Speaker
Description
The Event Horizon Telescope images of M87 and Sgr A give us a unique chance to test gravity in the strong field regime and assess deviations from the Kerr solution. We study the shadows of rotating nonsingular black holes—namely, generalized Ghosh black holes (GGBHs), given by the mass function $m(r) = M e^{-(k/r)^n}$, which reduces to the Kerr black hole ($k \to 0$) and to the original Ghosh model ($n = 1$). We calculate the photon's trajectory via the Hamilton-Jacobi method and construct the shadow seen by a distant observer. For a high spin $a/M = 0.9$, increasing $k/M$ from $0$ to $0.5$ reduces the shadow size by about $13\%$ and the shadow area by about $24\%$, while making the shape more distorted (up to $\sim 20\%$). For larger values $k/M \sim 1.0$, the area can decrease by nearly $30\%$. However, increasing $n$ reduces these effects, and for $n \geq 3$ the shadow becomes very close to the Kerr case (differences below $\sim 5\%$).
To compare with observations, we use two simple shadow quantities: the area $A$ and the oblateness $D$. Using EHT observational data, we find that for M87 ($17^\circ$ inclination, $37.8\ \mu\text{as}$ diameter), the parameter is constrained as $k/M \lesssim 0.133$ for $n=1$. For Sgr A ($50^\circ$ inclination, $48.7\ \mu\text{as}$ diameter), the bound is $k/M \lesssim 0.448$. These limits become weaker for larger $n$. Overall, the GGBHs are a viable alternative to the Kerr black hole, and future observations may help to detect or further constrain such deviations.