Average properties of hard X-ray AGN

The sample contains 42 AGN of which 10 are Seyfert 1, 9 intermediate Seyfert 1.5, 16 Seyfert 2, 5 blazars, 1 radio galaxy and 1 unidentified-type AGN (Fig. 5.4). For most of the objects, a simple model (absorbed or single power law) represents well the combined ISGRI, JEM-X and SPI data. In

Figure 5.4:Sky distribution in Galactic coordinates of the 42 AGN (plus 1 galaxy cluster) included in the first INTEGRALAGN catalogue.

6 cases a cut-off is required between 35 and 180 keV or a more complex model, including additional components (e.g. an iron line), is needed.

In order to compare the average properties of the hard X-ray spectra for different AGN classes, we concentrated on the analysis of the ISGRI spectra for the Seyfert galaxies. In Fig. 5.5 the distribution of the photon indices is shown, divided by Seyfert type. The INTEGRAL AGN have a large distribu-tion of photon indices between 1 and 3 and the Seyfert 2 spectra appear to be harder than those of Seyfert 1, as it has been observed in the past by other missions (Zdziarski et al. 1995; Malizia et al.

2003). To compute the average shape of the hard X-ray spectra as a function of the Seyfert type, we considered two different approaches: an average spectrum can be calculated and then fitted, or the parameters resulting from the single-source fits can be averaged. Furthermore, in each case the weighted or the arithmetic mean can be used.

The main problem of the arithmetic mean is the definition of the uncertainties. In fact, the stan-dard deviation of the mean does not take into account the error on the single measurements and the propagation of the errors (i.e. the quadratic sum of the measurement errors in this case) results here in too large error bars, preventing any meaningful determination of the fitting parameters with this method (Fig. 5.6, top left panel).

On the other hand, the weighted mean can be applied only to samples that have similar uncer-tainties, otherwise the average is dominated by the properties of the objects with better statistics. To minimize this problem, we have excluded from this analysis the two brightest objects of the sample, Centaurus A and NGC 4151, and we preferred to calculate the mean of the fitting parameters (Table 5 in Beckmann et al. 2006a). In fact, the effect of sources with different brightness seems to be stronger when the spectra are directly averaged. For example, in the case of Seyfert 1, the average photon index is Γ = 2.12±0.05, whereas the average spectrum is best fitted by a power law with photon indexΓ = 2.40±0.07. This is due to the fact that 2 out of the 3 brightest Seyfert 1 have very steep spectra (MR 2251-178 and GRS 1734-292, with photon index 2.3–2.7) determining the shape of the average spectrum.

The first INTEGRAL AGN catalogue 61

Figure 5.5:Histograms of the hard X-ray photon indices for the different Seyfert classes as obtained from the fit of the ISGRI spectra.

Besides, averaging the spectra results in globally steeper spectra than those found for the single-source fit, even though the general trend, i.e. Seyfert 2 with harder spectra than Seyfert 1, is main-tained. In Fig. 5.6, the average spectra (as calculated with the weighted mean) of the 10 Seyfert 1, 8 intermediate Seyfert 1.5 and 15 Seyfert 2 are shown, fitted with a simple power law model that results in photon indices ofΓ = 2.39±0.07, 2.24±0.04 and 2.09±0.03, respectively. As all the fits give χ²_red<1, an additional component, as for example a high-energy cut-off, is not required and cannot be constrained. Similar results are obtained when only a sub-sample of objects is selected for each class, narrowing the range of detection significances of the spectra to average. Choosing 5 Seyfert 1 with 9< σ < 21, 5 intermediate Seyfert 1.5 with 7< σ < 14 and 5 Seyfert 2 with 12< σ < 20, we find that the average spectra are also best fitted by a simple power law with photon indices of 2.40±0.07, 2.19±0.17 and 2.16±0.06, respectively, consistent with what we find with the complete samples.

The average spectrum of all Seyfert galaxies in the sample (excluding Cen A and NGC 4151) is best fitted by a power law with photon indexΓ =2.27±0.05 and the average photon index of all Seyfert is 2.04±0.04.

The average properties of this first INTEGRAL AGN sample confirm the past observations of Seyfert galaxies performed by Ginga, CGRO/OSSE and BeppoSAX (Zdziarski et al. 1995; Malizia et al. 2003). A study by Deluit & Courvoisier (2003) found instead that Seyfert 2 spectra are steeper (Γ =2.00^+0.05_−0.45) than Seyfert 1 (Γ =1.89^+0.28_−0.46); however, due to the large uncertainties, this difference is not significant. Later results from the Swift/BAT survey (Tueller et al. 2008; Ajello et al. 2008) also agrees with INTEGRAL findings. Ajello et al. (2008) confirm that the average spectrum of all Seyferts has a photon index 2.00±0.07 and that on average Seyfert 1 are characterised by a softer X-ray spectrum (Γ =2.23±0.11) than Seyfert 2 (Γ =1.86±0.10). If the stacked Seyfert 2 spectrum is best fitted by a simple power law, a reflection component seems to improve the fit of the Seyfert 1 average spectrum, but the photon index does not seem to be affected by the presence of a reflection component. This indicates that the steep Seyfert 1 spectrum is not an artifact due to the presence of

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Figure 5.6: Average ISGRI spectra of the different Seyfert classes for the objects in the first INTEGRAL catalogue. The continuous lines represent the best power-law fits to the data. Top left panel:arithmetic mean of the 10 Seyfert 1 spectra; red points show the errors calculated as standard deviation of the mean, whereas black points indicate those obtained through error propagation. Top right panel: weighted mean of the 10 Seyfert 1 spectra. Bottom left panel: weighted mean of the 8 intermediate Seyfert 1.5 spectra (NGC 4151 has been excluded). Bottom right panel: weighted mean of the 15 Seyfert 2 spectra (Centaurus A has been excluded).

a stronger reflection hump than in Seyfert 2 (in turn due to different viewing angles), but it seems to be more an intrinsic property. A hint for a cut-off is apparent only&100 keV in the Seyfert 1 average spectrum. A similar result, but only marginally significant, was found by Deluit & Courvoisier (2003), with a cut-off at 221₋₁₅₈^+∞ keV in Seyfert 1 galaxies.