A transition from mixed-fuel to pure-helium thermonuclear bursts in Terzan 5 X–3/Swift J174805.3–244637

We present a detailed analysis of seven thermonuclear X-ray bursts from Terzan 5 X–3/Swift J174805.3–244637 detected by NICER during the source’s 2023 outburst. Our analysis reveals a clear evolution of burst properties, identifying four non-photospheric radius expansion (non-PRE) bursts, one PRE candidate occurring in a mixed hydrogen–helium environment, and two powerful PRE bursts from pure helium ignition. The time-resolved burst spectra are described well by a model that includes a variable persistent emission component, quantified by a factor fa, due to the Poynting-Robertson drag. The strength of this interaction scales with burst luminosity: it is low in the faintest bursts (fa ≈ 1), becomes modest for the more luminous non-PRE burst and the PRE candidate (fa ≈ 1.5 − 2), and is very strong during the pure-helium PRE bursts (fa ≈ 6 − 8). This observed transition from mixed-fuel to pure-helium burning as the local mass accretion rate drops below ∼10% of the Eddington limit, ṁEdd, aligns with theoretical predictions. We verified this scenario with two independent methods. First, at the known distance to Terzan 5, the touchdown luminosities of both the pure helium PRE bursts and the mixed-fuel PRE candidate are consistent with reaching their respective, composition-dependent Eddington limits on the same plausible, massive neutron star of ∼2 M⊙. Second, the observed recurrence times of the non-PRE bursts were consistent with predictions for mixed-fuel burning.