2025MNRAS.537.1146S

We present a comprehensive spectral and timing analysis of 15 outbursts and 18 mini-outbursts from nine dynamically confirmed black hole X-ray transients with light curve and spectral data from RXTE, MAXI, and NuSTAR obtained from 1996 to 2024. Departed from the canonical fast-rise exponential decay (FRED) morphology, the most common morphology within our sample is triangular with similar rise and decay time-scale. In most outbursts, the spectral evolutions indicate the presence of limit-cycle instability, as predicted by the disc instability model (DIM). Even though almost all of the outbursts showed a similar canonical pattern, unique transition patterns are found in FRED outbursts. On the other hand, no spectral transition is found in any mini-outburst, which was observed in either hard or thermal-dominant (TD) state only. The Fe K $\alpha$ emission line is the most prominent feature in the hard state of the rising phase but none is found in the decay phase. Triangular outbursts are always in transition to the TD state, following a standard accretion disc, before the peak proceeds to match DIM prediction. This is unlike the FRED outburst which directly transitioned to the steep power law (SPL) state or high Eddington ratio TD state, resembling a slim accretion disc. Non-canonical spectral evolution as well as the rarity of FRED outburst in our sample, seem to add more challenges for DIM. Studying the morphology of outburst light curve may reveal more clues on the evolution of the disc instability at least during the time relevant to the burst.