tokens: counter rotation was produced in the first place, the formation of retrograde accretion discs has also been proposed to explain the long-term evolution of spin periods in X-ray binaries ( Nelson et al. 1997 ; Christodoulou et al. 2017 ). However, for the time window 2010–2015 we can only speculate for the evolution of Ṁ due to lack of observational coverage and because of the high absorption of the X-ray spectrum (see Fig. 3 of C18). Nevertheless, the P and Ṗ measurements during the 2014 Chandra observation are consistent with the constant Ṁ assumption (see Fig. 3 ), supporting our finding that the decrease of the observed flux between 2010 and 2015 is only due to high absorption. If Ṁ during that period was two orders of magnitude lower than after 2015 then it is possible that there was no NS spin reversal. In this case the NS spin period should have been larger than a few hundred seconds prior to the SN 2010da event. In general there are multiple ways to maintain the necessary Ṁ to fuel ULXPs. For systems fueled by Roche Lobe (RL) overflow, the accretion flow carries the angular momentum of the orbit. It is therefore implausible for an accretion disc to switch between retrograde and prograde rotation ( Chakrabarty et al. 1997 ), and any change between retrograde and prograde rotation of the NS disc system needs to be due to a change in the direction of the NS rotation. Therefore, for NGC 300 ULX1 it is more plausible that the accretion mechanism was drastically altered during the SN 2010da event, prior to which the NS could have been fueled via wind accretion. A possible explanation for such a phase transition to occur can be an episode of thermal-timescale mass transfer ( King et al. 2001 ). In that case the donor star has a radiative envelope and fills its RL as it expands across the Hertzsprung gap after the wind-fed X-ray phase ends. However, RL overflow is not the only way to explain such high accretion rates. Supergiant systems and luminous blue variables (LBV) can also exhibit mass loss rates in excess of 10 −6 M ⊙ yr −1 ( Smith 2014 ), and therefore for a binary system in a moderately close orbit the compact object can capture enough matter to sustain super-Eddington accretion. An LBV is also most probable according to Binder et al. (2016) while other authors have deemed this classification inconclusive ( Lau et al. 2016 ; Villar et al. 2016 ). We note that many of the arguments used in the classification of the system were based on the decay of the X-ray flux observed after the SN 2010da event. However, the decay in the X-ray flux was most likely a result of rapid increase in X-ray absorption (C18). 4. Conclusions We have studied the spin period evolution of NGC 300 ULX1, the most extreme spin-up NS powered by an accretion disc. We derived precise measurements of P and Ṗ for different epochs, and applied theoretical models of accretion theory in order to estimate the magnetic field strength of the NS. Even though many of the models
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