High mass X-ray binary SXP18.3 undergoes the longest Type II outburst ever seen in the Small Magellanic Cloud

TL;DR

This paper reports the longest observed Type II outburst from the high mass X-ray binary SXP18.3 in the Small Magellanic Cloud, enabling detailed analysis of its pulse timing and orbital characteristics.

Contribution

It provides the first extensive pulse timing study of an SMC Be/X-ray binary during a prolonged outburst, proposing a full orbital solution.

Findings

Longest Type II outburst observed in SMC HMXB

Proposed orbital period of 17.79 days for SXP18.3

First detailed pulse timing analysis over a 10-year span

Abstract

On 30th August 2006 SXP18.3 a high mass X-ray binary (HMXB) in the Small Magellanic Cloud (SMC) with an 18.3s pulse period was observed by Rossi X-ray Timing Explorer (RXTE). The source was seen continuously for the following 36 weeks. This is the longest Type II outburst ever seen from a HMXB in the SMC. During the outburst SXP18.3 was located from serendipitous XMM-Newton observations. The identification of the optical counterpart has allowed SXP18.3 to be classified as a Be/X-ray binary. This paper will report on the analysis of the optical and weekly RXTE X-ray data that span the last 10 years. The extreme length of this outburst has for the first time enabled us to perform an extensive study of the pulse timing of an SMC Be/X-ray binary. We present a possible full orbital solution from the pulse timing data. An orbital period of 17.79d is proposed from the analysis of the OGLE III light curve placing SXP18.3 on the boundary of known sources in the Corbet diagram.