Orbit and intrinsic spin-up of the newly discovered transient X-ray pulsar Swift J0243.6+6124

TL;DR

This paper reports the orbital parameters and spin-up behavior of the transient X-ray pulsar Swift J0243.6+6124, revealing a strongly magnetized neutron star at approximately 5 kpc distance during its 2017 outburst.

Contribution

It provides the first orbital solution and models the spin variations driven by accretion torque for Swift J0243.6+6124.

Findings

Neutron star is strongly magnetized.

Estimated distance to the source is about 5 kpc.

Observed spin-up correlates with accretion torque during outburst.

Abstract

We present the orbital solution for the newly discovered transient Be X-ray binary Swift J0243.6+6124 based on the data from gamma-ray burst monitor onboard Fermi obtained during the Oct 2017 outburst. We model the Doppler induced and intrinsic spin variations of the neutron star assuming that the later is driven by accretion torque and discuss the implications of the observed spin variations for the parameters of the neutron star and the binary. In particular we conclude that the neutron star must be strongly magnetized, and estimate the distance to the source at $\sim$5 kpc.