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

TL;DR

This study analyzes seven Type I X-ray bursts from Terzan 5 X-3 during the 2023 NICER outburst to probe fuel composition and burning regimes. Using time-resolved spectroscopy with a persistent-emission scaling factor f_a, the authors show a transition from mixed hydrogen/helium bursts (bursts 1–3, 5) to pure helium bursts (bursts 6–7) as the local accretion rate drops to about $0.1\dot{m}_{\rm Edd}$. Touchdown fluxes and a distance of $D=6.62\pm0.15$ kpc yield a neutron star mass around $M_{\rm NS} \approx 2\,M_\odot$, and the two PRE bursts are consistent with pure-He fuel, while the mixed-fuel PRE candidate suggests hydrogen depletion effects. Recurrence times and the observed anti-correlation with the local accretion rate further support the mixed-to-He burning transition, providing a rare within-outburst view of nuclear burning regimes on a neutron-star surface.

Abstract

We presented 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 were well described by a model including a variable persistent emission component, quantified by a factor $f_a$, due to the Poynting-Robertson drag. The strength of this interaction scales with burst luminosity: the enhancement is absent ($f_a \approx 1$) in the faintest bursts, becomes modest ($f_a \approx 1.5-2$) for the more luminous non-PRE burst and the PRE candidate, and is very strong ($f_a \approx 6-8$) during the pure-helium PRE bursts. This observed transition from mixed-fuel to pure-helium burning as the local mass accretion rate dropped below $\sim$10% of the Eddington limit, $\dot{m}_{\rm Edd}$, 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 $\sim 2 M_\odot$. Second, the observed recurrence times of the non-PRE bursts were consistent with predictions for mixed-fuel burning.

Figures (7)

• Figure 1: The 2023 outburst began on MJD 60002 (2023 February 27). The upper panel shows the light curves from NICER observations (0.5--10 keV, blue points), and MAXI (2--20 keV, black points). The lower panel shows the hardness ratio, defined as the count rate ratio between 3.8–6.8 keV and 2.0–3.8 keV. Each data point represents 64 seconds of NICER data, with all bursts excluded. The red dashed lines indicate the type I X-ray bursts observed by NICER while the green dashed line represents the burst observed by Chandra2023ATel15953....1H.
• Figure 2: The hardness-intensity diagram (HID) of the Terzan 5 X--3 during the 2023 outburst from NICER observations. All bursts are removed, and each point represents a segment of 64 s. The HID of the persistent emission before each NICER X-ray burst are marked as rad diamond points. The black arrow indicates the direction of HID evolution.
• Figure 3: Light curves of the seven X-ray bursts from Terzan 5 X--3 observed with NICER. Black lines are light curves in 0.5-10 keV with time bin of 0.5 s. The vertical dotted line marks the onset time of each burst. The gray dashed dot represents the persistent emission, which were subtracted from the bursts.
• Figure 4: The best-fitted parameters of the spectra from bursts #1-3 using the model TBabs$\times$ (bbodyrad + $f_a$$\times$ (diskbb + bbodyrad)). From top to bottom, we show the bolometric blackbody flux, $F_{\text{bb}}$, blackbody temperature, $kT_{\text{bb}}$, the blackbody radius, $R_{\rm bb}$ and the goodness of fit, $\chi_\nu^{2}$. In the bottom panel, the red and black lines indicate the $\chi_\nu^{2}$ with $f_a$ free and fixed at 1, respectively. Since allowing $f_a$ to vary did not improve the fit, it was fixed at 1.
• Figure 5: Same as Fig. \ref{['fig:fa1_3']}, but for bursts #4 (left) and #5 (right). In the forth panel, we show the factor $f_a$, which was set as free parameter improving the fit. The vertical dashed line indicates the touchdown moment for the PRE candidate, burst #4.