Scientists have identified a neutron star that is consuming material so fast it emits more x-rays than any other. Its extreme brightness can only be explained if the star has a complex multipolar magnetic field, the researchers say. Ultraluminous x-ray sources (ULXs) are seen in some nearby galaxies and shine brighter than any x-ray source in our own galaxy. Simple calculations show that, for such an intense amount of energy to be emitted, ULXs should be powered by black holes accreting surrounding material. Here, using the X-ray Multi-Mirror Mission (XMM-Newton) and Nuclear Spectroscopic Telescope Array (NuSTAR) space telescopes, Gian Luca Israel and colleagues detected periodic signals in x-rays emitted by a ULX in the nearby spiral galaxy NGC 5907, indicating that it is instead powered by a spinning neutron star. The star, known as NGC 5907 ULX, is accreting material so fast that its spin period is accelerating at astounding rates – it evolved from 1.43 seconds in 2003 to 1.13 seconds in 2014. Its peak luminosity exceeds the Eddington limit, the theoretical maximum set by the balance between the force of radiation acting outward and the gravitational force acting inward, by roughly 1,000 times what would be expected for a neutron star. The authors say that the only way to explain the data is if the neutron star does not have a simple (dipolar) magnetic field. Modelling shows that a strong, multipolar magnetic field could explain the extreme properties of NGC 5907 ULX and how it exceeds the Eddington limit.
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