Discovery of pulsations from NGC 300 ULX1 and its fast period evolution

TL;DR

This paper reports the discovery of pulsations from NGC 300 ULX1, a neutron star exhibiting the fastest observed spin-up rate among accreting neutron stars, confirming its classification as an ultraluminous X-ray pulsar.

Contribution

It provides the first detection of pulsations and rapid period evolution in NGC 300 ULX1, establishing it as a new ultraluminous X-ray pulsar with extreme spin-up behavior.

Findings

Detected 31.6 s pulsations confirming neutron star nature.

Observed the largest spin-up rate ever seen in an accreting neutron star.

Spectral analysis suggests intrinsic luminosity remained high despite flux variations.

Abstract

The supernova impostor SN 2010da located in the nearby galaxy NGC 300, later identified as a likely supergiant B[e] high-mass X-ray binary, was simultaneously observed by NuSTAR and XMM-Newton between 2016 December 16 and 20, over a total time span of 310 ks. We report the discovery of a strong periodic modulation in the X-ray flux with a pulse period of 31.6 s and a very rapid spin-up, and confirm therefore that the compact object is a neutron star. We find that the spin period is changing from 31.71 s to 31.54 s over that period, with a spin-up rate of -5.56 x 10-7 s s-1, likely the largest ever observed from an accreting neutron star. The 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 x 10^39 erg s-1. Applying our best-fit model successfully to the spectra of an XMM-Newton observation from 2010, suggests that the lower fluxes of NGC 300 ULX1 reported from observations around that time are caused by a large amount of absorption, while the intrinsic luminosity was similar as seen in 2016. A more constant luminosity level is also consistent with the long-term pulse period evolution approaching an equilibrium value asymptotically. We conclude that the source is another candidate for the new class of ultraluminous X-ray pulsars.