Neutron stars are small and dense, which gives them an intense gravitational field 鈥� one so powerful it can bend the light emitted on their far side around towards the front of the star. In other words, we could see all sides of them once.
When a massive star is destroyed in a supernova, it can leave behind a neutron star 鈥� a sphere the size of a city that is extremely dense. Hajime Sotani at the National Astronomical Observatory of Japan and his colleagues modelled three ultra-compact neutron stars each 10 kilometres in radius 鈥� one with the mass of two suns, one slightly more massive, and one slightly less.
They found that the light coming from the least massive star acted normally, meaning we would only see one side of it. But for the more dense neutron stars, photons from the backside are whipped around to the front by the star鈥檚 gravity. This effect should change the light signature from dense neutron stars.
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鈥淣ot only that, some of the light would get to you in two directions, coming around from either the right or the left side, so you鈥檇 get double-imaging. It鈥檇 be like having a distorted mirror in a funhouse,鈥� says Sharon Morsink at the University of Alberta, who works on NASA鈥檚 Neutron star Interior Composition Explorer (NICER) mission.
Morsink says the photons that are emitted at a right angle will still travel away from the star, but those leaving the surface at any other direction, will bent around to the front by the star鈥檚 gravity. That gravitational lensing would distort the light from our perspective, making a circular hot spot on the star look like an arc.
Most neutron stars we see are pulsars, which radiate intense beams of light and spin extremely fast, so their light looks like it is pulsing from our vantage point.
This extreme light bending will have to be taken into account when analysing the NICER data, says Morsink. The experiment, which is on board the International Space Station, can determine exactly when photons arrive in its detector to microsecond accuracy, Morsink says. From that, researchers can tease out which light came from the back and front sides of a neutron star, which will help calculate their radius.
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