High-precision mass measurement of ²⁴Si and a refined determination of the rp process at the A=22 waiting point

Type I x-ray bursts occur at astrophysical sites where a neutron star accretes H/He-rich matter from a companion star, leading to nuclear burning on the neutron star surface. The only observable is the x-ray burst light curve, which is used as a unique diagnostic of the outer layers of accreting neutron stars such as the accretion rate and fuel composition. In addition to the astrophysical conditions, the main determinant of the shape of the light curve is the nuclear physics involved. Variations within the uncertainty of the ²³Al(p,γ)²⁴Si reaction rate lead to significant shifts in simulated x-ray light curves, where the ground state mass of ²⁴Si is currently the dominant source of the reaction rate uncertainty (19 keV). A high-precision mass measurement of ²⁴Si was performed with the LEBIT facility at the National Superconducting Cyclotron Laboratory. The atomic mass excess, 10 753.8(37) keV, is a factor of 5 more precise than previous results. This substantially reduces the uncertainty of the ²³Al(p,γ)²⁴Si reaction rate, which is a key part of the rapid proton capture (rp) process powering type I x-ray bursts. The updated rate constrains the onset temperature of the (α,p) process at the ²²Mg waiting point to a precision of 9%.