Microlensing of stars, e.g. in the Galactic bulge and Andromeda galaxy (M31), is among the most robust, powerful method to constrain primordial black holes (PBHs) that are a viable candidate of dark matter. If PBHs are in the mass range $M_{rm PBH} lower.5exhbox{$; buildrel < over sim ;$} 10^{-10}M_odot$, its Schwarzschild radius ($r_{rm Sch}$) becomes comparable with or shorter than optical wavelength ($lambda)$ used in a microlensing search, and in this regime the wave optics effect on microlensing needs to be taken into account. For a lensing PBH with mass satisfying $r_{rm Sch}sim lambda$, it causes a characteristic oscillatory feature in the microlensing light curve, and it will give a smoking gun evidence of PBH if detected, because any astrophysical object cannot have such a tiny Schwarzschild radius. Even in a statistical study, e.g. constraining the abundance of PBHs from a systematic search of microlensing events for a sample of many source stars, the wave effect needs to be taken into account. We examine the impact of wave effect on the PBH constraints obtained from the $r$-band (6210AA) monitoring observation of M31 stars in Niikura et al. (2019), and find that a finite source size effect is dominant over the wave effect for PBHs in the mass range $M_{rm PBH}simeq[10^{-11},10^{-10}]M_odot$. We also discuss that, if a denser-cadence (10~sec), $g$-band monitoring observation for a sample of white dwarfs over a year timescale is available, it would allow one to explore the wave optics effect on microlensing light curve, if it occurs, or improve the PBH constraints in $M_{rm PBH}lower.5exhbox{$; buildrel < over sim ;$} 10^{-11}M_odot$ even from a null detection.
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