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 $L \gtrsim GM$ 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 $r \ll r_\text{ISCO}$, 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 ($M = 10M_\odot$) to 25 kHz ($M = 2M_\odot$). 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.