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Previously unremarkable, the extragalactic radio source GB 1310+487 showed agamma-ray flare on 2009 November 18, reaching a daily flux of ~10^-6photons/cm^2/s at energies E>100 MeV and becoming one of the brightest GeVsources for about two weeks. Its optical spectrum is not typical for a blazar,instead, it resembles those of narrow emission-line galaxies. We investigatechanges of the object's radio-to-GeV spectral energy distribution (SED) duringand after the prominent GeV flare with the aim to determine the nature of theobject and constrain the origin of the variable high-energy emission. The datacollected by the Fermi and AGILE satellites at gamma-ray energies, Swift atX-ray and ultraviolet, Kanata, NOT, and Keck telescopes at optical, OAGH andWISE at infrared, and IRAM 30m, OVRO 40m, Effelsberg 100m, RATAN-600, and VLBAat radio, are analysed together to trace the SED evolution on timescales ofmonths. The gamma-ray/radio-loud narrow-line active galactic nucleus (AGN) islocated at redshift z=0.638. It is shining through an unrelated foregroundgalaxy at z=0.500. The AGN light is likely amplified by a factor of a fewbecause of gravitational lensing. The AGN SED shows a two-humped structuretypical of blazars and gamma-ray-loud NLSy1 galaxies, with the high-energy(inverse-Compton) emission dominating by more than an order of magnitude overthe low-energy (synchrotron) emission during gamma-ray flares. The differencebetween the two SED humps is smaller during the low-activity state. Fermiobservations reveal a strong correlation between the gamma-ray flux andspectral index, with the hardest spectrum observed during the brightestgamma-ray state. If the gamma-ray flux is a mixture of synchrotron self-Compton(SSC) and external Compton (EC) emission, the observed GeV spectral variabilitymay result from varying relative contributions of these two emissioncomponents.

Two active states of the narrow-line gamma-ray-loud AGN GB 1310+487