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

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.

13 pages, 16 figures, accepted for publication in MNRAS

References in corpus (6)

• A Long Look at the Be/X-Ray Binaries of the Small Magellanic Cloud
• Spectral distribution of Be/X-ray binaries in the Small Magellanic Cloud
• Constraining the orbital history of the Magellanic Clouds: A new bound scenario suggested by the tidal origin of the Magellanic Stream
• On the metallicity dependence of HMXBs
• XMM-Newton observations of the Small Magellanic Cloud: Be/X-ray binary pulsars active between October 2006 and June 2007
• Fermi results on gamma-ray binaries

Cited by in corpus (6)

• Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud
• Unveiling the nature of INTEGRAL objects through optical spectroscopy. X. A new multi-year, multi-observatory campaign
• A Comprehensive Library of X-ray Pulsars in the Small Magellanic Cloud: Time Evolution of their Luminosities and Spin Periods
• The First Year of S-CUBED: The Swift Small Magellanic Cloud Survey
• Discovery of a 26.2 day period in the long-term X-ray light curve of SXP 1323: a very short orbital period for a long spin period pulsar
• An X-ray and optical study of the outbursting behaviour of the SMC Be X-ray binary SXP 91.1