Multi-band power color-color diagrams of three black hole X-ray binaries observed with Insight-HXMT

Abstract

Power color-color diagrams (PCCDs) provide a useful diagnostic tool for studying the evolution of outbursts in black hole X-ray binaries. In this paper, we present power color-color diagrams of three sources (MAXI J1348-630, MAXI J1820+070 and Swift J1727.8-1613) observed with Insight-HXMT in a wide energy range of 2-80 keV. We compared the hue regions defined by RXTE, which are associated with different spectral states, with the Insight-HXMT results. We find that, for hard and hard-intermediate states, the trajectories of Insight-HXMT in the power color-color diagrams are generally consistent with those of RXTE. In the soft and soft-intermediate states, weak variability generally prevents robust hue constraints. Nevertheless, a few points in MAXI J1348-630 deviate from the RXTE-defined regions, possibly because of averaged variable power spectra and the presence of a type-A QPO. The trajectories of MAXI J1348-630 and MAXI J1820+070 exhibited roughly consistent patterns over different energy bands, whereas Swift J1727.8-1613 was an exception during its very high state, caused by an additional low-frequency component in the low-energy band. We found that the very high state can be identified through the power color-color diagram, exhibiting a hue similar to that of the hard-intermediate state but not forming a loop pattern. We also investigated the relationship between hue and hardness and found that, although they are generally anti-correlated, they provide consistent timing for the spectral state transitions.