A search for X-ray burst thermonuclear burning ashes with NICER

Neutron stars are collapsed objects produced by supernova explosions of massive stars. They compress more than the Sun’s mass into a city-size volume, exhibiting some of the highest densities, spin rates, temperatures and magnetic fields that we can witness in the Universe. One of the key challenges in current research is the actual measurement of their mass and size ratio. Many neutron stars are observed in X-rays when they gravitationally attract matter from an orbiting stellar companion in a binary system. Type-I X-ray bursts are commonly detected that originate from the unstable thermonuclear burning of the accreted hydrogen and/or helium into heavier elements in the surface layers of neutron stars in low-mass X-ray binaries. The luminosity of the bursts does sometimes exceed the Eddington limit and temporarily drive the photosphere to large radii, which may lead to the ejection of nuclear burning ashes. The heavy elements thus exposed by thermonuclear explosions are supposed to engender absorption features in the X-ray burst spectra. The Neutron star Interior Composition Explorer (NICER) has been mounted on the International Space Station in summer 2017, and has since observed a number of X-ray bursters with both high timing and high energy resolutions. The soft (0.2-12 keV) energy passband of NICER is particularly well-suited for X-ray burst investigations. Detections in NICER burst spectra of photoionization edges from heavy elements are expected to display the thermonuclear burning and mixing processes under degenerate conditions. Moreover, the identification of gravitationally-redshifted edges would uniquely provide a
measure of the neutron star compactness, and thus constitute a probe of the ultra-dense matter equation of state.