# The SMC X-ray binary SXP4.78 : a new Type II outburst and the identification and study of the optical counterpart

## TL;DR

This study reports the detection of a new Type II outburst in the SMC X-ray binary SXP4.78, identifies its optical counterpart, and analyzes its long-term variability, disc growth, and spectral classification.

## Contribution

It provides the first accurate localization of SXP4.78 during an outburst, characterizes its optical counterpart, and investigates its disc variability and spectral type.

## Key findings

- Detected a new Type II outburst in 2018.
- Identified the optical counterpart and classified it as B0.5 IV-V.
- Observed long-term disc variability and growth during the outburst.

## Abstract

SXP4.78 was originally discovered in 2000 as a pulsar in the Small Magellanic Cloud (SMC) by the Rossi X-ray Timing Explorer (RXTE) but it was not spatially located at that time. A new detection in 2018 with the Neil Gehrels Swift Observatory during a Type II outburst permitted its position to be accurately located and its optical counterpart identified. We report X-ray and optical monitoring covering epochs before and during the outburst. Using photometric data we show the long-term variability of the Be disc where we present flux and colour changes associated with the disc growth and decay over a period of ~6000 days. We show evidence of disc growth during the recent outburst through an increase in the H-alpha equivalent width and photometric flux. Period analysis was performed using both optical photometric and spectroscopic data, but with no significant detection of an orbital period. A modest periodic signature of 2.65 days was detected from the OGLE I band data, however, but we attribute that to the non-radial pulsations (NRPs) of the Be star. We also obtained a blue spectrum from the Southern African Large Telescope (SALT) which permits us to classify the spectral type as B0.5 IV-V.