Broad-band analysis of X-ray pulsar 2S 1845$-$024

TL;DR

This study provides a comprehensive broadband X-ray analysis of the pulsar 2S 1845-024, revealing spectral properties, high absorption near periastron, and identifying its optical counterpart as an OB supergiant at over 15 kpc.

Contribution

First detailed broadband spectral analysis of 2S 1845-024 during outburst, including localization and optical counterpart classification.

Findings

No cyclotron absorption feature detected in spectra.

High hydrogen column density (~10^24 cm^-2) near periastron.

Optical counterpart identified as OB supergiant at >15 kpc.

Abstract

We present results of detailed investigation of the poorly studied X-ray pulsar 2S 1845$-$024 based on the data obtained with $NuSTAR$ observatory during the type I outburst in 2017. Neither pulse phase-averaged, nor phase-resolved spectra of the source show evidence for a cyclotron absorption feature. We also used the data obtained from other X-ray observatories ($Swift$, $XMM-Newton$ and $Chandra$) to study the spectral properties as a function of orbital phase. The analysis revealed a high hydrogen column density for the source reaching $\sim$10$^{24}$ cm$^{-2}$ around the periastron. Using high-quality $Chandra$ data we were able to obtain an accurate localization of 2S 1845$-$024 at R.A. = 18$^{h}$48$^{m}$16$^{s}$.8 and Dec. = $-$2$^{\circ}$25'25".1 (J2000) that allowed us to use infrared (IR) data to roughly classify the optical counterpart of the source as an OB supergiant at the distance of $\gtrsim$15 kpc.