SXP 7.92: A Recently Rediscovered Be/X-ray Binary in the Small Magellanic Cloud, Viewed Edge On

Abstract

We present a detailed optical and X-ray study of the 2013 outburst of the Small Magellanic Cloud Be/X-ray binary SXP 7.92, as well as an overview of the last 18 years of observations from OGLE, RXTE, Chandra and XMM-Newton. We revise the position of this source to RA(J2000)=00:57:58.4, Dec(J2000)=-72:22:29.5 with a 1σ uncertainty of 1.5 , correcting the previously reported position by Coe et al. (2009) by more than 20 arcminutes. We identify and spectrally classify the correct counterpart as a B1Ve star. The optical spectrum is distinguished by an uncharacteristically deep narrow Balmer series, with the Hα line in particular having a distinctive shell profile, i.e. a deep absorption core embedded in an emission line. We interpret this as evidence that we are viewing the system edge on and are seeing self obscuration of the circumstellar disc. We derive an optical period for the system of 40.0±0.3 days, which we interpret as the orbital period, and present several mechanisms to describe the X-ray/Optical behaviour in the recent outburst, in particular the "flares" and "dips" seen in the optical light curve, including a transient accretion disc and an elongated precessing disc.

Discovery of a magnetar candidate X-ray pulsar in the Large Magellanic Cloud

“…Accreting X-ray pulsars at similar ( ∼7 s) periods, instead, show a luminosity which is at least one order of magnitude higher ( ∼ 10 35 erg s −1 ), with the least luminous XRBs at these periods in the MCs being LXP 8.04 at L X 1 . 5 × 10 35 erg s −1 (Vasilopoulos et al 2014 ) and SXP 7.92 at L X 1 × 10 36 erg s −1 (Bartlett et al 2017 ).…”

“…Top panel: Distribution of the pulsed fraction of the magnetars reported in the MOOC catalogue (Coti Zelati et al 2018 ) at the peak of the outb urst. Bottom panel: Distrib ution of the pulsed fraction of the BeXRBs in the MCs during active phases (Coe & Kirk 2015 ; Antoniou & Zezas 2016 ;Bartlett et al 2017 ;Boon et al 2017 ;Haberl et al 2017Haberl et al , 2022a Haberl et al , b , 2023bKennea et al 2017 ;Vasilopoulos et al 2017 ; Koliopanos & Vasilopoulos 2018 ;Maitra et al 2018Maitra et al , 2021Maitra et al , 2023 La Palombara et al 2018a , b ; Carpano et al 2022 , and references therein) and the pulsars in LMXBs Her X-1, GRO J1744-28, IGR J16358-4726, 3XMM J181923.7-170616, and Swift J1843.5-0343. For these last sources, we refer to the LMXBs XRBCat (Avakyan et al 2023 ) and references therein.…”