  Superconducting resonators have to date been used for photon detection in a
non-equilibrium manner. In this paper, we demonstrate that such devices can
also be used in a thermal quasi-equilibrium manner to detect X-ray photons. We
have used a resonator to measure the temperature rise induced by an X-ray
photon absorbed in normal metal and superconducting absorbers on continuous and
perforated silicon nitride membranes. We observed two distinct pulses with
vastly different decay times. We attribute the shorter pulses to
non-equilibrium quasiparticle relaxation and the longer pulses to a thermal
relaxation process. In addition, we have measured the temperature dependence of
the X-ray induced temperature rise and decay times. Finally, we have measured
the resonator sensitivity and energy resolution. Superconducting resonators
used in a thermal quasi-equilibrium manner have the potential to be used for
X-ray microcalorimetry.
