An X-ray and optical study of the outbursting behaviour of the SMC Be X-ray binary SXP 91.1

Abstract

In this paper we report on the optical and X-ray behaviour of the Be X-ray binary, SXP 91.1, during a recent type I outburst. We monitored the outburst using the Neil Gehrels Swift Observatory. These data were supported by optical data from the Southern African Large Telescope (SALT) and the Optical Gravitational Lensing Experiment (OGLE) to show the circumstellar disc activity. Matter from this disc accretes onto the neutron star, giving rise to the X-ray outburst as seen in the synchronous evolution of the optical and X-ray lightcurves. Using data taken with OGLE we show that the circumstellar disc has exhibited stable behaviour over two decades. A positive correlation is seen between the colour and magnitude from the OGLE and MACHO observations, which indicates that the disc is orientated at relatively low inclination angles. From the OGLE and Swift data, we demonstrate that the system has shown relative phase offsets that have persisted for many years. The spin period derivative is seen to be at maximum spin-up at phases when the mass accretion rate is at maximum. We show that the neutron star in SXP 91.1 is an unusual member of its class in that it has had a consistent spin period derivative over many years, with the average spin-up rate being one of the highest for known SMC pulsars. The most recent measurements of the spin-up rate reveal higher values than the global trend, which is attributed to the recent mass accretion event leading to the current outburst.

arXiv commentAccepted for publication in MNRAS

References in corpus (22)

• The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission
• Astrophysics Source Code Library
• The Carnegie Hubble Program: The Distance and Structure of the SMC as Revealed by Mid-infrared Observations of Cepheids
• Giant Outbursts in Be/X-ray Binaries
• The Orbit of X Per and Its Neutron Star Companion
• MN48: a new Galactic bona fide luminous blue variable revealed by Spitzer and SALT
• Spin period change and the magnetic fields of neutron stars in Be X-ray binaries in the Small Magellanic Cloud
• Catalogue of Be/X-ray binary systems in the Small Magellanic Cloud: X-ray, optical & IR properties
• 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
• Discovery of a Cyclotron Resonant Scattering Feature in the RXTE Spectrum of 4U 0352+309 (X Per)
• Very Long-term Optical Variability of High Mass X-ray Binaries in the SMC
• The remarkable light and colour variability of Small Magellanic Cloud Be stars
• The Be/X-ray transient 4U 0115+63/V635 Cassiopeiae. III. Quasi-cyclic variability
• Precessing Warped Be Disk Triggering the Giant Outbursts in 2009 and 2011 in A 0535+262/V725 Tau
• Long-term variability of high-mass X-ray binaries. I.Photometry
• On the periodicities present in the optical light curves of SMC Be/X-ray binaries
• On the relationship between circumstellar disc size and X-ray outbursts in Be/X-ray binaries
• The First Year of S-CUBED: The Swift Small Magellanic Cloud Survey
• Discovery of a QPO in the X-ray pulsar 1A 1118-615: correlated spectral and aperiodic variability
• The SMC X-ray binary SXP4.78 : a new Type II outburst and the identification and study of the optical counterpart
• Contrasting behaviour from two Be/X-ray binary pulsars: insights into differing neutron star accretion modes

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• Optical and X-ray study of the peculiar high mass X-ray binary XMMU J010331.7-730144