Discovery of a putative supernova remnant around the long-period X-ray pulsar SXP 1323 in the Small Magellanic Cloud

We report the discovery of a circular shell centred on the Be X-ray binary (BeXB) SXP 1323 in the Small Magellanic Cloud. The shell was detected in an H α image obtained with the Very Large Telescope. Follow-up spectroscopy with the Southern African Large Telescope showed that the shell expands with a velocity of ≈ 100 \, km\, s⁻¹ and that its emission is due to shock excitation. We suggest that this shell is a remnant of the supernova explosion that led to the formation of SXP 1323’s neutron star ≈ 40\, 000 yr ago. SXP 1323 represents the second known case of a BeXB associated with a supernova remnant (the first one is SXP 1062). Interestingly, both of these BeXBs harbour long-period pulsars and are located in a low-metallicity galaxy.