The Fundamental Ringdown Mode of GW150914: A Berti-Fit Prediction Set Against the Published Test Values

GW150914 remains the reference event for ringdown work because its remnant, at about sixty-two solar masses with dimensionless spin near 0.67, rings squarely in the most sensitive part of the detector band. This paper asks a deliberately narrow question: starting only from the published remnant mass and spin, what frequency and damping time does the standard Kerr fitting formula predict for the fundamental quadrupole mode, and how far do those predictions sit from the values reported in the collaboration's own test of general relativity? The question is narrow, but it is the load-bearing comparison of the whole subject, because every later spectroscopy claim is built from the same two steps of predicting a mode from remnant parameters and comparing it with a ringdown-only measurement. For a remnant of 62 source-frame solar masses (67.8 in the detector frame at the published redshift) with spin 0.67, the fitting formula gives a dimensionless frequency M omega of 0.5227 and a quality factor of 3.167. Converted to physical units, the fundamental mode frequency is 249.2 Hz with a damping time of 4.04 ms, corresponding to 1.01 oscillation cycles per e-folding of amplitude decay. Sweeping the published mass and spin intervals gives a prediction band of 243.5 to 253.5 Hz in frequency and 4.20 to 3.88 ms in damping time. The predicted frequency and damping time fall within 0.11 and 0.06 of the published measurement uncertainties respectively, and the leading torsion-frame shift of 1.229e-40 Hz is far below any present measurement reach. This study is computed from stated assumptions and published catalogue values; it is simulated, not measured.