X-ray pulsar XTE J1858+034: discovery of the cyclotron line and the revised optical identification

TL;DR

This study reports the discovery of a cyclotron line at ~48 keV in the X-ray pulsar XTE J1858+034, refines its optical identification, and suggests it is a symbiotic binary rather than a Be X-ray binary.

Contribution

The paper presents the first detection of a cyclotron line in XTE J1858+034 and provides a revised optical counterpart identification, changing the understanding of the system's nature.

Findings

Cyclotron absorption feature at ~48 keV detected

Precise localization of the source achieved

Optical companion identified as a late-type star

Abstract

We present results of a detailed investigation of the poorly studied X-ray pulsar XTE J1858+034 based on the data obtained with the NuSTAR observatory during the outburst of the source in 2019. The spectral analysis resulted in the discovery of a cyclotron absorption feature in the source spectrum at ~48 keV both in the pulse phase averaged and resolved spectra. Accurate X-ray localization of the source using the NuSTAR and Chandra observatories allowed us to accurately determine the position of the X-ray source and identify the optical companion of the pulsar. The analysis of the counterpart properties suggested that the system is likely a symbiotic binary hosting an X-ray pulsar and a late type companion star of K-M classes rather than Be X-ray binary as previously suggested.