Evolution of the NS spin period of NGC 300 ULX1 since the SN 2010da event (blue vertical dashed line). The black solid line denotes the theoretical prediction assuming a constant accretion rate and stellar torque from disc-magnetosphere interactions (Wang 1995) with efficiency = 1.0, B = 5 × 10¹² G and L_x = 4   ×  10³⁹ erg s⁻¹. The grey shaded area marks the evolution path for a range of L_X, i.e. 3–5  ×  10³⁹ erg s⁻¹. The inset plot shows the predicted evolution of the spin period for the subsequent years, with the lone data point being the ∼19 s Swift measurement (MJD 58221).