Lepton accelerators near the event horizon, and the very-high-energy emission they produce.
KASI / UST and Academia Sinica, with Dr Kouichi Hirotani and Dr Satoki Matsushita · 2015 – 2018
Pulsars have a well-developed theory of particle acceleration in vacuum gaps — regions where the plasma fails to screen the electric field along the magnetic field lines, so charged particles are accelerated to enormous energies and radiate. The question we asked is whether the same machinery operates around a black hole, which has no surface, no crust, and no rotating magnet — only spacetime.
It does, and frame dragging is why. Near a rapidly rotating black hole, frame dragging reverses the sign of the Goldreich–Julian charge density. That reversal leaves an unscreened magnetic-field-aligned electric field, which accelerates electrons and positrons in opposite directions and drives a pair cascade. The result is a lepton accelerator in the immediate vicinity of the event horizon(Song et al., 2017).
Predicted spectrum for a 10 M☉ extremally rotating black hole at 3 kpc. The GeV–TeV emission rises above the Fermi-LAT and CTA sensitivity curves — which is what makes the prediction testable rather than merely interesting.
The observational prediction. The resulting gamma-ray flux is beamed towards the rotation axis, and it is strongly spin-dependent: it rises by more than an order of magnitude as the spin increases from \(a = 0.90M\) to \(a = 0.9999M\). That makes it a testable statement for Fermi-LAT and CTA, not merely a theoretical curiosity — a detection would constrain the spin of the emitting object.
Stellar-mass black holes too. Extending the same framework to a stellar-mass black hole traversing a dense gas cloud, we found that the pair cascade produces detectable very-high-energy emission if the hole is extremally rotating and within roughly 1 kpc (Hirotani et al., 2018).
This work ran across the Korea Astronomy and Space Science Institute and the University of Science and Technology, and a visiting studentship at the Academia Sinica Institute of Astronomy and Astrophysics in Taipei. The 2017 paper was my first peer-reviewed first-author publication, at nineteen.
Frame dragging near a rapidly rotating black hole reverses the sign of the Goldreich-Julian charge density, producing a magnetic-field-aligned electric field that accelerates electrons and positrons in opposite directions and drives a pair cascade. We show that a stellar-mass black hole moving through a gaseous cloud can emit a detectable very-high-energy flux if it is extremely rotating and within roughly 1 kpc.
@article{hirotani2018stellar,title={High-energy and Very High Energy Emission from Stellar-mass Black Holes Moving in Gaseous Clouds},author={Hirotani, Kouichi and Pu, Hung-Yi and Outmani, S. and Huang, H. and Kim, D. and Song, Yoogeun and Matsushita, Satoki and Kong, Albert K. H.},journal={The Astrophysical Journal},volume={867},number={2},pages={120},year={2018},doi={10.3847/1538-4357/aae47a},}
Applying pulsar outer-gap theory to black hole magnetospheres, we show that a lepton accelerator arises near the event horizon as a consequence of frame dragging. The resulting gamma-ray flux is beamed towards the rotation axis and is enhanced by more than an order of magnitude as the spin increases from a = 0.90M to a = 0.9999M, with implications for Fermi-LAT and CTA detectability.
@article{song2017gamma,title={Enhanced gamma radiation towards the rotation axis from the immediate vicinity of extremely rotating black holes},author={Song, Yoogeun and Pu, Hung-Yi and Hirotani, Kouichi and Matsushita, Satoki and Kong, Albert K. H. and Chang, Hsiang-Kuang},journal={Monthly Notices of the Royal Astronomical Society: Letters},volume={471},number={1},pages={L135--L139},year={2017},month=oct,doi={10.1093/mnrasl/slx119},}
