  Charge radii of mirror nuclei are calculated by implementing pairing effects
with the Hartree-Fock Bogoliubov approximation. Correlations between the
difference of charge radii ($\Delta R_{ch}$) and slope of nuclear symmetry
energy (L) are examined for different mirror nuclei pairs of varying masses
using 40 different Skyrme energy density functionals. $\Delta R_{ch}-L $
correlations are found to be robust for the binding constraints imposed on
density functionals. We observe that $\Delta R_{ch}$ and $L$ show better
correlations in relatively heavier pairs than those obtained in the lighter
pairs. Our calculations impose a constraint on the slope of nuclear symmetry
energy as -20 MeV $\leq L \leq$ 55 MeV with 68\% confidence band using
available measurements on charge radii. This is a moderately soft symmetry
energy, in contrast to stiff and soft symmetry energy indicated by PREX-II and
CREX measurements of neutron skin thickness in $^{208}Pb$ and $^{48}Ca$,
respectively. Our result is also in agreement with celestial constraints
obtained from observational data for neutron stars.
