An atypical low-frequency QPO detected in the hard state of MAXI J1348-630 with $Insight$-HXMT

TL;DR

This paper reports the discovery of a new atypical low-frequency QPO in the black hole binary MAXI J1348-630 using Insight-HXMT data, observed exclusively in the hard state with stable frequency and energy-dependent amplitude.

Contribution

The study identifies a novel type of LFQPO in MAXI J1348-630 and provides detailed observational characteristics, highlighting that existing models cannot explain this phenomenon.

Findings

Detected in all three Insight-HXMT instruments with 3-5 sigma significance.

Appears only in the hard state during both main and mini outbursts.

Frequency is stable between 0.08 and 0.15 Hz.

Abstract

Based on the $Insight$-HXMT archival data, we have detected a new atypical low-frequency quasi-periodic oscillation (LFQPO) in the black hole X-ray binary MAXI J1348$-$630. The new LFQPO is detected in all the three instruments of $Insight$-HXMT with a combined significance of 3--5 $\sigma$, covering a wide energy range of 1--100 keV. The fractional root-mean-square (RMS) seems decrease with energy. It exclusively appears in the hard state during both the main and mini outburst, spanning an X-ray intensity range by a factor of 10, and a very narrow hardness range. The frequency of this new type of LFQPO is moderately stable, in the range of 0.08--0.15 Hz. We discussed different models for the LFQPO, and found none is able to explain the observed properties of this new type of LFQPO.