An X-ray view of the 2021 outburst of SXP 15.6: constraints on the binary orbit and magnetic field of the Neutron Star

TL;DR

This study provides the first broadband X-ray spectral analysis of SXP 15.6 during its 2021 outburst, revealing insights into its orbital parameters and magnetic field strength, with implications for accretion physics in neutron star binaries.

Contribution

The paper presents the first simultaneous broadband X-ray spectra of SXP 15.6 and constrains its magnetic field and orbital parameters through spectral and temporal analysis.

Findings

Spectral analysis shows most energy above 10 keV with no cyclotron line.

The system exhibits a softer-when-brighter spectral evolution.

Orbital period is approximately 36 days with moderate eccentricity (~0.3).

Abstract

We conducted a spectral and temporal analysis of X-ray data from the Be X-ray binary pulsar SXP 15.6 located in the Small Magellanic Cloud based on NuSTAR, NICER and Swift observations during the 2021 outburst. We present for the first time the broadband X-ray spectra of the system based on simultaneous NuSTAR and NICER observations. Moreover we use monitoring data to study the spectral and temporal properties of the system during the outburst. Comparison of the evolution of the 2021 outburst with archival data reveals a consistent pattern of variability with multiple peaks occurring at time intervals similar to the orbital period of the system (~36 d). Our spectral analysis indicates that most of the energy is released at high energies above 10 keV while we found no cyclotron absorption line in the spectrum. Analysis of the spectral evolution during the outburst, we find that the spectrum is softer-when-brighter, which in turn reveals that the system is probably in the super-critical regime where the accretion column is formed. This places an upper limit to the magnetic field of the system of the order of 7$\times$10$^{11}$ G. The spin-evolution of the neutron star (NS) during the outburst is consistent with an NS with a low magnetic field ($\sim{5}\times$10$^{11}$ G), while there is evident orbital modulation which we modelled and derived the orbital parameters. We found the orbit to have a moderate eccentricity of ~0.3. Our estimates of the magnetic field are consistent with the lack of an electron cyclotron resonance scattering feature in the broadband X-ray spectrum.