SXP 15.6: X-ray spectral and temporal properties of a newly discovered pulsar in the Small Magellanic Cloud

We report on the X-ray spectral and temporal properties of the Be/X-ray binary system XMMU J004855.5-734946 located in the Small Magellanic Cloud. The system was monitored by Swift/XRT during a moderate outburst in 2016 July, while an unanticipated Chandra target of opportunity observation was triggered when the luminosity of the system was greater than 10³⁶ erg s^-1 allowing a detailed study of X-ray properties of the systems. Specifically, its X-ray spectrum, as observed during the outburst, is well modelled by an absorbed power law (Γ = 0.58). Timing analysis of the collected photon events revealed coherent X-ray pulsations with a period of ∼15.64 s, thus confirming XMMU J004855.5-734946 as a high-mass X-ray binary pulsar. By analysing archival XMM-Newton observations, we determined the long-term spin period evolution of the neutron star, showing that the compact star has spun-up by \dot{P}∼ -0.0028 s yr^{-1}. By modelling the X-ray pulsed emission as detected by Chandra, we set constraints on the inclination of the magnetic and rotation axis of the neutron star, as well to its compactness (I.e. (M/M_⊙)/(R/km) = 0.095 ± 0.007).