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Astron. Astrophys. 327, 72-80 (1997)

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1. Introduction

The class of narrow-line Seyfert 1 (NLS1) galaxies is characterized by the properties of their optical emission lines (Osterbrock & Pogge 1985; Goodrich 1989): the permitted lines are narrower ([FORMULA] 1 000 km s-1 full width at half-maximum, FWHM) than in typical Seyfert 1 galaxies ([FORMULA] 5 000 km s-1, FWHM), though still slightly broader than the forbidden lines; the [OIII]/H [FORMULA] ratio is much smaller than in Seyfert 2; narrow lines of highly ionized atoms are detected; the optical FeII emission is among the strongest detected in active galactic nuclei (AGN).

The radio properties of NLS1 galaxies (Ulvestad, Antonucci & Goodrich 1995) are similar to those found in other Seyfert galaxies. The only exception refers to the orientation of the radio axes with respect to the optical polarization: Mrk 766 and Mrk 1126 are the only two known Seyfert galaxies where the radio axis is perpendicular to the polarization. Spectropolarimetric observations of a sample of 17 NLS1 galaxies (Goodrich 1989) revealed that 6 of them show significant intrinsic polarization.

It has been claimed (Boller et al. 1996 and references therein) that in the ROSAT band, NLS1 show both steeper spectra and stronger variability than typical broad line Seyfert 1 galaxies. Moreover, the soft X-ray variability time scales found in some NLS1 are among the fastest of Seyfert galaxies: the doubling times are shorter than 15000 seconds for 4 out of 30 NLS1. In spite of the different spectral shapes, the soft X-ray luminosities of NLS1 are similar to those of typical broad line Seyfert 1s. At higher energies, only three objects have been reported to be observed so far: IRAS 13224-3809 shows a hard ([FORMULA] 1.3) power law from 2 to 10 keV while below 2 keV the spectrum is dominated by a soft excess (Otani 1995); in contrast, RE 1034+39 shows a very steep spectrum ([FORMULA] 2.6) in the range 2-10 keV (Pounds, Done & Osborne 1995); finally, Leighly et al. (1996) find rapid variations (doubling time scale [FORMULA] 1 000 seconds) in the ASCA data (0.4-10.5keV) of Mrk 766. These authors model the combined ROSAT -ASCA spectra of Mrk 766 with a power law, warm absorber and soft excess. With this model, the spectral variability observed can be attributed to changes in the spectral index of the power law and in the ionization degree of the warm absorber.

The narrowness of the optical permitted lines has been interpreted as an orientation effect of a disc shaped BLR (Osterbrock & Pogge 1985; Goodrich 1989; Stephens 1989; Puchnariewitz et al. 1992). If the clouds are confined to move in a plane, the emission lines will be much narrower when viewed from a line of sight nearly perpendicular to the plane. In this pole-on model, the rapid X-ray variability might be understood in terms of relativistic beaming effects if we were looking down an outflow from the central engine. If such beaming effects are not present, a rapid intrinsic variability can only result from a very compact emitting region.

Another possibility, that does not consider orientation effects as playing a major role, is based on the steep spectrum generally observed in the soft X-rays. If it is the high energy tail of the emission from an accretion disc, such "hot" spectrum will be produced when the central black hole has a relatively small mass compared to those in normal Seyfert 1's (Ross & Fabian 1993). In this scenario, the rapid variability observed in the X-rays would be naturally explained by the smaller dimension of the emitting region. Moreover, if the gravitational force from the central black hole dominates the BLR kinematics, smaller black holes would result in lower cloud velocities, assuming these clouds are kept at distances similar to those in normal Seyfert 1's. This situation may occur if the larger ionization parameter (the ratio of the density of ionizing photons to hydrogen atoms), implied by the harder accretion disc spectrum, hinders the formation of BLR clouds close to the central source.

It is remarkable that all these hypotheses about the nature of NLS1 are based on the narrowness of the optical permitted lines. However, it is known that the emission lines that better trace the innermost regions of the BLR lie in the UV domain (Peterson 1994, and references therein). Crenshaw et al. (1991) have reported IUE observations of three NLS1 and found that the line ratios are similar to those found in normal Seyfert 1 galaxies, but nothing is mentioned about the line widths.

In the very last years, IUE has observed a number of NLS1 galaxies, increasing noticeably the sample discussed by Crenshaw et al. (1991). Moreover, most of the results on the X-ray properties of NLS1 have come out also in the current decade. Our aim is then to revise the UV properties of NLS1, based on a larger sample than in previous works, and link these properties to those in other spectral ranges in order to constrain the models currently proposed for this sub-class of AGN.

In Sect. 2 we describe the analyzed sample as well as its UV properties. In this section we also combine the continuum measurements with far-infrared (FIR) fluxes and soft X-ray properties from the literature to study the spectral energy distribution of NLS1. The implications of the emission lines analysis on the emitting-gas conditions are discussed in Sect 3 and the main results are finally summarized in Sect. 4.

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© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998