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 \times 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 \sim 45 s to \sim 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 \times 10 ^ { 39 } 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 .