Revisiting the shadow of braneworld black holes

In the Randall-Sundrum braneworld scenario, there has been no exact rotating black hole solutions throughout the whole spacetime so far. Instead, a Kerr-Newman-like metric approximately describing the region near the black hole and a linearized metric portraying the far region has been found. In the previous studies, only the Krr-Newman-like metric was considered to analyze the black hole shadows observed by distant observers in the braneworld scenario, while the linearized metric has been ignored. In this work, we revisit the shadows of rotating braneworld black holes in the Randall-Sundrum type II model, by considering not only the metric in the near region of the black hole but also the linearized metric in the far region where the observer stays. Our study is significantly different from previous studies. From our study, for black holes with an event horizon, we identify a critical angle ${\ensuremath{\theta}}_{c}$ which decides the shadow curve is open or closed; the shadow curve would be open if the observational angle ${\ensuremath{\theta}}_{c}<{\ensuremath{\theta}}_{o}\ensuremath{\le}\ensuremath{\pi}/2$, and the curve becomes closed when $0<{\ensuremath{\theta}}_{o}\ensuremath{\le}{\ensuremath{\theta}}_{c}$. In contrast, the open shadow curves were found only for braneworld black holes with naked singularities in the literature. We study how various parameters affect the shape of the shadow curve. We furthermore apply our analysis to the data of M87* from the Event Horizon Telescope and obtain a new constraint on the parameters of the braneworld black holes, which is also different from the previous results.

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