SRG/ART-XC, Swift, NICER and NuSTAR study of different states of the transient X-ray pulsar MAXI J0903-531

The results of the broadband spectral and timing study of the recently discovered transient X-ray pulsar MAXI J0903-531 in a wide range of luminosities differing by a factor of ~30 are reported. The observed X-ray spectrum in both states can be described as a classical pulsar-like spectrum consisting of the power-law with the high-energy cutoff. We argue that absence of the spectrum transformation to the two-hump structure expected at low fluxes points to a relatively weak magnetic field of the neutron star below (2-3)$\times10^{12}$ G. This estimate is consistent with other indirect constraints and non-detection of any absorption features which can be interpreted as a cyclotron absorption line. Timing analysis of the NuSTAR data revealed only slight variations of a single-peaked pulse profile of the source as a function of the energy band and mass accretion rate. In both intensity states the pulsed fraction increases from 40% to roughly 80% with the energy. Finally we were also able to obtain the orbital solution for the binary system using data from the Fermi/GBM, NICER and NuSTAR instruments.