Bayesian Modeling of NICER Cometary X-ray Spectra: A Legacy Survey of Solar-Wind Charge Exchange

arXiv:2605.21684 · astro-ph.EP, astro-ph.HE · Submitted 2026-05-20 · Read on arXiv

astro-ph.EP, astro-ph.HE

Submitted: 2026-05-20

Updated: 2026-05-20

Comments: Accepted for publication in Icarus

License: http://creativecommons.org/licenses/by/4.0/

The gist: We present a uniform, epoch-resolved analysis of soft X-ray observations of eight comets obtained with NICER, using Bayesian statistics to identify charge-exchange line components, measure relative

Terminology

Geostationary Operational Environmental Satellites — 24× in this paper
Bayesian Color Model — 19× in this paper
Degeneracy — 10× in this paper · explained in 3 episodes
Calibration — 9× in this paper · explained in 7 episodes
Abundance — 8× in this paper · explained in 1 episode
Trace — 6× in this paper · explained in 2 episodes
Robustness — 5× in this paper · explained in 5 episodes
Accuracy — 3× in this paper · explained in 1 episode
Consistency — 3× in this paper · explained in 3 episodes
Fidelity — 3× in this paper · explained in 2 episodes
Bayesian Information Criterion — 2× in this paper
High Energy Astrophysics Science Archive Research Center — 2× in this paper
Interpretability — 2× in this paper · explained in 15 episodes
X-ray Timing Instrument — 2× in this paper
Aperture — 1× in this paper · explained in 1 episode
Bayesian Inference — 1× in this paper · explained in 5 episodes
Benchmark — 1× in this paper · explained in 2 episodes
Benchmarking — 1× in this paper · explained in 3 episodes
Distribution Shift — 1× in this paper · explained in 5 episodes
Global Oscillation Network Group — 1× in this paper
Signal-to-noise ratio (SNR) — 1× in this paper · explained in 2 episodes
Spectral Signature — 1× in this paper · explained in 2 episodes
Systematic Bias — 1× in this paper · explained in 3 episodes
alpha — 1× in this paper · explained in 1 episode

Abstract

We present a uniform, epoch-resolved analysis of soft X-ray observations of eight comets obtained with NICER, using Bayesian statistics to identify charge-exchange line components, measure relative ion fluxes, and infer nominal solar-wind freeze-in temperatures. The sample exhibits recurring spectral morphologies that fall into distinct families: carbon-dominated, intermediate, and nitrogen-/oxygen-dominated. Epoch-resolved flux ratios yield a robust separation between diagnostics: carbon-derived freeze-in temperatures cluster near T freeze(C) about 1.4-1.7 MK, while nitrogen- and oxygen-derived diagnostics are systematically higher, typically T freeze(N,O) about 2.0-2.3 MK. Short-timescale variability in inferred freeze-in conditions is common, indicating that instantaneous solar-wind charge-state fluctuations, rather than large changes in coma composition, dominate the spectral differences. We discuss instrumental and modeling limitations, demonstrate how our Bayesian fitting method mitigates degeneracies via physically motivated priors and Bayesian model selection, and recommend laboratory measurements and coordinated high-resolution X-ray observations to refine charge-exchange diagnostics and validate low-resolution inferences.

Bayesian Modeling of NICER Cometary X-ray Spectra: A Legacy Survey of Solar-Wind Charge Exchange

Terminology

Abstract

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