Mass measurement of ²⁷P to constrain type-I x-ray burst models and validate the isobaric multiplet mass equation for the A=27, T=3/2 isospin quartet

Background: Light curves are the primary observable of type-I x-ray bursts. Computational x-ray burst models must match simulations to observed light curves. Most of the error in simulated curves comes from uncertainties in r p process reaction rates, which can be reduced via precision mass measurements of neutron-deficient isotopes in the r p process path. Purpose: Perform a precise atomic mass measurement of 27P. Use this new measurement to calculate r p process reaction rates and input these rates into an x-ray burst model to reduce simulated light curve uncertainty. Use the mass measurement of 27P to validate the isobaric multiplet mass equation (IMME) for the A = 27 T = 3 2 isospin quartet which 27P belongs to. Method: High-precision Penning trap mass spectrometry utilizing the time-of-flight ion cyclotron resonance technique was used to determine the atomic mass of 27P. The MESA code (Modules for Experiments in Stellar Astrophysics) was then used to simulate x-ray bursts using a one-dimensional multizone model to produce updated light curves. Results: The mass excess of 27P was measured to be -670.7(6) keV, a 14-fold precision increase over the mass reported in the 2020 Atomic Mass Evaluation (AME2020). The 26Si(p, gamma ) 27P - 27P(gamma , p) 26Si rate equilibrium has been determined to a higher precision based on the precision mass measurement of 27P. x-ray burst light curves were produced with the MESA code using the new reaction rates. Changes in the mass of 27P seem to have minimal effect on light curves, even in burster systems tailored to maximize impact. Conclusion: The mass of 27P does not play a significant role in x-ray burst light curves. It is important to understand that more advanced models do not just provide more precise results, but often qualitatively different ones. This result brings us a step closer to being able to extract stellar parameters from individual x-ray burst observations. In addition, the IMME has been validated for the A = 27, T = 3/2 quartet. The normal quadratic form of the IMME using the latest data yields a reduced chi 2 of 2.9. The cubic term required to generate an exact fit to the latest data matches theoretical attempts to predict this term.