On the nature of the X-ray pulsar XTE J1859+083 and its broadband properties

TL;DR

This study analyzes the broadband spectral and timing properties of the X-ray pulsar XTE J1859+083 during its 2015 outburst, revealing its complex pulse profile, typical spectral shape, and likely Be star companion, with implications for its magnetic field.

Contribution

The paper provides the first detailed broadband spectral and timing analysis of XTE J1859+083, estimates its magnetic field strength, and identifies its optical companion as a Be star.

Findings

Pulse profile is energy-dependent and complex.

Pulse fraction remains constant at ~35%, atypical for X-ray pulsars.

Optical companion is likely a Be star with specific spectral lines.

Abstract

This work is devoted to the study of the broadband 0.8-79 keV spectral and timing properties of the poorly studied X-ray pulsar XTE J1859+083 during its 2015 outburst based on the data from the NuSTAR and Swift observatories. We show that the source pulse profile has complex shape that depends on the energy band. Pulse fraction of XTE J1859+083 has constant value around 35% in the broad energy band, this behaviour is atypical for X-ray pulsars. At the same time its energy spectrum is typical of this class of objects and has a power-law shape with an exponential cutoff at high energies. No cyclotron absorption line was discovered in the source spectrum. On the basis of indirect method and the absence of a cyclotron line, an estimation was made for the magnetic field strength as less than $5\times10^{11}$ G or belonging to the interval from $5\times10^{12}$ to $2.0^{+0.9}_{-1.2}\times10^{13}$ G. Data from the NOT and SALT telescopes as well as optical and IR sky surveys allowed us also to study the nature of its optical companion. We have proposed and studied new possible candidates for the optical companion of XTE J1859+083 and the most likely candidate was identified. The results of the optical and IR photometry and spectroscopy of these possible companions showed that the system is a Be X-ray binary, showing Br$\gamma$, He I and strong H$\alpha$ spectral lines.