NICER / ISS Science Nugget
for February 8, 2024

Tracking dynamic plasmas

RS Ophiuchi is a recurrent nova - a binary star system in which a white dwarf (the remnant of a star comparable to our Sun that has run out of nuclear fuel and collapsed) draws matter from a companion star, accumulating it to the point where it periodically undergoes a runaway thermonuclear reaction, producing explosive brightening across the electromagnetic spectrum. In the case of RS Oph, these outbursts are seen approximately every 15 years, with the most recent beginning on August 8, 2021. One property of RS Oph that makes it unique is the nature of the companion star: a red giant that loses mass in a dense "wind" of gas primarily along its equator, with this material ultimately encircling the binary system. When the thermonuclear explosions on the white dwarf's surface drive ejecta into this circumbinary gas, a high-temperature plasma emits X-rays that reveal the chemistry and energetics of the outflow.

NICER's dense sampling of the 2021 outburst of RS Oph provided exceptional coverage of the event over 90 days, as reported in a peer-reviewed paper by N. Islam (Univ. of Maryland Baltimore County/NASA GSFC) and collaborators recently published in The Astrophysical Journal. High-energy X-rays followed visible-light and gamma-ray trends in peaking near the start of the outburst, when these emissions were dominated by the hot (278 million Kelvin) plasma. Within a few days, as the plasma cooled and dissipated, low-energy X-rays from the still-burning white-dwarf surface began to outshine the plasma. In analyzing the NICER spectra at various stages of the outburst, the team confronted the data with models of X-ray emission from plasmas that are collisionally ionized - i.e., in which collisions between atoms in the hot gas are sufficiently energetic to knock electrons out of their orbits around the atomic nuclei. Additional collisions between atoms and freed electrons liberate still more electrons. At the same time, some electrons recombine with previously-ionized atoms. Depending on the timescales of these interactions and dynamic inputs to the gas - such as from an explosive shock wave passing through, or slow cooling - ionization and recombination can reach an equilibrium or can be well out of equilibrium. Islam et al. find that non-equilibrium ionization models fit the early NICER data much better than do equilibrium models. They also find that up to 40% of the early-time emission is obscured by relatively cold gas intervening along our sight-line, but that this "covering" fraction decreases over time, evidence that the ejecta first interact with the companion star's dense equatorial wind but later encounter less-dense polar regions as they expand outward.

Much remains to be explored in this rich dataset, including the evolving properties of the soft X-ray emission from the white-dwarf's surface, which has previously been shown to be pulsed with that star's approximately 35-second rotation period.