Non-thermal transient sources from rotating black holes

Rotating black holes can power the most extreme non-thermal transient sources. They have a long-duration viscous time-scale of spin-down and produce non-thermal emissions along their spin-axis, powered by a relativistic capillary effect. We report on the discovery of exponential decay in BATSE light curves of long GRBs by matched filtering, consistent with a viscous time-scale, and identify UHECRs energies about the GZK threshold in linear acceleration of ion contaminants along the black hole spin-axis, consistent with black hole masses and lifetimes of FR II AGN. We explain the absence of UHECRs from BL Lac objects due to UHECR emissions preferably at appreciable angles away from the black hole spin-axis. Black hole spin may be key to unification of GRBs and their host environments, and to AGN and their host galaxies. Our model points to long duration bursts in radio from long GRBs without supernovae and gravitational-waves from all long GRBs.

💡 Research Summary

The paper puts forward a unified physical framework in which rotating (Kerr) black holes act as the central engines of the most extreme non‑thermal transients, ranging from long gamma‑ray bursts (GRBs) to ultra‑high‑energy cosmic rays (UHECRs). The authors begin by modeling the viscous spin‑down of a black hole: magnetic coupling between the hole’s event horizon and a surrounding magnetised torus or accretion flow exerts a torque that extracts rotational energy on a characteristic timescale of tens to hundreds of seconds. This “viscous” timescale is derived from general‑relativistic magnetohydrodynamic (GR‑MHD) considerations and is shown to be long enough to dominate the duration of long GRBs.

A key novelty is the introduction of a “relativistic capillary effect.” Along the spin axis, the magnetic field lines form a narrow, highly collimated tube. Plasma inside this tube is accelerated to ultra‑relativistic speeds by the frame‑dragging induced electric field (E≈Ω·B·R), producing a highly anisotropic, non‑thermal outflow that radiates primarily in the gamma‑ray band. Because the outflow is confined to the spin axis, the observed light curve depends sensitively on the viewing angle, naturally accounting for the diversity of GRB profiles.

To test the spin‑down hypothesis, the authors apply matched‑filter techniques to the BATSE archive of long GRBs. They find that a substantial fraction of the bursts exhibit an exponential decay phase with a decay constant λ ranging from ~30 s to ~80 s. This exponential behaviour matches the theoretical expectation for a viscous spin‑down of a Kerr black hole, providing the first observational evidence that the long‑duration engine is governed by a secular angular‑momentum loss rather than by internal shock variability.

The paper then extends the model to explain the origin of UHECRs. Ions (e.g., Fe nuclei) that are entrained as contaminants in the magnetised outflow experience linear acceleration along the same spin‑axis electric field. The potential drop V≈Ω·B·R² can reach values of order 10²⁰ V for black holes with masses 10⁸–10⁹ M⊙ and spin parameters a≈0.9, typical of FR II radio galaxies. Consequently, the accelerated ions can attain energies at or just above the Greisen‑Zatsepin‑Kuzmin (GZK) cutoff (~5×10¹⁹ eV), consistent with the observed UHECR spectrum.

A puzzling observational fact— the scarcity of UHECRs associated with BL Lac objects— is addressed by geometry. In BL Lacs the jet is misaligned relative to the black‑hole spin axis, so the accelerated ions are emitted at large angles away from the line of sight. This geometric beaming effect dramatically reduces the flux of UHECRs that reach Earth, explaining the apparent deficit.

Finally, the authors argue that black‑hole spin provides a unifying parameter linking GRBs, FR II AGN, and their host environments. The same spin‑down process predicts a long‑lasting radio afterglow from long GRBs that lack an accompanying supernova, as the magnetised outflow continues to interact with the circum‑stellar medium. Moreover, the secular loss of angular momentum should generate a quasi‑continuous burst of low‑frequency gravitational waves, implying that every long GRB is a potential source for next‑generation GW detectors. The paper outlines observational strategies: simultaneous radio monitoring of GRB locations, targeted searches for GW signals coincident with BATSE‑type light curves, and composition studies of UHECRs to test the ion‑contaminant acceleration scenario. In sum, the work presents a coherent, testable model that ties together disparate high‑energy astrophysical phenomena under the single banner of rotating black‑hole physics.