Neutron stars emit radiation across the spectrum, but observing in the X-ray band offers the greatest insights into their structure, the ultimate stability of their pulses as precise clock "ticks," and the high-energy, dynamic phenomena that they host, including starquakes, thermonuclear explosions, and the most powerful magnetic fields known in the universe.
NICER's array of 56 telescopes will collect X-rays generated both from hotspots on a neutron star's surface and from its tremendously strong magnetic field. There are two hotspots on a neutron star, one at each magnetic pole, where the star's intense magnetic field emerges from the surface. Here, particles trapped in the magnetic field rain down and generate X-rays when they strike the surface. As a hotspot spins through our line of sight, we perceive a rise and fall in X-ray brightness.
This subgroup of neutron stars rotates rapidly, emitting from their magnetic poles powerful beams of light that sweep around as the star spins, much like a lighthouse. At Earth, these beams are seen as flashes of light, pulsing on and off at intervals from seconds down to milliseconds, giving rise to the stars' alternate name, pulsars.
Because of their predictable pulsations—especially millisecond pulsars, which are the mission's prime target and are found to spin as much as 700 times per second—"they are extremely reliable celestial clocks" and can provide high-precision timing just like the atomic clock signals supplied through the 26-satellite, military-operated Global Positioning System (GPS), an Earth-centric service that weakens the farther one travels out beyond Earth orbit and into the solar system, Arzoumanian said.As a result, NICER/SEXTANT also will demonstrate the viability of pulsar-based navigation. "The hardware needed for neutron star science is identical to that needed for pulsar-based navigation," Gendreau said. "In fact, the mission's two goals share many of the same targets and the same operational concept. The differences are on the back end in terms of how the data will be used."