  Sombor index is a novel topological index, which was introduced by Gutman and
defined for a graph $G$ as $SO(G)=\sum\limits_{uv\in
E(G)}\sqrt{d_{u}^{2}+d_{v}^{2}}$, where $d_{u}=d_{G}(u)$ denotes the degree of
vertex $u$ in graph $G$.
  Extremal problems on the Sombor index for trees with a given diameter has
been considered by Chen et al. [H. Chen, W. Li, J. Wang, Extremal values on the
Sombor index of trees, MATCH Commun. Math. Comput. Chem. 87 (2022) 23--49] and
Li et al. [S. Li, Z. Wang, M. Zhang, On the extremal Sombor index of trees with
a given diameter, Appl. Math. Comput. 416 (2022) 126731]. As an extension of
results introduces above, we determine the maximum Sombor indices for unicyclic
graphs with a fixed order and given diameter.
