Counts lightcurves for GRB 150101B as observed by Fermi GBM (top) and Swift BAT (bottom) in the 15–150 keV (right) and >150 keV energy ranges (left). Shown are 4 ms lightcurves in black and a background estimate in red. Time axis values are relative to the T₀ times of each instrument (GBM on top with T₀ defined as the trigger time, and BAT on bottom with T₀ defined as the start of the T100 interval since BAT did not trigger on-board). The lightcurves are aligned by correcting for light travel time between Fermi and Swift (a difference of 1.35 ms, with the burst arriving at Swift first). The standard duration measure of each instrument are shown with the BAT T100 interval in blue and the GBM T₉₀ interval in green.