A population of radio-loud narrow-line Seyfert 1 galaxies with blazar-like properties?

Blazars having strong emission lines were found to be associated mostly with broad-line type 1 active galactic nuclei (AGNs). Hitherto, evidence for blazars identified with narrow-line Seyfert 1 (NLS1) AGNs was limited to only a very few individual cases. Here we present a comprehensive study of a sample of 23 genuine radio-loud NLS1 AGNs with radio loudness larger than 100. The radio sources of the sample are ubiquitously compact, unresolved on scales of several arcseconds. Some of these objects show interesting radio to X-ray properties that are unusual to most of the previously known radio-loud NLS1 AGNs, but are reminiscent of blazars. These include flat radio spectra, large-amplitude flux and spectral variability, compact VLBI cores, very high variability brightness temperatures, enhanced optical continuum emission, flat X-ray spectra, and blazar-like spectral energy distributions (SEDs). We interpret them as signatures of the postulated blazar nature of some of the objects, which likely possess at least moderately relativistic jets. Intrinsically, some of them have relatively low radio power and would have been classified as radio-intermediate AGNs. The black hole masses are estimated to be within 10(6)-10(8) M-circle dot, and the Eddington ratios close to unity, as in normal NLS1 AGNs. The results imply that radio-loud AGNs may be powered by black holes of moderate masses (similar to 10(6)-10(7) M-circle dot) accreting at high rates. Some of the objects, despite having strong emission lines, resemble high-energy-peaked BL Lac objects (HBLs) in their SEDs. Given the peculiarities of blazar-like NLS1 AGNs, questions arise as to whether they are plain downsizing extensions of normal radio-loud AGNs, or form a previously unrecognized population.