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We report on the monitoring of the final stage of the outburst from the firstGalactic ultraluminous X-ray pulsar Swift J0243.6+6124, which reached $\sim$40Eddington luminosities at the peak of the outburst. The main aim of themonitoring program with the {\it Swift}/XRT telescope was to measure themagnetic field of the neutron star using the luminosity of transition to the"propeller" state. The visibility constraints, unfortunately, did not permit usto observe the source down to the fluxes low enough to detect such atransition. The tight upper limit on the propeller luminosity $L_{\rmprop}<6.8\times10^{35}$ erg s$^{-1}$ implies the dipole component of themagnetic field $B<10^{13}$ G, under plausible assumptions on the effectivemagnetosphere size. On the other hand, the observed evolution of the pulseprofile and of the pulsed fraction with flux points to a change of the emissionregion geometry at the critical luminosity $L_{\rm crit}\sim3\times10^{38}$ ergs$^{-1}$ both in the rising and declining parts of the outburst. We associatethe observed change with the onset of the accretion column, which allows us toget an independent estimate of the magnetic field strength close to the neutronstars surface of $B>10^{13}$ G. Given the existing uncertainty in the effectivemagnetosphere size, we conclude that both estimates are marginally compatiblewith each other.

On the magnetic field of the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124