The first direct observation of a binary neutron star ( BNS ) merger was a watershed moment in multi-messenger astronomy .
However , gravitational waves from GW170817 have only been observed prior to the BNS merger , but electromagnetic observations all follow the merger event .
While post-merger gravitational wave signal in general relativity is too faint ( given current detector sensitivities ) , here we present the first tentative detection of post-merger gravitational wave ‘ ‘ echoes ’ ’ from a highly spinning ‘ ‘ black hole ’ ’ remnant .
The echoes may be expected in different models of quantum black holes that replace event horizons by exotic Planck-scale structure and tentative evidence for them has been found in binary black hole merger events .
The fact that the echo frequency is suppressed by \log M ( in Planck units ) puts it squarely in the LIGO sensitivity window , allowing us to build an optimal model-agnostic search strategy via cross-correlating the two detectors in frequency/time .
We find a tentative detection of echoes at f _ { echo } \simeq 72 Hz , around 1.0 sec after the BNS merger , consistent with a 2.6-2.7 M _ { \odot } ‘ ‘ black hole ’ ’ remnant with dimensionless spin 0.84 - 0.87 .
Accounting for all the ‘ ‘ look-elsewhere ’ ’ effects , we find a significance of 4.2 \sigma , or a false alarm probability of 1.6 \times 10 ^ { -5 } , i.e .
a similar cross-correlation within the expected frequency/time window after the merger can not be found more than 4 times in 3 days .
If confirmed , this finding will have significant consequences for both physics of quantum black holes and astrophysics of binary neutron star mergers .