Blazars with strong emission lines were found to be associated mostly with broad-line type 1 Active Galactic Nuclei (AGN). Hitherto, evidence for blazars identified with Narrow Line Seyfert 1 (NLS1) AGN was limited to very few individual cases. Here we present a comprehensive study of a sample of 23 genuine radio-loud NLS1 galaxies which have the radio-loudness parameters, the ratio of radio (21 cm) to optical (4400Å) luminosity, greater than 100. The sample, drawn from the SDSS and FIRST, is homogeneous and the largest of this kind. A significant fraction of the sample objects show interesting radio to X-ray properties that are unusual to most of the previously known radio-loud NLS1 AGN, but are reminiscent of blazars. These include flat radio spectra, large amplitude flux and spectral variability, compact VLBI cores, very high brightness temperatures (10 11−14 K) derived from variability, enhanced optical emission in excess of the normal ionising continuum, flat X-ray spectra, and blazar-like SEDs. We interpret them as evidence for the postulated blazar nature of these very radio-loud NLS1 AGN, which might possess at least moderately relativistic jets. We suggest that those steep spectrum radio-loud NLS1 AGN in the sample are of the same population but with their radio jets aligned at large angles to the lines-of-sight. Intrinsically, some of the objects have relatively low radio power and would have been classified as radio-intermediate AGN.The black hole masses, estimated from the broad Balmer line width and luminosity, are within 10 6−8 M ⊙ , and the inferred Eddington ratios are around unity. Unless the black hole masses are largely under-estimated, our result stretches the low mass end of the black holes of luminous, fast accreting radio-loud AGN to a smaller mass regime (the order of 10 6 M ⊙ ) in the black hole mass-radio-loudness space where other normal AGN are seldom found. The results imply that radio-loud AGN may be powered by black holes with moderate masses (∼ 10 6−7 M ⊙ ) accreting at high rates (Eddington ratios up to unity or higher). The host galaxies of a few nearby objects appear to be disk-like or merger; and some of the objects show imprints of young stellar populations in their SDSS spectra. We find that some of the objects, despite having strong emission lines, resemble high-energy peaked BL Lacs in their SED with the synchrotron component peaked at around the UV; such objects constitute an intriguingly high fraction of the sample. The radio sources of the sample are ubiquitously compact. They are smaller than at most several tens of kilo-parsecs, suggesting a possible link with compact steep-spectrum radio sources. Given the peculiarities of blazar-like NLS1 galaxies, questions arise as to whether they are plain downsizing extensions of normal radio-loud AGN, or whether they form a previously unrecognised population.

The dependence of the long-term optical/UV variability on the spectral and the fundamental physical parameters for radio-quiet active galactic nuclei (AGNs) is investigated. The multi-epoch repeated photometric scanning data in the Stripe-82 region of the Sloan Digital Sky Survey (SDSS) are exploited for two comparative AGN samples (mostly quasars) selected therein, a broad-line Seyfert 1 (BLS1) type sample and a narrow-line Seyfert 1 (NLS1) type AGN sample within redshifts 0.3-0.8. Their spectral parameters are derived from the SDSS spectroscopic data. It is found that on rest-frame timescales of several years the NLS1-type AGNs show systematically smaller variability compared to the BLS1-type. In fact, the variability amplitude is found to correlate, though only moderately, with the Eigenvector 1 parameters, i.e., the smaller the Hβ linewidth, the weaker the [O III] and the stronger the Fe II emission, the smaller the variability amplitude is. Moreover, an interesting inverse correlation is found between the variability and the Eddington ratio, which is perhaps more fundamental. The previously known dependence of the variability on luminosity is not significant, and that on black hole mass-as claimed in recent papers and also present in our data-fades out when controlling for the Eddington ratio in the correlation analysis, though these may be partly due to the limited ranges of luminosity and black hole mass of our samples. Our result strongly supports that an accretion disk is likely to play a major role in producing the opitcal/UV variability.

Narrow-line Seyfert 1 galaxies (NLS1s) with very small broad-line widths (say, FWHM(Hβ) 1200 km s −1 ) represent the extreme type of Seyfert 1 galaxies that have small black hole masses (M BH ) and/or high Eddington ratios (L/L Edd ). Here we study the X-ray properties of a homogeneously and optically selected sample of 13 such objects, termed as very narrow line Seyfert 1 galaxies (VNLS1s), using archival XMM-Newton data. It is found that the Fe Kα emission line is at most weak in these objects. A soft X-ray excess is ubiquitous, with the thermal temperatures falling within a strict range of 0.1-0.2 keV. Our result highlights the puzzling independence of the thermal temperature by extending the relations to even smaller FWHM(Hβ), i.e., smaller M BH (∼ 10 6 M ⊙ ) and/or higher L/L Edd . The excess emission can be modeled by a range of viable models, though the disk reflection and Comptonization models generally give somewhat better fits over the smeared absorption and the p-free models. At the Eddington ratios around unity and above, the X-ray spectral slopes in the 2-10 keV band are systematically flatter than the Risaliti et al.'s predictions of the relationship with L/L Edd suggested previously. Short timescale (1-2 hours) X-ray variability is common, which, together with the variability amplitude computed for some of the objects, are supportive of the scenario that NLS1s are indeed AGN with relatively small M BH .