Neutron stars provide a window into the properties of dense nuclear matter. Several recent observational and theoretical developments provide powerful constraints on their structure and internal composition. Among these are the first observed binary neutron star merger, GW170817, whose gravitational radiation was accompanied by electromagnetic radiation from a short γ-ray burst and an optical afterglow believed to be due to the radioactive decay of newly minted heavy r-process nuclei. These observations give important constraints on the radii of typical neutron stars and on the upper limit to the neutron star maximum mass and complement recent pulsar observations that established a lower limit. Pulse-profile observations by the Neutron Star Interior Composition Explorer (NICER) X-ray telescope provide an independent, consistent measure of the neutron star radius. Theoretical many-body studies of neutron matter reinforce these estimates of neutron star radii. Studies using parameterized dense matter equations of state (EOSs) reveal several EOS-independent relations connecting global neutron star properties.
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Mass measurements show slowdown of rapid proton capture process at waiting-point nucleus 64Ge
Abstract X-ray bursts are among the brightest stellar objects frequently observed in the sky by space-based telescopes. A type-I X-ray burst is understood as a violent thermonuclear explosion on the surface of a neutron star, accreting matter from a companion star in a binary system. The bursts are powered by a nuclear reaction sequence known as the rapid proton capture process (rp process), which involves hundreds of exotic neutron-deficient nuclides. At so-called waiting-point nuclides, the process stalls until a slower β + decay enables a bypass. One of the handful of rp process waiting-point nuclides is 64 Ge, which plays a decisive role in matter flow and therefore the produced X-ray flux. Here we report precision measurements of the masses of 63 Ge, 64,65 As and 66,67 Se—the relevant nuclear masses around the waiting-point 64 Ge—and use them as inputs for X-ray burst model calculations. We obtain the X-ray burst light curve to constrain the neutron-star compactness, and suggest that the distance to the X-ray burster GS 1826–24 needs to be increased by about 6.5% to match astronomical observations. The nucleosynthesis results affect the thermal structure of accreting neutron stars, which will subsequently modify the calculations of associated observables.
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- Award ID(s):
- 1927130
- PAR ID:
- 10464371
- Author(s) / Creator(s):
- ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; more »
- Date Published:
- Journal Name:
- Nature Physics
- Volume:
- 19
- Issue:
- 8
- ISSN:
- 1745-2473
- Page Range / eLocation ID:
- 1091 to 1097
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Type I X-ray bursts are rapid-brightening transient phenomena on the surfaces of accreting neutron stars (NSs). Some X-ray bursts, called clocked bursters, exhibit regular behavior with similar light-curve profiles in their burst sequences. The periodic nature of clocked bursters has the advantage of constraining X-ray binary parameters and physics inside the NS. In the present study, we compute numerical models, based on different equations of state and NS masses, which are compared with the observations of a recently identified clocked burster, 1RXS J180408.9−342058. We find that the relation between the accretion rate and the recurrence time is highly sensitive to the NS mass and radius. We determine, in particular, that 1RXS J180408.9−342058 appears to possess a mass less than 1.7
M ⊙and favors a stiffer nuclear equation of state (with an NS radius ≳12.7 km). Consequently, the observations of this new clocked burster may provide additional constraints for probing the structure of NSs. -
Liu, W. ; Wang, Y. ; Guo, B. ; Tang, X. ; Zeng, S. (Ed.)In Type-I X-ray bursts (XRBs), the rapid-proton capture (rp-) process passes through the NiCu and ZnGa cycles before reaching the region above Ge and Se isotopes that hydrogen burning actively powers the XRBs. The sensitivity study performed by Cyburt et al . [1] shows that the 57 Cu(p, γ ) 58 Zn reaction in the NiCu cycles is the fifth most important rp-reaction influencing the burst light curves. Langer et al . [2] precisely measured some low-lying energy levels of 58 Zn to deduce the 57 Cu(p, γ ) 58 Zn reaction rate. Nevertheless, the order of the 1 + 1 and 2 + 3 resonance states that dominate at 0:2 ≲ T (GK) ≲ 0:8 is not confirmed. The 1 + 2 resonance state, which dominates at the XRB sensitive temperature regime 0:8 ≲ T (GK) ≲ 2 was not detected. Using isobaric-multipletmass equation (IMME), we estimate the order of the 1 + 1 and 2 + 3 resonance states and estimate the lower limit of the 1 + 2 resonance energy. We then determine the 57 Cu(p, γ ) 58 Zn reaction rate using the full pf -model space shell model calculations. The new rate is up to a factor of four lower than the Forstner et al . [3] rate recommended by JINA REACLIBv2.2. Using the present 57 Cu(p, γ ) 58 Zn, the latest 56 Ni(p, γ ) 57 Cu and 55 Ni(p, γ ) 56 Cu reaction rates, and 1D implicit hydrodynamic K epler code, we model the thermonuclear XRBs of the clocked burster GS 1826–24. We find that the new rates regulate the reaction flow in the NiCu cycles and strongly influence the burst-ash composition. The 59 Cu(p, γ ) 56 Ni and 59 Cu(p, α ) 60 Zn reactions suppress the influence of the 57 Cu(p, γ ) 58 Zn reaction. They strongly diminish the impact of the nuclear reaction flow that bypasses the 56 Ni waiting point induced by the 55 Ni(p, γ ) 56 Cu reaction on burst light curve.more » « less
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Abstract Type I X-ray bursts are rapidly brightening phenomena triggered by thermonuclear burning on the accreting layers of a neutron star (NS). The light curves represent the physical properties of NSs and the nuclear reactions on the proton-rich nuclei. The numerical treatments of the accreting NS and physics of the NS interior are not established, which shows uncertainty in modeling for observed X-ray light curves. In this study, we investigate theoretical X-ray burst models compared with burst light curves with GS 1826-24 observations. We focus on the impacts of the NS mass and radius and base heating on the NS surface using the MESA code. We find a monotonic correlation between the NS mass and the parameters of the light curve. The higher the mass, the longer the recurrence time and the greater the peak luminosity. While the larger the radius, the longer the recurrence time, the peak luminosity remains nearly constant. In the case of increasing base heating, both the recurrence time and peak luminosity decrease. We also examine the above results with a different numerical code, HERES , based on general relativity and consider the central NS. We find that the burst rate, energy, and strength are almost the same in two X-ray burst codes by adjusting the base heat parameter in MESA (the relative errors ≲5%), while the duration and rise times are significantly different between (the relative error is possibly ∼50%). The peak luminosity and the e-folding time change irregularly between two codes for different accretion rates.more » « less
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ABSTRACT We present a new method of matching observations of Type-I (thermonuclear) X-ray bursts with models, comparing the predictions of a semi-analytic ignition model with X-ray observations of the accretion-powered millisecond pulsar SAX J1808.4–3658 in outburst. We used a Bayesian analysis approach to marginalize over the parameters of interest and determine parameters such as fuel composition, distance/anisotropy factors, neutron star mass, and neutron star radius. Our study includes a treatment of the system inclination effects, inferring that the rotation axis of the system is inclined $\left(69^{+4}_{-2}\right)^\circ$ from the observers line of sight, assuming a flat disc model. This method can be applied to any accreting source that exhibits Type-I X-ray bursts. We find a hydrogen mass fraction of $0.57^{+0.13}_{-0.14}$ and CNO metallicity of $0.013^{+0.006}_{-0.004}$ for the accreted fuel is required by the model to match the observed burst energies, for a distance to the source of $3.3^{+0.3}_{-0.2}\, \mathrm{kpc}$. We infer a neutron star mass of $1.5^{+0.6}_{-0.3}\, \mathrm{M}_{\odot }$ and radius of $11.8^{+1.3}_{-0.9}\, \mathrm{km}$ for a surface gravity of $1.9^{+0.7}_{-0.4}\times 10^{14}\, \mathrm{cm}\, \mathrm{s}^{-2}$ for SAX J1808.4–3658.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1038/s41567-023-02034-2
