Hotspot Images Driven by Magnetic Reconnection in Kerr-Sen black hole

TL;DR

This work addresses detecting energy extraction via magnetic reconnection in Kerr-Sen black holes using hot-spot imaging. It applies the Comisso-Asenjo reconnection framework in Kerr-Sen spacetime and models emission with backward ray-tracing in a ZAMO frame to predict observable flare signatures. A key finding is a three-flare pattern, where the first flare linked to a negative-energy plasmoid may indicate ongoing energy extraction, while subsequent flares arise from positive-energy plasmoids and Doppler effects; the observability of this signature depends on the expansion parameter r_0 and the spin a, with near-extremal spins potentially reducing to two detectable flares. The results inform horizon-scale observational strategies and tests of Penrose-process energy extraction in string-theory inspired black holes, highlighting limitations and avenues for future multi-band emissivity modeling and non-geodesic dynamics.

Abstract

In the Kerr-Sen black hole, this study investigates the changes in hotspot images before and after the occurrence of magnetic reconnection. After reviewing the Comisso-Asenjo magnetic reconnection process and introducing the hotspot imaging method, we examine the temporal evolution of hotspot intensity, including when energy extraction occurs, when it does not occur, and when the observer's azimuthal angle is altered. We also discuss the influence of the black hole's expansion parameter and spin on hotspot imaging. The results indicate that the first flare may serve as a potential signature of ongoing energy extraction: changing the observer's azimuthal angle may alter the time interval between the first and second flares: a larger expansion parameter makes it more difficult to identify the energy extraction signal, and a higher spin also makes it more challenging to detect the energy extraction signal.

Figures (7)

• Figure 1: Time evolution of the plasma hot spot intensity distribution in the Kerr-Sen black hole
• Figure 2: The left panel displays the trajectory of the plasmoid in a two-dimensional Cartesian coordinate system. The blue curve represents the case without magnetic reconnection, the red curve represents the accelerated plasma, and the green curve represents the decelerated plasma. The black circle denotes the black hole. The middle panel depicts the observed hot spot intensity distribution, showing the time-averaged radiation intensity on the observation plane, normalized by $I/I_{MAX}$. The outer ring in the middle panel represents secondary or higher-order images. The inner ring and the tailed stripe in the middle panel are the primary images. The right panel shows the light curve of the hot spot emission, revealing the variation of the total flux with observation time.
• Figure 3: The left panel illustrates the normalized hot spot intensity distribution generated solely by the decelerated plasma. The middle panel shows light curves, the blue solid line represents $\varepsilon_{+}$ and the before reconnection intensity, the red solid line represents the intensity from only $\varepsilon_{-}$, and the green dashed line represents the total light curve observed. The right panel displays the photon trajectories corresponding to the first flare.
• Figure 4: In the absence of energy extraction, the top-left panel shows the trajectory of the plasmoid, the top-right panel depicts the normalized intensity distribution of the hot spot observed, the bottom-left panel displays the light curve of the hot spot emission, and the bottom-right panel presents the three types of light curves.
• Figure 5: When $\phi_0=0$, the left panel depicts the normalized intensity distribution of the hot spot observed, the middle panel shows the light curve of the hot spot emission, and the right panel displays the light curves for the three cases.