Probing the Penrose Process: Images of Split Hotspots and Their Observational Signatures

TL;DR

Addresses the lack of observational evidence for the Penrose process by evaluating a magnetic-reconnection–driven mechanism in Kerr spacetime that accelerates a plasmoid into an escaping positive-energy fragment and an infalling negative-energy fragment. The authors implement a hotspot model with isotropic, broadband emission and perform backward ray tracing in a ZAMO frame to produce time-resolved images and light curves for spins $a = 0.94$ and $a = 0.99$ at observer azimuths $\phi_o=\pi/2$ and $\phi_o=0$. They find that plasmoid fragmentation yields distinctive, energy-sign–dependent flares, offering observable signatures of energy extraction and highlighting the role of viewing geometry. The study lays a framework for future, more comprehensive explorations, including other parameter regimes and the collisional Penrose process.

Abstract

While theoretically established for decades, the Penrose process - energy extraction from rotating black holes - still lacks clear observational evidence. A promising theoretical framework posits magnetic reconnection in the ergosphere as a trigger, causing a plasmoid to separate into an escaping positive-energy fragment and an infalling negative-energy one. In this work, we investigate the observational imprints of this scenario. We treat the energized plasmoid as a hotspot and calculate its light curves for a realistic plasma magnetization. In particular, we further compare with the scenario in which the plasmoid, after fragmentation, falls into the black hole with positive energy, while all other conditions remain unchanged. Our results reveal that the process of fragmentation generates distinct flares, whose characteristics depend heavily on whether the infalling fragment carries negative or positive energy. We propose that these differences serve as identifiable signatures of the Penrose process.

Figures (9)

• Figure 1: The schematic diagram of the magnetic reconnection-driven Penrose process.
• Figure 2: Image features in the absence of magnetic reconnection. The top-left panel presents a schematic of the plasmoid's trajectory in a two-dimensional Cartesian coordinate system, where the coordinates are defined as $x = r \sin\theta \cos\phi$, $y = r \sin\theta \sin\phi$. The black circular region in the image represents a Kerr black hole. The top-right panel depicts the intensity distribution of the hotspot as seen by the observer, illustrating the time-averaged radiation intensity across the observational plane; the intensity values are normalized by $I/I_{\mathrm{max}}$ . The bottom-left panel shows the temporal evolution of the centroid position, with colour encoding observational time (measured in minutes) and a temporal resolution of 2 minutes between successive data points. The bottom-right panel displays the light curve of the hotspot emission, revealing the variation of total flux over observational time.
• Figure 3: Temporal evolution of the hotspot intensity distribution in the magnetic-reconnection-driven Penrose process, composed of eight frames showcasing key moments. The timestamp in the upper left corner of each frame indicates the corresponding observational time in minutes. The sequence vividly reveals the dynamic spatial shifts of the hotspot as well as the evolving characteristics of its brightness distribution.
• Figure 4: Features of the magnetic-reconnection-driven Penrose process. The upper-left panel presents a schematic diagram of plasmoid trajectories in a two-dimensional Cartesian coordinate system. The black circular region denotes the event horizon of a Kerr black hole. The green solid line represents the trajectory of the plasmoid prior to reconnection, the blue solid line indicates the path of the ejected plasmoid after reconnection, and the red solid line shows the trajectory of the plasmoid falling into the black hole. The black dot marks the location where the reconnection event occurs. The upper-right panel displays the intensity distribution of the hotspot as perceived by a distant observer. The lower-left panel depicts the evolution of the brightness centroid as a function of observational time, with a uniform time interval of two minutes between successive points. The lower-right panel shows the light curve corresponding to the radiation emitted by the hotspot.
• Figure 5: Comparison plots of the intensity distribution for positive- and negative-energy plasmoids plunging into the black hole are shown. The upper row corresponds to negative-energy plasmoids with $\epsilon_{-} \approx -0.50$, while the lower row depicts positive-energy plasmoids with $\epsilon_{+} \approx 0.50$. The first column presents the normalized intensity distributions generated solely by the plasmoids as they fall into the black hole. The second column displays schematic trajectories of the infalling plasmoids. The color gradient along each trajectory illustrates the evolution of the plasmoid's position as a function of its proper time $\tau$. Specifically, we mark the position corresponding to approximately $\tau = 0.06$ in the plot at the centre of the first row, which precisely identifies the origin of the flare within the underlying light curves. The third column offers the light curves arising from hotspot emission, providing a comparative view of the total flux versus time for three scenarios: pre-reconnection and escaping plasmoids (solid red curve), infalling plasmoids (solid blue curve), and the overall process (dashed green curve).