Low evidence for ringdown overtone in GW150914 when marginalizing over time and sky location uncertainty

Understand

Tests of the no-hair theorem using astrophysical black holes involve the detection of at least two quasi-normal modes (QNMs) in the gravitational wave emitted by a perturbed black hole.

• A detection of two modes -- the dominant, $(\ell, m, n) = (2,2,0)$, mode and its first overtone, the $(2,2,1)$ mode -- in the post-merger signal of the binary black hole merger GW150914 was claimed in Isi et al.
• [arXiv:1905.00869], with further evidence provided in Isi & Farr [arXiv:2202.02941].
• However, Cotesta et al.