Detection of a cyclotron line in SXP 15.3 during its 2017 outburst

TL;DR

This study reports the detection of a cyclotron line in the X-ray spectrum of SXP 15.3 during its 2017 outburst, revealing the neutron star's magnetic field strength through broadband timing and spectral analysis.

Contribution

First broadband detection of a cyclotron line in SXP 15.3, providing direct measurement of its magnetic field during an outburst.

Findings

Detection of a ~5 keV cyclotron line indicating a magnetic field of 6x10^{11} G.

Energy-dependent pulse profile changing from double peaked to single broad pulse.

Spectral hardening during intensity dip explains pulse profile morphology.

Abstract

We report the results of AstroSat and NuSTAR observations of the Be/X-ray binary pulsar SXP 15.3 in the Small Magellanic Cloud during its outburst in late 2017, when the source reached a luminosity level of ~ 10^{38} erg s^{-1}, close to the Eddington limit. The unprecedented broadband coverage of the source allowed us to perform timing and spectral analysis between 3 and 80 keV. The pulse profile exhibits a significant energy dependence, and morphs from a double peaked profile to a single broad pulse at energies >15 keV. This can be explained by a spectral hardening during an intensity dip seen between the two peaks of the pulse profile. We detect a Cyclotron Resonance Scattering Feature (CRSF) at ~5 keV in the X-ray spectrum, independent of the choice of the continuum model. This indicates a magnetic field strength of 6x10^{11} G for the neutron star.