Light curves and hardness ratios of EXO 1846−031 during its 2019 outburst as functions of MJD. Top panel shows the light curves in the soft bands: 0.3 − 4 keV for NICER/XTI and Swift/XRT; 2 − 4 keV for MAXI/GSC. Middle panel shows the light curves in hard bands: 4 − 6 keV for NICER/XTI, 4 − 10 keV for Swift/XRT and MAXI/GSC. Bottom panel shows the corresponding hardness ratio for NICER/XTI (4 − 6 keV/0.3 − 4 keV), Swift/XRT (4 − 10 keV/0.3 − 4 keV), and MAXI/GSC (4 − 10 keV/2 − 4 keV), respectively. The NICER (black circles) and Swift data (open circles) are depicted on the left y-axis, and the MAXI data (blue triangles) are depicted on the right y-axis. The green and red bars represent the times of the NuSTAR and HXMT observations.