Blazar nuclei in radio-loud narrow-line Seyfert 1?
Introduction
The radio-loudness phenomenon, i.e. the dominance of radio over optical emission, is still poorly understood. Basically, it is known that a small percentage of Active Galactic Nuclei (AGN) displays this type of activity, which is commonly ascribed to the presence of a relativistic jet (Urry and Padovani, 1995), but the physics necessary to explain the generation, collimation, and evolution of the jet is yet to be known (e.g. Blandford, 2001). Among the possible ways to investigate this problem, one is to study the population of radio-loud narrow-line Seyfert 1 galaxy (NLSy1). Indeed, NLSy1 – as a peculiar subclass of Seyferts – are generally radio-quiet, but a small percentage (<7%, Komossa et al., 2006) of this population is radio-loud, i.e., with the ratio R between the 5 GHz and B filter (4400 Å) flux densities greater than 10, according to the definition by Kellermann et al. (1989). Therefore, it is worth investigating this type of sources to understand the origin of the radio loudness and, if due to a relativistic jet, how this is possible in a population of active nuclei that are mostly radio quiet.
After a few isolated cases of radio-loud narrow-line Seyfert 1 galaxies (e.g. Padovani et al., 2002) and early surveys (e.g. Zhou and Wang, 2002, Komossa et al., 2006, Whalen et al., 2006), the advent of the Sloan Digital Sky Survey (SDSS) made it possible to perform large scale surveys (Zhou et al., 2006, Yuan et al., 2008). Particularly, Komossa et al. (2006), with a sample including also sources with weak radio loudness (R ≳ 10), found that most of radio loud NLSy1 are resembling to GHz-Peaked/Compact Steep Spectrum sources (GPS/CSS), which in turn are believed to be the young stage of quasars (e.g. Guainazzi et al., 2006, Stawarz et al., 2008). Yuan et al. (2008) analyzed a sample of strong radio loud objects (R ≳ 50) and suggested similarities with high-energy peaked BL Lac Objects (HBL). Specifically, Yuan et al. (2008) claim to have found examples of “high-frequency peaked flat spectrum radio quasars” (HFSRQ, see Padovani, 2007), which are quasars with synchrotron peak in the X-rays. This type of source is not foreseen by the classical “blazar sequence” (Fossati et al., 1998, Ghisellini et al., 1998) and therefore the discovery of such types of sources can represent a failure in the “sequence”. Although some examples have been suggested (see Padovani, 2007 for a review), no conclusive evidence has been presented. In addition, Ghisellini and Tavecchio (2008) recently suggested that such type sources can exist and can be in agreement with the sequence by assuming that the jet is dissipating most of its energy beyond the broad-line region.
Therefore, since the study of radio-loud NLSy1 could have impact on the understanding of radio-loudness and the blazar sequence, we engaged in a study of a sample of these sources, by taking advantage of the power of XMM-Newton and Swift in giving simultaneous optical/UV/X-rays data. This allowed us to perform multiwavelength variability studies and to build Spectral Energy Distributions (SED), which can be reproduced with some models generally applied to blazars.
Section snippets
Sample selection and analysis
We performed a cross-correlation between the radio-loud NLSy1 reported in Zhou et al., 2007, Komossa et al., 2006, plus some specific source like 1H 0323 + 342 (Zhou et al., 2007), and the archives of publicly available data of XMM-Newton and Swift. We found data for 10 sources, but not all were useful. Indeed, the XMM-Newton observation of IRAS 01506+2554 was severely affected by soft proton flares (anomalous high background) and no useful data remained after a proper screening. B3 1702+457 and
Discussion
Fig. 1, Fig. 2 (left column) show the X-ray flux density at 1 keV versus Γ, UV and optical flux densities of RGB J1629+401, 1H 0323+342, RX J0134.2+4258. To compare with the behavior of a typical NLSy1 radio-quiet, we have analyzed the data of Mkn 766. We note that, as expected, Seyfert X-ray spectra become steeper with higher fluxes (see Fig. 2, right column; cf. with, e.g., Papadakis et al., 2002). Blazars show harder spectra with increasing flux, but this is not true for all the types of
Conclusions
The analysis of the optical-to-X-ray data presented here suggests that in the class of radio-loud narrow-line Seyfert 1 galaxies there are some sources with interesting behavior, in some way similar to blazars. There is no “unified way” for radio-loud NLSy1 to be similar to blazars: basically, in most of the analyzed cases, the X-ray spectrum, generally soft, shows a break with the emergence of a hard component. In one case, the X-ray spectrum can be fitted with a single power-law model with a
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