Determination of Transition Energies and Excitation Cross Sections of Rydberg Transitions in S XV and S XVI Using a Transition Edge Sensor Microcalorimeter

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Recent high-resolution spectral observations with XRISM have revealed strong emissions of highly charged sulfur ions. To interpret them, precise modeling not only of the K α and K β emissions but also of the entire Rydberg series up to its series limit is needed. We observe these emissions by performing high-resolution spectroscopy of sulfur ions, focusing primarily on S xvi and S xv . By attaching an advanced X-ray spectrometer based on a transition-edge sensor array with frequency-division multiplexing readout to a cryogenic electron beam ion trap (EBIT), we resolve Rydberg transitions up to n = 13, determining their transition energy with uncertainties ranging from 60 to 110 meV for isolated lines, corresponding to velocity uncertainties between 6 and 10 km s −1 . We also calibrate their collisional excitation cross sections with emissions arising from radiative recombination of monoenergetic electrons in the EBIT and correct for contributions to the transition intensities due to charge exchange processes with additional state-resolved measurements.