Radio-to-gamma-ray Monitoring of the Narrow-Line Seyfert 1 Galaxy PMN
  J0948+0022 from 2008 to 2011
  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 at
energies above 100 MeV and therefore can be considered the prototype of this
emerging new class of gamma-ray emitting active galactic nuclei (AGN). The
observations performed from 2008 August 1 to 2011 December 31 confirmed that
PMN J0948+0022 generates a powerful relativistic jet, able to develop an
isotropic luminosity at gamma-rays of the order of 10^48 erg s^-1, at the level
of powerful quasars. The evolution of the radiation emission of this source in
2009 and 2010 followed the canonical expectations of relativistic jets, with
correlated multiwavelength variability (gamma-rays followed by radio emission
after a few months), but it was difficult to retrieve a similar pattern in the
light curves of 2011. The comparison of gamma-ray spectra before and including
2011 data suggested that there was a softening of the high-energy spectral
slope. We selected five specific epochs to be studied by modelling the
broad-band spectrum, characterised by an outburst at gamma-rays or very
low/high flux at other wavelengths. The observed variability can largely be
explained either by changes in the injected power, the bulk Lorentz factor of
the jet or the electron spectrum. The characteristic time scale of
doubling/halving flux ranges from a few days to a few months, depending on the
frequency and the sampling rate. The shortest doubling time scale at gamma-rays
is 2.3+-0.5 days. These small values underline the need of highly-sampled
multiwavelength campaigns to better understand the physics of these sources.
