  Observational evidence points to a red supergiant (RSG) progenitor for SN
2023ixf. The progenitor candidate has been detected in archival images at
wavelengths (>0.6 micron) where RSGs typically emit profusely. This object is
distinctly variable in the infrared (IR). We characterize the variability using
pre-explosion mid-IR (3.6 and 4.5 micron) Spitzer and ground-based near-IR
(JHKs) archival data jointly covering 19 yr. The IR light curves exhibit
significant variability with RMS amplitudes in the range of 0.2-0.4 mag,
increasing with decreasing wavelength. From a robust period analysis of the
more densely sampled Spitzer data, we measure a period of 1091+/-71 days. We
demonstrate using Gaussian Process modeling that this periodicity is also
present in the near-IR light curves, thus indicating a common physical origin,
which is likely pulsational instability. We use a period-luminosity relation
for RSGs to derive a value of M_K=-11.58+/-0.31 mag. Assuming a late M spectral
type, this corresponds to log(L/L_sun)=5.27+/-0.12 at T_eff=3200 K and to
log(L/L_sun)=5.37+/-0.12 at T_eff=3500 K. This gives an independent estimate of
the progenitor's luminosity, unaffected by uncertainties in extinction and
distance. Assuming the progenitor candidate underwent enhanced dust-driven
mass-loss during the time of these archival observations, and using an
empirical period-luminosity-based mass-loss prescription, we obtain a mass-loss
rate of around (2-4)x10^-4 M_sun/yr. Comparing the above luminosity with
stellar evolution models, we infer an initial mass for the progenitor candidate
of 20+/-4 M_sun, making this one of the most massive progenitors for a Type II
SN detected to-date.
