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

We report on the monitoring of the final stage of the outburst from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124, which reached ∼40 Eddington luminosities. The main aim of the monitoring program with the Swift/XRT telescope was to measure the magnetic field of the neutron star using the luminosity of transition to the `propeller' state. The visibility constraints, unfortunately, did not permit us to observe the source down to the fluxes low enough to detect such a transition. The tight upper limit on the propeller luminosity L_prop < 6.8 × 10³⁵ erg s^-1 implies the dipole component of the magnetic field B < 10¹³ G. On the other hand, the observed evolution of the pulse profile and of the pulsed fraction with flux points to a change of the emission region geometry at the critical luminosity L_crit ∼ 3 × 10³⁸ erg s^-1 both in the rising and declining parts of the outburst. We associate the observed change with the onset of the accretion column, which allows us to get an independent estimate of the magnetic field strength close to the neutron stars surface of B > 10¹³ G. Given the existing uncertainty in the effective magnetosphere size, we conclude that both estimates are marginally compatible with each other.