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We revisit the Tidally Excited Oscillations (TEOs) in the A-type,main-sequence, eccentric binary KOI-54, the prototype of heartbeat stars.Although the linear tidal response of the star is a series of orbital-harmonicfrequencies which are not stellar eigenfrequencies, we show that thenon-linearly excited non-orbital-harmonic TEOs are eigenmodes. By carefullychoosing the modes which satisfy the mode-coupling selection rules, a periodspacing ($\Delta P$) pattern of quadrupole gravity modes ($\Delta P \approx2520-2535$ sec) can be discerned in the Fourier spectrum, with a detectionsignificance level of $99.9\%$. The inferred period spacing value agreesremarkably well with the theoretical $l=2,m=0$ g modes from a stellar modelwith the measured mass, radius and effective temperature. We also find that thetwo largest-amplitude TEOs at $N=90, 91$ harmonics are very close to resonancewith $l=2,m=0$ eigenmodes, and likely come from different stars.  Previous works on tidal oscillations primarily focus on the modeling of TEOamplitudes and phases, the high sensitivity of TEO amplitude to the frequencydetuning (tidal forcing frequency minus the closest stellar eigenfrequency)requires extremely dense grids of stellar models and prevents us fromconstraining the stellar physical parameters easily. This work, however, opensthe window of real tidal asteroseismology by using the eigenfrequencies of thestar inferred from the non-linear TEOs and possibly very-close-to-resonancelinear TEOs. Our seismic modeling of these identified eigen g-modes shows thatthe best-matching stellar models have ($M \approx 2.20, 2.35 M_{\odot}$) andsuper-solar metallicity, in good agreement with previous measurements.

A New Window to Tidal Asteroseismology: Non-linearly Excited Stellar Eigenmodes and the Period Spacing Pattern in KOI-54