SXP 65.8: A Refined Orbital Period and Evidence for a Brief Occultation

ATel #13033; P. C. Schmidtke (Arizona State University), A. P. Cowley (Arizona State University), A. Udalski (Warsaw University Observatory)
on 16 Aug 2019; 19:48 UT
Credential Certification: Paul Schmidtke (Paul.Schmidtke@asu.edu)

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The SMC X-ray pulsar SXP 65.8 was discovered by McGowan et al. (2007, MNRAS, 376, 759). The spin period of the neutron star has been measured four additional times, with no significant change in value (Yang et al., 2017, ApJ, 839:119). At present there has been no determination of the orbital period from long-term X-ray observations (Galache et al. 2008, ApJS, 177, 189; Kennea et al. 2018, ApJ, 868:47).

The position of SXP 65.8 is coincident with the emission-line star [MA93] 1619 (Meyssonnier & Azzopardi, 1993, A&AS, 102, 451). Using MACHO and OGLE-II observations, Schmidtke & Cowley (2007, BAAS, 39, 724) found a recurring signal with P=110.6 d, which they proposed is the orbital period. The optical light curve folded on this period has relatively large scatter but shows a weak outburst (~0.03 mag), with a fast rise and slow decline. Unrecognized at the time was a slight depression that occurs ~0.3P before the outburst.

From the first 3 seasons of OGLE-IV data (see http://ogle.astrouw.edu.pl/ogle4/xrom/sxp65.8.html ), Schmidtke, Cowley, & Udalski (2013, MNRAS, 431, 252; hereafter SCU13) derived P(orb)=110.74 +/- 0.11 d. A similar periodicity in OGLE data was found by Bird et al. (2012, MNRAS, 423, 3663). The folded light curve shows considerable, but repeatable, structure (see Fig. 12 of SCU13). The primary maximum (phase zero) is an outburst whose profile is narrower (~0.1P) and larger in amplitude (~0.15 mag) compared to earlier optical observations. SCU13 also noted a set of faint points forming a narrow dip at phase 0.7.

We have made a new analysis of data from all 9 OGLE-IV seasons. The same general structure seen in the first 3 seasons is still present in the expanded data set. A plot of the folded light curve for all OGLE-IV observations is shown in http://www.public.asu.edu/~atpcs/SXP/SXP65.8_OGLEIV.pdf . In particular, the coherence of new data near phase 0.7 reinforces the interpretation of a brief occultation (<0.01P), lasting last than a day. Aligning the lowest points, all taken during different orbital cycles, we derive a refined period of 110.72 +/- 0.01 d. With phase zero set to the time of maximum light (JD 2445999 +/- 2), the occultation falls at phase 0.706. A possible explanation for this dip is the transit of the neutron star and its associated accretion disk/region in front of the Be star. If this is correct, then further observations of SXP 65.8 may advance the understanding of mass transfer in Be/X-ray binary systems.