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We present more than three years of observations at different frequencies,from radio to high-energy gamma-rays, of the Narrow-Line Seyfert 1 (NLS1)Galaxy PMN J0948+0022 (z=0.585). This source is the first NLS1 detected atenergies above 100 MeV and therefore can be considered the prototype of thisemerging new class of gamma-ray emitting active galactic nuclei (AGN). Theobservations performed from 2008 August 1 to 2011 December 31 confirmed thatPMN J0948+0022 generates a powerful relativistic jet, able to develop anisotropic luminosity at gamma-rays of the order of 10^48 erg s^-1, at the levelof powerful quasars. The evolution of the radiation emission of this source in2009 and 2010 followed the canonical expectations of relativistic jets, withcorrelated multiwavelength variability (gamma-rays followed by radio emissionafter a few months), but it was difficult to retrieve a similar pattern in thelight curves of 2011. The comparison of gamma-ray spectra before and including2011 data suggested that there was a softening of the high-energy spectralslope. We selected five specific epochs to be studied by modelling thebroad-band spectrum, characterised by an outburst at gamma-rays or verylow/high flux at other wavelengths. The observed variability can largely beexplained either by changes in the injected power, the bulk Lorentz factor ofthe jet or the electron spectrum. The characteristic time scale ofdoubling/halving flux ranges from a few days to a few months, depending on thefrequency and the sampling rate. The shortest doubling time scale at gamma-raysis 2.3+-0.5 days. These small values underline the need of highly-sampledmultiwavelength campaigns to better understand the physics of these sources.

Radio-to-gamma-ray Monitoring of the Narrow-Line Seyfert 1 Galaxy PMN J0948+0022 from 2008 to 2011