Correlation in the Soft Energy Range

A significant correlation between the slope of the 0.1-2.4 keV X-ray continua and the FWHM of the H^ line was found for Seyfert 1 galaxies [4]. This work was based on a systematic investigation of 46 NLS1s with the X-ray satellite ROSAT. The 46 NLS1s were selected from available data in the literature (in 1996). 32 of these 46 NLS1s are located within the field of view of public ROSAT pointed observations. 31 of these 32 narrow-line Seyfert 1 galaxies show significant X-ray emission in ROSAT pointed observations, and only one object does not show detectable X-ray emission. For these 31 X-ray-detected NLS1s, power-law fits were obtained. The photon index r is compared with the corresponding value found in broad-line Seyfert 1 galaxies, analyzed using ROSAT All-Sky Survey data [55]. The relation between the photon index r and the H^ line widths is presented in Fig. 22.3. The results of power-law fits to the ROSAT data of narrow- and broad-line Seyfert 1 galaxies can be summarized as follows:

2000 4000

2000

4000

6000

6000

2000 4000

2000

4000

6000

Fig. 22.3 Left: Photon index in the 0.1—2.4 keV energy range as a function of the FWHM line width of the H^ line. The photon index serves as a measurement of the steepness of the X-ray continuum. All objects in the diagram are Seyfert 1 galaxies. The narrow-line Seyfert 1 galaxies H^ line widths smaller than 2000 km s—1 are taken from [4]. The broad-line Seyfert 1 galaxies were investigated by [55]. A significant correlation between the slope of the X-ray continuum and the FWHM H^ line width is found. Seyfert 1 galaxies with large line widths (FWHM H^ greater than 2000 km s—1) show a relatively small dispersion in their values of the photon index with a mean of about 2.3. NLS1s show a large dispersion in their values of the photon index. Some of these objects exhibit values of the photon index up to about 5. A region of the diagram, with values of the photon index larger than 3 and optical line widths larger than about 3000 km s— 1 is not occupied by Seyfert galaxies. This region is sometimes called as 'zone of avoidance'. There is probably an underlying physical parameter, which causes the absence of objects in this region. One possible explanation is, that this physical parameter is the ratio of the accretion rate to the black hole mass. Right: Photon index in the 2—10 keV energy range as a function of the FWHM H^ line width discovered by [10]. The measurements were obtained with the Japanese X-ray satellite ASCA. Also in the hard X-ray band NLS1s show a stronger dispersion in their photon indices than previously thought. Typical values of the photon index in the hard X-ray energy band range between about 1.7 and 2.6. In this energy range, the spectrum is dominated by the power law component. The correlation between the slope of the X-ray spectral energy distribution and the optical line width is expected to allow statements on the physical conditions in the accretion disc corona

6000

- Seyfertl galaxies with H^ line widths greater then 3000 km s-1 display flat X-ray continua with a mean value of about r = 2.3. The dispersion from the mean value is low. The minimum value of r is 1.3 and the maximum value is 2.6.

- NLSls show a significantly stronger dispersion in the values of the photon index compared with broad-line Seyfert 1 galaxies. The photon index reaches values of up to about 5. Steep X-ray continua with values of the photon index greater than 3 are found only in NLS1s. As a class, NLS1s show therefore steeper X-ray continua than broad-line Seyfert 1 galaxies.

- A region in the r - FWHM plane with r > 3 and FWHM greater than 3000 km s-1 is not occupied by Seyfert 1 galaxies. This region is sometimes called the 'zone of avoidance'. There are no obvious selection effects which could cause the absence of objects in this region.

- The distribution of the values r and FWHM H^ show a continuous increase in the slope of the spectral continuum distribution with decreasing FWHM H^ line width. This suggests that narrow- and broad-line Seyfert 1 galaxies form essentially the same class of objects and that there might be an underlying physical parameter which controls the distribution of objects shown in Fig. 22.3.

- In the 0.1 -2.4 keV energy range, the soft excess component as well as the power-law component contribute to the measured spectral energy distribution (in contrast to the 2-10 keV energy range, where only the power-law component is contributing to the continuum emission). Therefore, in the 0.1-2.4 keV energy range, the photon index serves as a measure of the relative ratio of the emission of the soft excess- to the power-law component. As the soft X-ray excess component is thought to arise primarily in the accretion disc, the distribution of photon indices at soft X-rays should allow us to make statements regarding the physical conditions in the accretion disc. In addition, as the soft excess component is probably the high energy tail of the big blue bump, the photon index in the soft energy range is probably a measure of the strength of the ionizing radiation from the innermost regions of active galactic nuclei. The distribution of the data points in Fig. 22.3(a) suggests that the width of the H^ line is determined by the strengths of ionizing radiation and the lower masses of NLS1s with respect to broad-line Seyfert 1's [4]. This strength of the ionizing radiation may (as one possible explanation) determine the size of the region, which is emitting the permitted optical lines.

0 0

Post a comment