NGC 300 ULX1: A new ULX pulsar in NGC 300

NGC 300 ULX1 is the fourth to be discovered in the class of the ultra-luminous X-ray pulsars. Pulsations from NGC 300 ULX1 were discovered during simultaneous XMM-Newton / NuSTAR observations in Dec. 2016. The period decreased from 31.71 s to 31.54 s within a few days, with a spin-up rate of -5.56×10^-7 s s^-1, likely one of the largest ever observed from an accreting neutron star. Archival Swift and NICER observations revealed that the period decreased exponentially from 45 s to 17.5 s over 2.3 years. The pulses are highly modulated with a pulsed fraction strongly increasing with energy and reaching nearly 80% at energies above 10 keV. The X-ray spectrum is described by a power-law and a disk black-body model, leading to a 0.3-30 keV unabsorbed luminosity of 4.7×10³⁹ erg s^-1. The spectrum from an archival XMM-Newton observation of 2010 can be explained by the same model, however, with much higher absorption. This suggests, that the intrinsic luminosity did not change much since that epoch. NGC 300 ULX1 shares many properties with supergiant high mass X-ray binaries, however, at an extreme accretion rate.