On the optical emission in the mini-outburst of the black hole X-ray binary MAXI J1348-630

We investigate the optical emission of the black hole X-ray binary MAXI J1348-630 during its 2019 minioutburst. Using optical data from the Las Cumbres Observatory Global Telescope and X-ray data from Insight-HXMT, we performed time delay analysis, optical-X-ray correlation analysis, and spectral energy distribution (SED) fitting. Our key findings are as follows: (1) The X-ray Comptonization flux lags behind the optical emission by about 8.5 days, a delay naturally explained by the disk instability model (DIM). (2) The optical and X-ray fluxes show a power-law correlation with a slope about 0.4, which lies between the predicted values for viscous heating and X-ray reprocessing, consistent with the DIM framework. (3) SED fitting with the irradiated disk model successfully reproduces the quasi-simultaneous optical and X-ray data, and the contribution of the jet is negligible. Our results indicate that the optical emission during the mini-outburst originates from the disk, rather than the jet or hot accretion flow, and highlight the critical role of the DIM in understanding the mini-outburst of X-ray binaries.