SDSS J211852.96-073227.5: a new {\gamma}-ray flaring narrow-line Seyfert
  1 galaxy
  We report on the identification of a new {\gamma}-ray-emitting narrow-line
Seyfert 1 (NLS1) galaxy, SDSS J211852.96-073227.5 (hereinafter J2118-0732). The
galaxy, at a redshift of 0.26, is associated with a radio source of
flat/inverted spectrum at high radio frequencies. The analysis of its optical
spectrum obtained in the Sloan Digital Sky Survey (SDSS) revealed a small
linewidth of the broad component of the H{\beta} line (full width at
half-maximum = 1585 km s^{-1}), making it a radio-loud NLS1 galaxy -- an
intriguing class of active galactic nuclei with exceptional multiwavelength
properties. A new {\gamma}-ray source centred at J2118-0732 was sporadically
detected during 2009--2013 in form of flares by the Fermi-LAT. Our XMM-Newton
observations revealed a flat X-ray spectrum described by a simple power law,
and a flux variation by a factor of ~2.5 in five months. The source also shows
intraday variability in the infrared band. Its broad-band spectral energy
distribution can be modelled by emission from a simple one-zone leptonic jet
model, and the flux drop from infrared to X-rays in five months can be
explained by changes of the jet parameters, though the exact values may be
subject to relatively large uncertainties. With the NLS1-blazar composite
nucleus, the clear detection of the host galaxy, and the synchronous variations
in the multiwavelength fluxes, J2118-0732 provides a new perspective on the
formation and evolution of relativistic jets under the regime of relatively
small black hole masses and high accretion rates.
