2S1553-542: a Be/X-ray binary pulsar on the far side of the Galaxy

TL;DR

This study identifies and characterizes the distant Be/X-ray binary pulsar 2S1553-542 using multi-wavelength observations, revealing its optical counterpart, spectral type, and placing it on the far side of the Milky Way at over 15 kpc.

Contribution

First identification of the optical/infrared counterpart of 2S1553-542 and estimation of its distance and spectral type using combined X-ray, optical, and infrared data.

Findings

Optical/infrared counterpart identified for the first time.

Distance to the system estimated at over 15 kpc.

Spectral type of the companion star is B1-2V.

Abstract

We report the results of a comprehensive analysis of X-ray (Chandra and Swift observatories), optical (Southern African Large Telescope, SALT) and near-infrared (the VVV survey) observations of the Be/X-ray binary pulsar 2S1553-542. Accurate coordinates for the X-ray source are determined and are used to identify the faint optical/infrared counterpart for the first time. Using VVV and SALTICAM photometry, we have constructed the spectral energy distribution (SED) for this star and found a moderate NIR excess that is typical for Be stars and arises due to the presence of circumstellar material (disk). A comparison of the SED with those of known Be/X-ray binaries has allowed us to estimate the spectral type of the companion star as B1-2V and the distance to the system as $>15$ kpc. This distance estimation is supported by the X-ray data and makes 2S1553-542 one of the most distant X-ray binaries within the Milky Way, residing on the far side in the Scutum-Centaurus arm or even further.