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

TL;DR

This study investigates the origin of optical emission during the 2019 mini-outburst of the black hole X-ray binary MAXI J1348-630 by combining optical data from the Las Cumbres Observatory with X-ray data from Insight-HXMT. Using ICCF time-delay analysis, the X-ray Comptonization flux is found to lag the optical light by about 8.5 days, and the optical–X-ray flux follows a shallow power-law with beta ≈ 0.40–0.43, consistent with a disk-dominated scenario under the disk instability model (DIM). SEDs constructed from quasi-simultaneous optical and X-ray data are well described by an irradiated outer disk with negligible jet contribution, supporting the view that optical emission during the mini-outburst arises from the disk rather than the jet or hot accretion flow. Collectively, these results reinforce the DIM as a key mechanism for faint, mini-outbursts in LMXBs and demonstrate the crucial role of irradiation and disk physics in shaping multiwavelength emission and state-transition behavior, with optical monitoring providing essential diagnostics for disk evolution.

Abstract

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.

Figures (6)

• Figure 1: The light curves of MAXI J1348-630 during the mini-outburst. The top panel shows the X-ray count rates observed by Insight-HXMT and the middle panel presents the optical magnitudes observed by LCO. The bottom panel displays the Comptonization flux.
• Figure 2: Cross-correlation analysis between the Comptonization flux and different optical bands ($g'$, $r'$, and $i'$ bands for the top, middle, and bottom panels, respectively). In all panels, the red line indicates the cross-correlation function (left axis), and the green and blue histograms are the cross-correlation peak delay distribution and the centroid delay distribution (right axis).
• Figure 3: The power-law correlation between the Comptonization flux and different optical bands (see the legend). The solid lines are the best fits, and the shaded regions indicate uncertainties at the $1\, \sigma$ level.
• Figure 4: Best-fit unabsorbed SED of MAXI J1348-630 at MJD 58643. The blue and red data points correspond to the observation of Insight-HXMT and LCO. The radio data (green points) were observed by ACTA, presented by Carotenuto2021. The black line represents the best-fit of model tbabs*diskir. The green dotted line is a power-law fit to radio data. The bottom-right panel is a zoom-in of the X-ray and optical fits, along with the fit residuals.
• Figure 5: Best-fit unabsorbed SED of MAXI J1348-630. The blue and red data points correspond to the observation of Insight-HXMT and LCO. The black line represents the best-fit of model tbabs*diskir.