Neutral particle collisions near Gibbons-Maeda-Garfinkle-Horowitz-Strominger black holes after shadow observations

A Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) black hole with a magnetic charge (or an electric charge) has noteworthy features that its scalar curvature near the event horizon of the black hole with the almost maximal charge can be extremely large. The large curvature, which is related with the gravity on a finite-sized object or between two points, causes high center-of-mass energy for two neutral particles near the almost maximally charged GMGHS black hole. Recently, the Event Horizon Telescope Collaboration gave the bound on the charge of black holes from the shadow and mass observations of black holes under an assumption that the diameter of observed rings are proportion to that of photon spheres. The photon sphere would be less related with the curvature, since it is determined by the behavior of one photon or one ray neither two photons nor two rays. Thus, the high-energy neutral particle collision and the black hole shadow observations would be complementary to distinguish the GMGHS black hole from other black hole solutions. In this paper, we investigate a new way to compare the center-of-mass energy for neutral particle collisions in the GMGHS spacetime and other black hole spacetimes. From the shadow observations and the mass observations under the assumptions on the effect of black hole charges, we can put constraints on the center-of-mass energy of the particles. We apply our method to shadow and mass observations of M87* and Sagittarius~A*. We find that the center-of-mass energy of neutral particles near the GMGHS black holes cannot be extremely large under the observational constraints, and conclude that the GMGHS spacetimes are hardly distinguishable from the Reissner-Nordstr\"{o}m spacetimes by the particle collisions if we apply the shadow and mass observations at $1 \sigma$ probability.