Constraining electromagnetic signals from black holes with hair

We constrain a broad class of "hairy" black hole models capable of directly sourcing electromagnetic radiation during a binary black hole merger. This signal is generic and model-independent since it is characterized by the black hole mass (M ) and the fraction of that mass released as radiation (ε ). For field energy densities surpassing the Schwinger limit, this mechanism triggers pair production to produce a gamma-ray burst. By cross-referencing gravitational wave events with gamma-ray observations, we place upper bounds of ε <10^-5- 10^-4 for 10 - 50 M_⊙ black holes depending on the black hole mass. We discuss the weak detection of a gamma-ray burst following GW150914 and show that this event is consistent with rapid electromagnetic emission directly from a hairy black hole with ε ∼10^-7- 10^-6 . Below the Schwinger limit, ambient charged particles are rapidly accelerated to nearly the speed of light by the strong electromagnetic field. For 1 - 50 M_⊙ black holes and ε ranging from 10^-20 to 10^-7, the typical proton energies are ∼20 GeV - 20 TeV and electron energies are ∼0.01 - 10 GeV . At these energies, cosmic ray protons and electrons quickly diffuse into the Milky Way's background magnetic field, making it difficult to identify a point source producing them. Overall, constraining ε in this less energetic regime becomes difficult and future constraints may need to consider specific models of hairy black holes.