Insight-HXMT observations of Swift J0243.6+6124 during its 2017-2018 outburst

TL;DR

This study uses Insight-HXMT data to analyze the broadband spectrum of the neutron star transient Swift J0243.6+6124 during its 2017-2018 outburst, estimating its magnetic field and luminosity without detecting cyclotron lines.

Contribution

It provides the first broadband spectral analysis of Swift J0243.6+6124 during outburst and estimates its magnetic field and luminosity without observing cyclotron features.

Findings

Magnetic field estimated at ~10^13 G

Peak luminosity exceeds 10^39 erg/s

No cyclotron line detected up to 150 keV

Abstract

The recently discovered neutron star transient Swift J0243.6+6124 has been monitored by {\it the Hard X-ray Modulation Telescope} ({\it Insight-\rm HXMT). Based on the obtained data, we investigate the broadband spectrum of the source throughout the outburst. We estimate the broadband flux of the source and search for possible cyclotron line in the broadband spectrum. No evidence of line-like features is, however, found up to $\rm 150~keV$. In the absence of any cyclotron line in its energy spectrum, we estimate the magnetic field of the source based on the observed spin evolution of the neutron star by applying two accretion torque models. In both cases, we get consistent results with $B\rm \sim 10^{13}~G$, $D\rm \sim 6~kpc$ and peak luminosity of $\rm >10^{39}~erg~s^{-1}$ which makes the source the first Galactic ultraluminous X-ray source hosting a neutron star.