Detection of a previously unknown Be/X-ray binary in the SMC, Swift J010902.6-723710 = SXP 182

ATel #16321; Coe M. J. (Southampton University), Kennea J. A. (PSU), Gaudin T. M. (PSU), Monageng I. (SAAO/UCT), Townsend L. (SALT), Buckley D. A. (SAAO), Udalski A. (Warsaw University), Evans P. (Leicester University)
on 6 Nov 2023; 14:23 UT
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Credential Certification: Malcolm Coe (mjcoe@soton.ac.uk)

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bservations carried out as part of the regular S-CUBED monitoring of the Small Magellanic Cloud (Kennea et al, 2018) detected weak X-ray emission from Swift J010902.6-723710 on 9 October 2023 and again on 30 October 2023. This source had previously been detected by S-CUBED three times in July 2020, but on no other occasions since the start of S-CUBED in July 2016. Each detection revealed a Swift/XRT count rate of approximately 0.1 counts/sec, which implies an X-ray luminosity of around 3 x 10E36 erg/s for a source in the SMC.

The X-ray position was measured to be: RA (J2000): 01h 09m 02.72s (17.26132) Dec (J2000): 72deg 37arcmin 11.7arcsec (-72.61991) 90% Error radius: 2.1arcsec

Two longer Swift observations were carried out on 2 and 4 November 2023. The source remained pretty stable during those observations around the count rate value quoted above. But the longer observations (5ksec each) enabled a more sensitive timing search with the result that a period of 182s was detected on both occasions. We attribute this period to the spin period of the neutron star.

Within 1.7 arcsec of the centre of this position lies the Be star candidate OGLE J010902.25-723710.1. I-band data from 14 years of OGLE IV observations were investigated and revealed a strong eclipsing modulation with a period of 60.670d. Overall these data also revealed that the source was currently at its I-band brightest for this whole period of time. This bright phase was confirmed independently by extracting the Swift UVOT lightcurve. We therefore propose that this 60.670d modulation is the binary period of a new Be/X-ray binary system. The combination of the 60d binary period and the 182s spin period will give this source the additional classification of SXP 182, following the accepted convention detailed in Coe and Kirk (2015).

An optical spectrum of the OGLE candidate was obtained on the 2 November 2023 from the Southern African Large Telescope (SALT). The spectrum revealed Balmer lines weakly in emission, with an Equivalent Width value of approximately 6.5 Angstroms for the H alpha line.

An ephemeris for the optical eclipses was established from the OGLE data which revealed that the X-ray detections occurred at a range of binary phases (0.47, 0.56, 0.36, 0.06, 0.40). In between the detections were several non-detections. At this point in time it is not clear whether we are seeing a series of Type I outbursts or some other behavioural pattern.

Further detailed X-ray and optical observations are scheduled.