Consistent Modeling of GS 1826-24 X-Ray Bursts for Multiple Accretion Rates Demonstrates the Possibility of Constraining rp-process Reaction Rates

Abstract

Type-I X-ray burst light curves encode unique information about the structure of accreting neutron stars and the nuclear reaction rates of the rp-process that powers bursts. Using the first model calculations of hydrogen/helium burning bursts for a large range of astrophysical conditions performed with the code MESA, this work shows that simultaneous model-observation comparisons for bursts from several accretion ratesṀ are required to remove degeneracies in astrophysical conditions that otherwise reproduce bursts for a singleṀ and that such consistent multi-epoch modeling could possibly limit the 15 O(α, γ) 19 Ne reaction rate. Comparisons to the year 1998, 2000, and 2007 bursting epochs of the neutron star GS 1826-24 show thatṀ must be larger than previously inferred and that the shallow heating in this source must be below 0.5 MeV/u, providing a new method to constrain the shallow heating mechanism in the outer layers of accreting neutron stars. Features of the light curve rise are used to demonstrate that a lower-limit could likely be placed on the 15 O(α, γ) reaction rate, demonstrating the possibility of constraining nuclear reaction rates with X-ray burst light curves. arXiv:1805.05552v1 [astro-ph.HE]

Influence of Nuclear Reaction Rate Uncertainties on Neutron Star Properties Extracted from X-Ray Burst Model–Observation Comparisons

“…One could imagine that the C increase from increasing Q b could be mitigated by a modification of X(H). However, it is apparent that an X(H) increase would only modestly reduce C while also substantially reducing E. As such, to explain a source such as GS 1826-24, there appears to be a unique solution in terms oḟ M , X(H), and Q b , which Meisel (2018) used to demonstrate that shallow heating in neutron star outer layers can be constrained with X-ray burst light curve modelobservation comparisons.…”

“…Astrophysical conditions which were varied between different model calculations include the accretion ratė M , metallicity of the accreted composition Z, accretionbased heating at the base of the envelope Q b , and hydrogen mass fraction X(H). The helium mass fraction Y was adjusted to enforce X(H) + Y + Z = 1.Ṁ = 0.05, 0.07, 0.08, 0.11, 0.15, 0.17Ṁ E , whereṀ E = 1.75 × 10 −8 M /yr is the Eddington accretion rate , were explored to sampleṀ for the observed GS 1826-24 epochs from 1998from , 2000from , and 2007from (Galloway et al 2008) and the observedṀ scaled up by ∼2 (Meisel 2018). Z = 0.01, 0.02 were used to investigate the solar Z favored by previous investigations of GS 1826-24 (Galloway et al 2004Heger et al 2007) and a slight reduction from that value.…”

“…The light curves corresponding to a sequence (typically 10 − 20 bursts) were combined to achieve an average light curve and an uncertainty band, where fewer bursts in a sequence will generally result in a larger band. See Meisel (2018) for an example of this process.…”

“…3. NUCLEAR REACTION RATES VARIED Nuclear reaction rates were varied for calculations employing the astrophysical conditions recently found to best reproduce light curve features of GS 1826-24 (Meisel 2018):Ṁ = 0.17Ṁ E , Z = 0.02, Q b = 0.1 MeV/u, X(H) = 0.70. Due to computational expense, the set of reaction rates under investigation was limited to the 19 rate variations found to have a significant impact on Xray burst light curves calculated with KEPLER for similar astrophysical conditions (Cyburt et al 2016).…”

“…To date, it has not been directly explored how nuclear physics uncertainties influence astrophysical constraints derived from model-observation comparisons for a specific source. We investigate this in the present work, employing the model calculations described in Meisel (2018) and focusing on conditions similar to those required to reproduce the observed features of GS 1826-24. This is presented as follows.…”