Photon emissions from near-horizon extremal and near-extremal Kerr equatorial emitters

Abstract

We consider isotropic and monochromatic photon emissions from equatorial emitters moving along future-directed timelike geodesics in the near-horizon extremal Kerr (NHEK) and near-horizon near-extremal Kerr (near-NHEK) regions, to asymptotic infinity. We obtain numerical results for the photon escaping probability (PEP) and derive analytical expressions for the maximum observable blueshift (MOB) of the escaping photons, both depending on the emission radius and the emitter's proper motion. In particular, we find that for all antiplunging or deflecting emitters that can eventually reach to asymptotic infinity, the PEP is greater than 50% while for all plunging emitters the PEP is less than 55%, and for the bounded emitters in the (near-)NHEK region, the PEP is always less than 59%. In addition, for the emitters on unstable circular orbits in the near-NHEK region, the PEP decreases from 55% to 50% as the orbital radius decreases from the one of the innermost stable circular orbit to the one of the horizon. Furthermore, we show how the orientation of the emitter's motion along the radial or azimuthal direction affects the PEP and the MOB of the emitted photons.