Flash light curve of XMM J202154.7+402855 obtained from MOS 1/2. The epoch zero of the merged light curve corresponds to MJD 56028.31 related to the selected GTI of XMM archive investigated in 2012. The light curve of XMM J202154.7+402855 was binned with 200 s labeled by cross signs. The highest peak reaches to   ~  0.4 counts s⁻¹, and all the error bars of data points indicate the 1σ uncertainty. Data points labeled by diamonds demonstrate the cooling tail of the burst, and the red and blue lines are the best fits to a PL model with a free index and a fixed index at −5/3, respectively. All the obtained parameters are presented in Table 2.