  A sunspot is an ideal waveguide for a variety of magnetohydrodynamic waves,
which carry a significant amount of energy to the upper atmosphere and could be
used as a tool to probe magnetic and thermal structure of a sunspot. In this
study, we used the New Vacuum Solar Telescope and took high-resolution image
sequences simultaneously in both TiO (7058$\pm$10 \AA) and H$_\alpha$
(6562$\pm$2.5 \AA) bandpasses. We extracted the area and total emission
intensity variations of sunspot umbra and analyzed the signals with
synchrosqueezing transform. We found that the area and emission intensity
varied with both three and five minute periodicity. Moreover, the area and
intensity oscillated in phase with each other, this fact hold in both TiO and
H$_\alpha$ data. We interpret this oscillatory signal as propagating slow
sausage wave. The propagation speed is estimated at about 8 km$\cdot$s$^{-1}$.
We infer that this sunspot's umbra could have temperature as low as 2800--3500
K.
