Measuring the Magnetic Field Strength of a Transient X-ray Pulsar 4U 1901+03 by Its Variability on Different Time Scales

By combining data from several X-ray telescopes (Swift/XRT, NICER, Chandra) we reconstructed a profile of the 2019 outburst of accreting X-ray pulsar 4U 1901+03 from its peak down to the return to the ‘‘low’’ state. Softening of the X-ray spectrum and disappearance of the pulsations at a later stages of the outburst tentatively indicates that the source transitioned to the ‘‘propeller’’ state at luminosity about $$10^{36}$$ erg s $${}^{-1}$$ , which corresponds to a magnetic field strength of $$B\lesssim 10^{12}$$ G at the neutron star surface. We also investigated the fast X-ray variability that was observed during the peak of 2003 outburst. The shape of the power spectrum could be described with a broken power law, with a break frequency of 7.5 Hz. Assuming that this frequency corresponds to a viscous frequency at a magnetospheric boundary we estimated magnetic field strength as $$B\approx 7\times 10^{11}$$ G, which is similar to the estimate obtained from observed transition to the ‘‘propeller’’ regime.