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Lyman-$\alpha$ spheres, i.e. regions around the first stars which areilluminated by Lyman-$\alpha$ photons and show 21cm absorption feature againstthe CMB, are smoking guns at the dawn of the reionization epoch. Thoughoverwhelming radio foreground makes their detections extremely difficult, wepointed out that, strong gravitational lensing can significantly improve theirobservational feasibility. Since Lyman-$\alpha$ spheres have ~10" sizes,comparable to the caustic size of galaxy clusters, individual images of eachstrongly lensed Lyman-$\alpha$ sphere often merge together and form singlestructures in the 21cm sky with irregular shapes. Using high-resolution N-bodyLCDM simulations, we found that the lensing probability to have magnificationbigger than 10 is ~10^{-5}. This results in $\ga 10^6$ strongly lensedLyman-$\alpha$ spheres across the sky, which should be the primary targets forfirst detections of Lyman-$\alpha$ spheres. Although the required total radioarray collecting area for their detection is large (~100 km^2), the design oflong fixed cylindrical reflectors can significantly reduce the total cost ofsuch array to the level of the square kilometer array (SKA) and makes thedetection of these very first objects feasible.Comment: 5 pages, 3 figures, accepted by the Astrophysical Journa

Detecting First‐Star Lyα Spheres through Gravitational Telescopes