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In this work, we study the black hole light echoes in terms of the two-photonautocorrelation and explore their connection with the quasinormal modes. It isshown that the above time-domain phenomenon can be analyzed by utilizing thewell-known frequency-domain relations between the quasinormal modes andcharacteristic parameters of null geodesics. We found that the time-domaincorrelator, obtained by the inverse Fourier transform, naturally acquires theecho feature, which can be attributed to a collective effect of the asymptoticpoles through a weighted summation of the squared modulus of the relevantGreen's functions. Specifically, the contour integral leads to a summationtaking over both the overtone index and angular momentum. Moreover, thedominant contributions to the light echoes are from those in the eikonal limit,consistent with the existing findings using the geometric-optics arguments. Forthe Schwarzschild black holes, we demonstrate the results numerically byconsidering a transient spherical light source. Also, for the Kerr spacetimes,we point out a potential difference between the resulting light echoes usingthe geometric-optics approach and those obtained by the black hole perturbationtheory. Possible astrophysical implications of the present study are addressed.

Quasinormal modes in two-photon autocorrelation and the geometric-optics approximation