No Observational Constraints from Hypothetical Collisions of Hypothetical Dark Halo Primordial Black Holes with Galactic Objects

It was suggested by several authors that hypothetical primordial black holes (PBHs) may contribute to the dark matter (DM) in our Galaxy. There are strong constraints based on the Hawking evaporation that practically exclude PBHs with masses m _pbh ~ 10¹⁵to10¹⁶ g and smaller as significant contributors to the Galactic DM. Similarly, PBHs with masses greater than about 10²⁶ g are practically excluded by the gravitational lensing observation. The mass range between 10¹⁶ g <m _pbh < 10²⁶ g is unconstrained. In this paper, we examine possible observational signatures in the unexplored mass range, investigating hypothetical collisions of PBHs with main-sequence stars, red giants, white dwarfs, and neutron stars in our Galaxy. This has previously been discussed as possibly leading to an observable photon eruption due to shock production during the encounter. We find that such collisions are either too rare to be observed (if the PBH masses are typically larger than about 10²⁰ g), or produce too little power to be detected (if the masses are smaller than about 10²⁰ g).