Contrasting behaviour from two Be/X-ray binary pulsars: insights into differing neutron star accretion modes

TL;DR

This study identifies and characterizes two Be/X-ray binary pulsars in the Small Magellanic Cloud, revealing different neutron star accretion behaviors and providing new orbital and spectral data for one of the systems.

Contribution

The paper presents the first detailed characterization of a new Be/X-ray binary pulsar and clarifies the identity of a previously known pulsar, highlighting differing accretion modes.

Findings

Discovery of a new Be/X-ray binary pulsar with orbital period 87 days.

Confirmation that the second pulsar is the previously known SXP91.1.

Evidence of continuous spin-up during X-ray quiescence for SXP91.1.

Abstract

In this paper we present the identification of two periodic X-ray signals coming from the direction of the Small Magellanic Cloud (SMC). On detection with the Rossi X-ray Timing Explorer (RXTE), the 175.4s and 85.4s pulsations were considered to originate from new Be/X-ray binary (BeXRB) pulsars with unknown locations. Using rapid follow-up INTEGRAL and XMM-Newton observations, we show the first pulsar (designated SXP175) to be coincident with a candidate high-mass X-ray binary (HMXB) in the northern bar region of the SMC undergoing a small Type II outburst. The orbital period (87d) and spectral class (B0-B0.5IIIe) of this system are determined and presented here for the first time. The second pulsar is shown not to be new at all, but is consistent with being SXP91.1 - a pulsar discovered at the very beginning of the 13 year long RXTE key monitoring programme of the SMC. Whilst it is theoretically possible for accreting neutron stars to change spin period so dramatically over such a short time, the X-ray and optical data available for this source suggest this spin-up is continuous during long phases of X-ray quiescence, where accretion driven spin-up of the neutron star should be minimal.