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

Abstract

During a systematic search for new X-ray pulsators in the XMM-Newton archive, we discovered a high amplitude ($PF\simeq 86~{{\ \rm per\ cent}}$) periodic ($P\simeq 7.25\, \mathrm{s}$) modulation in the X-ray flux of 4XMM J045626.3-694723 (J0456 hereafter), a previously unclassified source in the Large Magellanic Cloud (LMC). The period of the modulation is strongly suggestive of a spinning neutron star (NS). The source was detected only during one out of six observations in 2018-2022. Based on an absorbed power-law spectral model with photon slope of Γ ≃ 1.9, we derive a 0.3-10 keV luminosity of $L_\mathrm{X}\simeq 2.7\times 10^{34}\, \mathrm{erg}\, \mathrm{s}^{-1}$ for a distance of 50 kpc. The X-ray properties of J0456 are at variance with those of variable LMC X-ray pulsars hosted in high-mass X-ray binary systems with a Be-star companion. Based on Southern African Large Telescope (SALT) spectroscopic observations of the only optical object that matches the X-ray uncertainty region, we cannot completely rule out that J0456 is an NS accreting from a late-type (G8-K3) star, an as-yet-unobserved binary evolutionary outcome in the Magellanic Clouds (MCs). We show that the source properties are in better agreement with those of magnetars. J0456 may thus be the second known magnetar in the LMC after SGR 0526-66.