A photon number density operator in the covariant formulation of quantum electrodynamics

We introduce a new photon number density operator in the Gupta-Bleuler quantized photon field and show that the corresponding total number operator counts transverse photons in the new model. It is also shown that the suggested operator interpreted in the usual Hilbert space counts, in adition, a number of gauge photons for the free field. In the case of two fixed charges it also counts an extra number of scalar photons due the charges.