  We present simultaneous multiwavelength observations of the 4.66 ms redback
pulsar PSR J1048+2339. We performed phase-resolved spectroscopy with the Very
Large Telescope (VLT) searching for signatures of a residual accretion disk or
intra-binary shock emission, constraining the companion radial velocity
semi-amplitude ($K_2$), and estimating the neutron star mass ($M_{\rm NS}$).
Using the FORS2-VLT intermediate-resolution spectra, we measured a companion
velocity of $291 < K_2 < 348$ km s$^{-1}$ and a binary mass ratio of $0.209 < q
< 0.250$. Combining our results for $K_2$ and $q$, we constrained the mass of
the neutron star and the companion to $(1.0 < M_{\rm NS} < 1.6){\rm
sin}^{-3}i\,M_{\odot}$ and $(0.24 < M_2 < 0.33){\rm sin}^{-3}i\,M_{\odot}$,
respectively, where $i$ is the system inclination. The Doppler map of the
H$\alpha$ emission line exhibits a spot feature at the expected position of the
companion star and an extended bright spot close to the inner Lagrangian point.
We interpret this extended emission as the effect of an intra-binary shock
originating from the interaction between the pulsar relativistic wind and the
matter leaving the companion star. The mass loss from the secondary star could
be either due to Roche-lobe overflow or to the ablation of its outer layer by
the energetic pulsar wind. Contrastingly, we find no evidence for an accretion
disk. We report on the results of the SRT and the LOFAR simultaneous radio
observations at three different frequencies (150 MHz, 336 MHz, and 1400 MHz).
No pulsed radio signal is found in our search. This is probably due to both
scintillation and the presence of material expelled from the system which can
cause the absorption of the radio signal at low frequencies. Finally, we report
on an attempt to search for optical pulsations using IFI+Iqueye mounted at the
1.2 m Galileo telescope at the Asiago Observatory.
