NICER / ISS Science Nugget
for June 19, 2025

Gently burbling nuclear burning

The build-up of matter on the surface of a neutron star, conveyed from an orbiting companion star by tidal and other forces, can produce densities and temperatures that are conducive to nuclear fusion reactions, the merging of light-element atoms to form heaver ones. Depending on factors such as the rate of mass transfer, the rotation of the neutron star, and the presence of a strong magnetic field, the nuclear fuel may burn continuously, explosively, or somewhere in between - not quite stably but cyclically. In each case, time-varying X-ray emission traces the heat of the fusion reactions: persistent emission for continuous burning, "Type I" bursts of X-rays for explosive burning, and quasi-periodic brightness oscillations (QPOs) for cyclic burning. A handful of neutron-star binary systems exhibit the QPOs that are thought to represent marginally stable burning, with cycling timescales on the order of a few minutes (frequencies conveniently described in millihertz [mHz]), and the variety of conditions under which it occurs is surprisingly broad.

The neutron-star binary GS 1826-238 was already known to be a source of Type I thermonuclear bursts - with such regularity that it was called the "Clocked Burster" - but in 2018 NICER discovered that it sometimes also produces mHz QPOs. A peer-reviewed paper by H. Xiao (Univ. of Turku, Finland, and Sun Yat-sen Univ., PRC) and collaborators, recently published in The Astrophysical Journal, describes a comprehensive study of all mHz oscillations detected by NICER in its archive of observations from 2017 to 2022 to assess their properties and better understand the conditions in which nuclear burning on the star is marginally stable vs. explosive. The team identifies 37 instances of mHz QPOs in 106 observations, with no apparent correlation with the persistent X-ray brightness (expected to be proportional to overall mass-transfer rate). The oscillations span a range of frequencies between 3 and 17 mHz, and are more pronounced for higher X-ray photon energies than for lower. A closer look at the distribution of X-ray photon energies during the mHz oscillations shows a pattern that matches a prediction of the theory of marginally stable burning. The authors apply a model in which high-energy X-ray emission originates in a hot plasma that boosts low-energy "seed" photons from the neutron star's surface, and infer the temperatures and intensities of these seed photons as a function of the oscillation phase. As expected for marginally stable burning, peak X-ray brightness coincides with maximum temperature and minimum seed-photon intensity, while the oscillation minimum occurs for lower seed temperature and higher intensity. The pattern reflects the origin of the cyclic burning: rising temperature results in more rapid consumption of fuel, reducing its thickness and promoting cooling, but the continuous supply of fresh fuel soon increases the temperature again.