Active galactic nuclei (AGN) are divided according to their optical line properties into subclasses, like Seyfert 1 galaxies, narrow-line Seyfert 1 galaxies, Seyfert 2's, LINERs, BL Lacs, or QSOs. Narrow-line Seyfert 1 galaxies (NLS1) are a very interesting group of AGN showing Full Width at Half Maximum much smaller (500 - 1500 km/s) than the one typically observed (1500 - 10000 km/s). NLS1 are clearly Seyfert type 1 as is evident from the presence of strong iron lines, which arise in a high density region, known to be present in AGN. The reasons for the unusual line width were not well understood before ROSAT observations on narrow-line Seyfert 1 galaxies. One explanation was that the part of the broad-line region producing the very broad component is obscured from the direct view. Another possibility had to do with the observes viewing angle and the geometry and dynamics of the broad-line region, e.g. a flat rotating system seen face-on.
Figure 1:X-ray continuum slope, obtained from a power-law fit, versus FWHM of Hß for 'normal' Seyfert 1 galaxies (vertical bars) and narrow-line Seyfert 1s (filled dots). The anticorrelation is remarkable as the X-ray spectra of most Seyfert 1 type galaxies are formed predominantly within ~50 Schwarzschild radii of their black holes, while Seyfert 1 optical permitted lines are formed in a separate and significantly larger region.
ROSAT has shown many narrow-line Seyfert 1 galaxies to have remarkable X-ray properties. The ROSAT spectra of NLS1 have generally strong soft excess components compared to Seyfert 1 galaxies with broader optical permitted lines. When simple power-law models are fit to the data, photon indices reach values up to about 5, much higher than the photon index of about 2 seen in Seyfert 1's. A clear anticorrelation is found between the ROSAT spectral softness and the line width of the optical permitted lines (see Figure 1). This is remarkable as the X-ray spectra are formed within only a few Schwarzschild radii of their black holes, while the optical permitted lines are formed in a significantly larger region. Narrow-line Seyfert 1 galaxies thus provide a unique bench-mark for understanding several of the more fundamental properties of AGN, such as the relationship between the X-ray properties and the absence of broad optical emission lines. As the gravitational force from the central black hole is dominant in causing the motions of Seyfert broad-line region clouds, narrower optical lines will result from smaller black hole masses due to smaller cloud Keplerian velocities. Narrow-line Seyfert 1 galaxies with smaller black holes would have to be accreting at higher fractions of the Eddington rates to maintain their observed luminosities, which are similar to that of Seyfert 1's. Therefore it has been suggested that NLS1 may be those Seyfert 1's which are accreting at relatively high fractions of the Eddington rate. The steep X-ray spectra found in narrow-line Seyfert 1 galaxies have established the soft X-ray excess as a class property of Seyfert galaxies with narrow optical lines
Boller Th., Brandt W.N., Fink H., 1996, A&A, 305, 53
NLS1 can also show remarkably rapid and large-amplitude X-ray variability. One spectacular object, the radio-quiet, ultrasoft NLS1 IRAS 13224-3809, shows persistent giant-amplitude variability events by factors of 35--60 on timescales of just a few days (see Figure~2). This is remarkable as the timescales for such variations found before are of the order of few years. The rapid observed variations suggest that the X-ray emission originates close to the supermassive black hole. In this region, the emitting particles will be moving with relativistic bulk velocities. The most plausible model is the strongly forward collimated radiation of a "hot" gas bubble orbiting a central supermassive black hole. These relativistic motions in an accretion disc will lead to strong Doppler boosting. In addition, gravitational lensing by the black hole will be important. These effects lead to strong apparent flux variations if the emission region is not steady and homogeneous.
Figure 2: ROSAT HRI light curve obtained during a 30-day monitoring observation (between January 11, 1996 and February 9, 1996) of the narrow-line Seyfert~1 galaxy IRAS 13224-3809. The X-axis label gives the Julian date minus 2450093.523 days. The data points are plotted at the middle of the exposure time intervals from which they were obtained. The total exposure time is 111.313 ks, and the source is centered on-axis in the HRI field of view. The dashed curve indicates the background counting rate within the source extraction circle as a function of time. At least five giant-amplitude count rate variations from IRAS 13224-3809 are visible. Giant-amplitude variability has also been seen by ASCA, and this argues against any instrumental errors. We have carefully verified that the highly variable X-ray source is associated with the distant galaxy IRAS 13224-3809.
Boller Th., Brandt W.N., Fabian A.C., Fink H. 1997, MNRAS 289, 393
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