The 3-min counting-rate profiles of the NM-64 for one X-class and three M-class flares. The left- and right-upper panels and the left-lower panel indicate those for the M5.4 (2010/11/06), M6.6 (2011/02/13), and M3.3 (2012/06/03) flares, respectively. The right-lower panel corresponds to the X2.8 (2013/05/13) flare. The interval of the signal window is indicated by horizontal and vertical violet lines. The red thick line represents the average background for 20 min. The red dashed lines which follow the average background are ±1σ curves. The dashed blue line and the label time show the flare onset (t
0) according to GOES-SXR. The abscissa in all plots represent the time in UT.