  We study the mass spectra and radiative decays of doubly heavy baryons within
the diquark picture in a relativized quark model. The mass of the $J^P=1/2^+$
$\Xi_{cc}$ ground state is predicted to be 3606 MeV, which is consistent with
the mass of $\Xi_{cc}^{++}(3621)$ newly observed by the LHCb collaboration. The
predicted mass gap between two $S$ wave states, $\Xi_{cc}^*$ ($J^P=3/2^+$) and
$\Xi_{cc}$ ($J^P=1/2^+$), is 69 MeV. Furthermore, the radiative transitions of
doubly heavy baryons are also estimated by using the realistic wave functions
obtained from relativized quark model. The radiative decay widths of
$\Xi_{cc}^{*++} \to \Xi_{cc}^{++}\gamma$ and $\Xi_{cc}^{*+} \to
\Xi_{cc}^+\gamma$ are predicted to be about 7 and 4 keV, respectively. These
predictions of doubly heavy baryons can provide helpful information for future
experimental searches.
