency is related to a proxy of the orbital frequency in the accretion flow as the Lense–Thirring precession model suggests. Furthermore, the apparent higher coherence of the optical QPO favors that it is a more original signal as compared with the X-ray QPO.