Broad-band noises in GX 339-4 during the 2021 outburst observed with Insight-HXMT and NICER

Rapid X-ray variability of GX 339−4 including the low-frequency quasi-periodic oscillations (LFQPOs) and broad-band noises were observed with the Hard X-ray Modulation Telescope (Insight-HXMT) and Neutron star Interior Composition Explorer (NICER) during the 2021 outburst. Here we present a systematic study of the evolution and energy dependence properties of such broad-band noises (BBNs). The outburst from February to March of 2021 can be divided into three stages: the low hard state (LHS), the hard intermediate state (HIMS), and soft intermediate state (SIMS). In the PDSs of the LHS and HIMS, the broad-band noises were well fitted with three Lorentzian components: a low-frequency component L 1 , a middle-frequency component L 2 , and a high-frequency component L 3 . The increasing trend of the characteristic frequencies for L 1 and L 2 and the relation between the QPO frequency and characteristic BBN frequency are reported. We find that the energies corresponding to the peaks and shapes of the rms spectra for three BBN components are different. The comparison among three BBN components indicates that energy-dominant bands of these BBN components are distinct. Our results can be explained with the truncated disc and hot flow model with a large variable disc and a small hot inner flow. A possible description of the accretion structure and its evolution from the LHS to the SIMS is proposed. Further research is still required to probe such an accretion structure in GX 339-4.