Novel very-high-frequency quasi-periodic oscillations of compact, non-singular objects

We report on a novel set of very-high-frequency quasi-periodic oscillations
(VHFQPO’s) in the context of compact, non-singular horizonless objects. Focussing on the static, spherically symmetric case we utilize metrics of non-singular black holes that are accompanied by a regulator length scale L > 0. The choice L ≳ GM generically removes the horizon from these metrics leading to compact, horizonless but non-singular objects. This generically guarantees the existence of a stable orbit at small radii r ≪ rISCO, independent of the angular momentum of the massive particle. Crucially, the absence of a horizon allows the resulting VHFQPO’s to escape to infinity, spanning the range from 1kHz (M = 10M⊙) to 25 kHz (M = 2M⊙). Within the paradigm of non-singular spacetime geometries, the absence of such VHFQPO’s from X-ray binary spectra implies the presence of a horizon around the central, compact object.