A comparative study on the spectra of normal outburst and mini-outbursts of black hole X-ray binary XTE J1550-564

One normal outburst and three mini-outbursts have been detected by Rossi X-ray Timing Explorer (RXTE) satellite after 2000 in the well-known black hole X-ray binary XTE J1550-564. In our work, we explored the energy spectral properties of the four outbursts and the three mini-outbursts retained in the Low/Hard state. The results show that the X-ray spectra of the three mini-outbursts are harder than that of normal outburst and have no disk component. In the correlation analysis, the $\Gamma -L_X/L_{\mathrm{Edd}}$ anti-relationship of the three mini-outbursts suggested that it may be a result of the incomplete evolutionary success of Shakura-Sunyaev disk. The whole X-ray spectral correlation evolution shows well the state transition from the Shakura-Sunyaev disk to the advection-dominated accretion flow. We think that the three mini-outbursts may originated from a small discrete accretion event.