Chandra and Swift X-ray Observations of the X-ray Pulsar SMC X-2 During the Outburst of 2015

TL;DR

This paper presents Chandra and Swift observations of the 2015 outburst of the SMC X-2 pulsar, confirming its optical counterpart, deriving orbital parameters, and analyzing X-ray dips and outflow effects during the outburst.

Contribution

It unambiguously identifies the optical companion, provides precise orbital parameters, and discusses the outburst evolution and possible eclipse phenomena.

Findings

Confirmed O-type star as the optical counterpart

Derived accurate orbital parameters from TOA analysis

Identified X-ray dips possibly caused by eclipses or wind shadows

Abstract

We report the Chandra/HRC-S and Swift/XRT observations for the 2015 outburst of the high-mass X-ray binary (HMXB) pulsar in the Small Magellanic Cloud, SMC X-2. While previous studies suggested that either an O star or a Be star in the field is the high-mass companion of SMC X-2, our Chandra/HRC-S image unambiguously confirms the O-type star as the true optical counterpart. Using the Swift/XRT observations, we extracted accurate orbital parameters of the pulsar binary through a time of arrivals (TOAs) analysis. In addition, there were two X-ray dips near the inferior conjunction, which are possibly caused by eclipses or an ionized high-density shadow wind near the companion's surface. Finally, we propose that an outflow driven by the radiation pressure from day ~10 played an important role in the X-ray/optical evolution of the outburst.