SDSS J211852.96$-$073227.5: a new blazar-like narrow-line Seyfert 1 galaxy with flaring $\gamma$-ray radiation

We report on the identification of a new $\gamma$-ray-emitting narrow-line Seyfert 1 (NLS1) galaxy, SDSS J211852.96$-$073227.5 (hereafter J2118$-$0732). The galaxy, at a redshift of 0.26, is associated with a radio source of flat/inverted spectrum at high radio frequencies. The analysis of its optical spectrum obtained from the SDSS revealed a small line width of the broad component of the H$\beta$ line (FWHM = 1590 km\,s$^{-1}$), making it a radio-loud NLS1 galaxy --- an intriguing class of AGN with exceptional multi-wavelength properties. A new $\gamma$-ray source centred at J2118$-$0732 was sporadically detected during 2009-2013 in form of flares by the {\it Fermi}-LAT. Our {\it XMM-Newton} observations revealed a flat X-ray spectrum described by a simple power law, and a flux variation by a factor of $\sim$2.4 in 5~months. The source also shows intraday variability in the IR band. The drop of the broad-band fluxes from the IR, optical to X-ray in 5 months can be modeled by a drop of both the non-thermal jet emission and the emission from an accretion disk, with a decrease of the Eddington ratio by several times, suggesting the coupling of the disk accretion and the jet launching. With the NLS1-blazar composite nucleus, the clear detection of the host galaxy and the synchronous variations in the multiwavelength fluxes and the disk component, J2118$-$0732 provides a new perspective on the formation and evolution of relativistic jets as well as the disk-jet coupling mechanism.