An accreting pulsar with extreme properties drives an ultraluminous x-ray source in NGC 5907

TL;DR

This paper reports the discovery of a neutron star in NGC 5907 ULX with extreme luminosity and magnetic field properties, challenging existing models of ultraluminous X-ray sources and suggesting some may host neutron stars instead of black holes.

Contribution

It provides the first evidence of a neutron star powering an ultraluminous X-ray source with extreme properties, expanding the understanding of ULX diversity.

Findings

NGC 5907 ULX is an accreting neutron star.

The neutron star's spin period evolved from 1.43 s to 1.13 s between 2003 and 2014.

Its luminosity exceeds the Eddington limit by about 1000 times.

Abstract

Ultraluminous x-ray sources (ULXs) in nearby galaxies shine brighter than any X-ray source in our Galaxy. ULXs are usually modeled as stellar-mass black holes (BHs) accreting at very high rates or intermediate-mass BHs. We present observations showing that NGC5907 ULX is instead an x-ray accreting neutron star (NS) with a spin period evolving from 1.43~s in 2003 to 1.13~s in 2014. It has an isotropic peak luminosity of about 1000 times the Eddington limit for a NS at 17.1~Mpc. Standard accretion models fail to explain its luminosity, even assuming beamed emission, but a strong multipolar magnetic field can describe its properties. These findings suggest that other extreme ULXs (x-ray luminosity > 10^{41} erg/s) might harbor NSs.