  Mechanisms of particle heating are crucial to understanding the shock physics
in supernova remnants (SNRs). However, there has been little information on
time variabilities of thermalized particles so far. Here, we present a
discovery of a gradually-brightening thermal X-ray emission found in Chandra
data of Tycho's SNR obtained during 2000--2015. The emission exhibits a
knot-like feature (Knot1) with a diameter of $\simeq0.04$~pc located in the
northwestern limb, where we also find localized H$\alpha$ filaments in an
optical image taken with the Hubble Space Telescope in 2008. The model with the
solar abundance reproduces the spectra of Knot1, suggesting that Knot1
originates from interstellar medium; this is the first detection of thermal
X-ray emission from swept-up gas found in Tycho's SNR. Our spectral analysis
indicates that the electron temperature of Knot1 has increased from $\sim0.30$
keV to $\sim0.69$ keV within the period between 2000 and 2015. These results
lead us to ascribe the time-variable emission to a small dense clump recently
heated by the forward shock at the location of Knot1. The electron-to-proton
temperature ratio immediately downstream the shock ($\beta_{0}\equiv T_e/T_p$)
is constrained to be $m_e/m_p\leq\beta_{0}\leq0.15$ to reproduce the data,
indicating the collisionless electron heating with efficiency consistent with
previous H$\alpha$ observations of Tycho and other SNRs with high shock
velocities.
