General Physical Properties of Gamma-Ray-emitting Narrow-line Seyfert 1 Galaxies

We present the results of a detailed multiwavelength study of a sample of 16 narrow-line Seyfert 1 (NLSy1) galaxies known in γ-rays so far. This includes a focused X-ray spectral analysis, to compare them with a more general NLSy1 population, and a broadband spectral energy distribution (SED) modeling considering the leptonic radiative processes to understand their similarity/dissimilarity with blazars. The X-ray spectra of γ-NLSy1 galaxies exhibit similarities to both typical NLSy1 sources below ≲2 keV and blazars at higher energies. We also find weak evidences of excess absorption in the soft X-ray spectra of three sources and a tentative detection of the Fe line in 1H 0323+342. A comparison of the broadband parameters derived from the SED modeling reveals the similarity of γ-NLSy1 objects to blazars, in particular to flat-spectrum radio quasars. The γ-NLS1 galaxies host relatively low power jets with small bulk Lorentz factors with respect to blazars, and this explains the paucity of such sources in the γ-ray catalogs and identification of even fewer parent population members. Many of the observed features of these intriguing objects suggest that they host low-mass black holes. The accretion rate in Eddington units can be considered as the driving factor to explain the physical properties of γ-NLSy1 sources, similar to that which is known for the general blazar population.