NICER / ISS Science Nugget
for November 14, 2019

Key results depend on sophisticated numerical simulations

This week, The Astrophysical Journal Letters accepted for publication a paper by the NICER science team (Bogdanov et al. 2019, Paper II) that describes a key method that we use to extract neutron star radius and mass measurements from NICER's X-ray observations of millisecond pulsars. This method, which drove the NICER design and mission concept, makes use of the gravitational bending of light expected due to the high density of neutron stars.

Many neutron stars appear to pulse because hot spots on their surfaces rotate in and out of our view as seen from the Earth. When the star's rotation places a hot spot on the side of the star nearer to the Earth, it appears to brighten. As the star rotates and the hot spot moves to the far side, as seen from the Earth, it fades. The millisecond pulsars that NICER observes rotate hundreds of times each second, leading to pulses every few milliseconds.

In strong gravity, Einstein's theory of general relativity predicts that light bends. The denser the object, the higher the degree of gravitational light bending. The effect with pulsars is that as a hot spot rotates out of our field of view, we continue to see some of its emission, depending on the ratio of the star's mass to its radius. This affects the depth of modulation of the pulses.

The visibility of antipodal hotspots changes with mass and radius
of the neutron star due to relativistic effects

Figure: A rotating neutron star with a pair of antipodal hotspots as seen by a distant observer without light-bending (left) and for strong gravity (M/R = 0.25, right). The visibility of the far-side hotspot provides a measure of M/R through light curve analysis with NICER. (Adapted from Nollert et al. 1989.)

The paper describing our detailed numerical modeling of this effect will be published alongside NICER's first definitive results on neutron star mass and radius in the coming weeks.

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