Single-quark electromagnetic form factors of charmonium up to $J=2$

We calculate the single-quark electromagnetic form factors of a broad subset of charmonium, including $\eta_c(1S)$, $\eta_c(2S)$, $\chi_{c0}(1P)$, $\chi_{c0}(2P)$, $J/\psi(1S)$, $J/\psi(2S)$, $\chi_{c1}(1P)$, $\chi_{c1}(2P)$, $h_c(1P)$, $h_c(2P)$, $\chi_{c2}(1P)$ and $\chi_{c2}(2P)$, via a relativized quark model. The reference frame dependence of the results is estimated as the computational error. We compare our results with those of the lattice quantum chromodynamics (LQCD), the Dyson-Schwinger equation (DSE) and the basis light front quantization (BLFQ) approaches where available and we find that most of our results agree with the other results. We also predict the single-quark electromagnetic form factors of $\chi_{c0}(2P)$, $\chi_{c1}(2P)$, $h_c(1P)$, $h_c(2P)$, $\chi_{c2}(1P)$ and $\chi_{c2}(2P)$, where no direct comparisons are available.