General-relativistic magnetohydrodynamic (BHAC) simulations of magnetically arrested discs, post-processed with general-relativistic radiative transfer (BHOSS), are used to model the multi-wavelength variability of Sgr A*. We compare electron-heating prescriptions (R-beta, turbulent, and magnetic-reconnection heating) across black hole spins a = -0.94 to +0.94 and assess their observational signatures.
@article{song2026sgra,title={Multi-wavelength variability of {Sagittarius A*} from {GRMHD} simulations with electron heating},author={Song, Yoogeun and Younsi, Ziri and Mizuno, Yosuke and Wu, Kinwah and Fromm, Christian M.},journal={Monthly Notices of the Royal Astronomical Society},year={2026},note={Draft in preparation},}
A zero-angular-momentum-observer (ZAMO) treatment of Blandford-Znajek energy extraction from rotating black holes, in which the frame-dragging force-free magnetosphere mediates the outward transport of electromagnetic energy.
@article{okamoto2024kerr,title={Electromagnetic Energy Extraction in {Kerr} Black Holes through Frame-Dragging Magnetospheres},author={Okamoto, Isao and Uchida, Toshio and Song, Yoogeun},journal={arXiv preprint},year={2024},}
The force-free magnetosphere of a Kerr black hole is divided at the null surface, and electromagnetic energy is shown to be self-extracted across it – a reformulation of the Blandford-Znajek process in terms of frame dragging.
@article{okamoto2019selfextraction,title={Energy self-extraction of a {Kerr} black hole through its frame-dragged force-free magnetosphere},author={Okamoto, Isao and Song, Yoogeun},journal={arXiv preprint},year={2023},note={v7},}
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},}
Raychaudhuri-type and shear-evolution equations are derived for geodesic congruences in a Bianchi-I universe, and used to study how anisotropic shear imprints itself on primordial gravitational waves.
@article{song2016shear,title={The Influence of the Shear on the Gravitational Waves in the Early Anisotropic Universe},author={Song, Yoo Geun and Park, Seok Jae},journal={arXiv preprint},year={2016},}