  We report on multifrequency observations performed during 2012 December-2013
August of the first narrow-line Seyfert 1 galaxy detected in gamma rays, PMN
J0948+0022 ($z$ = 0.5846). A gamma-ray flare was observed by the Large Area
Telescope on board Fermi during 2012 December-2013 January, reaching a daily
peak flux in the 0.1-100 GeV energy range of (155 $\pm$ 31) $\times$10$^{-8}$
ph cm$^{-2}$ s$^{-1}$ on 2013 January 1, corresponding to an apparent isotropic
luminosity of about 1.5$\times$10$^{48}$ erg s$^{-1}$. The gamma-ray flaring
period triggered Swift and VERITAS observations in addition to radio and
optical monitoring by OVRO, MOJAVE, and CRTS. A strong flare was observed in
optical, UV, and X-rays on 2012 December 30, quasi-simultaneously to the
gamma-ray flare, reaching a record flux for this source from optical to gamma
rays. VERITAS observations at very high energy (E > 100 GeV) during 2013
January 6-17 resulted in an upper limit of F (> 0.2 TeV) <
4.0$\times$10$^{-12}$ ph cm$^{-2}$ s$^{-1}$. We compared the spectral energy
distribution (SED) of the flaring state in 2013 January with that of an
intermediate state observed in 2011. The two SEDs, modelled as synchrotron
emission and an external Compton scattering of seed photons from a dust torus,
can be modelled by changing both the electron distribution parameters and the
magnetic field.
