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X-ray spectral properties of Seyfert 2s observed with BeppoSAX
Nuclear Physics B (Proc. Suppl.) 132 (2004) 229–231 www.elsevierphysics.com
X-ray spectral properties of Seyfert 2s observed with BeppoSAX G. Risalitia a
∗
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
We present the results of a homogeneous analysis of BeppoSAX observations of 19 bright Compton-thin Seyfert 2s. Using the wide spectral coverage of BeppoSAX it is possible to determine the single spectral components with unprecedented precision. We find that the intrinsic emission of Seyfert 2s is well reproduced by a power law with a photon index Γ = 1.79 ± 0.01, and a dispersion σ = 0.23. A reflection component is present in most spectra. We discuss these results in the view of the unified model, and we show a stacked spectrum obtained adding up the 1-200 keV spectra of all the sources of the sample.
1. INTRODUCTION The hard X-ray spectrum of Seyfert 2 galaxies has been widely studied in the past years with the instruments on board EXOSAT, Ginga, ASCA [5,2,6] and are also being studied with the new generation satellites XMM-Newton and Chandra. However, a basic limitation in all these observatories is that the narrow energy range of the observations (typically 1-10 keV) prevents from removing the degeneracy between the spectral components. For example, the parameters of a model with an intrinsic power law, a cold reflection component and an iron Kα line, are hardly determined in the 4-10 keV range even in the cases with the best statistics available. For this reason, the wide spectral coverage of BeppoSAX (0.1-200 keV) provides a unique tool to study the complex spectra of Seyfert 2s, and disentangle the different components with unprecedented precision. Here we present the results of a spectral analysis of 30 BeppoSAX observations of 19 bright Seyfert 2 galaxies. The sample consists of all the sources in the BeppoSAX public archive with a 2-10 keV flux F2−10 > 10−11 erg s−1 cm−2 , absorbing column density NH < 1024 cm−2 and optically classified as Seyfert 1.8, 1.9 or 2. The same model has been used for all spectra, obtaining good fits in almost all cases. The model com∗ Also INAF - Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, I-50125 Firenze, Italy
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.04.040
ponents are an intrinsic power law with an exponential high-energy cut-off, and a low-energy photoelectric cut-off, a cold reflection component (we used the PEXRAV model in XSPEC, derived by the Compton-reflection model of Magdziarz & Zdziarski [1]), a warm reflection component (modeled with a power law with the same photon index as the intrinsic component) an iron Kα emission line, and thermal emission at low energy. More details on this analysis can be found in Risaliti et al. [4]. 2. RESULTS The main results of our spectral fitting can be summarized as follows: • All the spectra can be adequately fitted with the model described above (in 28 out of 30 we obtained a reduced χ2 lower than 1.10, while in the remaining two cases χ2 ∼ 1.25). The measured NH ranges between 1022 and 7 × 1023 cm−2 . This confirms that sources optically classified as type 2 Seyferts show absorption in the X-rays. In only one case (NGC 7679) do we not measure a column density in excess of the Galactic value. • The average photon index of the intrinsic power law is 1.79±0.01. The dispersion is σΓ = 0.23. • A high energy cut-off is present in 70% of the spectra. In the remaining 30% we do
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G. Risaliti / Nuclear Physics B (Proc. Suppl.) 132 (2004) 229–231
Table 1 The sample of Compton thin, X-ray bright Seyfert 2 observed with BeppoSAX Name z Class. NGC 526a 0.0295 Sy 2 NGC 1365 0.0055 Sy 1.8 IRAS 05189-2524 0.0426 Sy 2 NGC 2110 0.0076 Sy 2 NGC 2992 0.0077 Sy 1.9 MCG-5-23-16 0.0083 Sy 2 NGC 4258 0.0015 Sy 1.9 NGC 4388 0.0084 Sy 2 NGC 4507 0.0118 Sy 2 IRAS 13197-1627 0.0172 Sy 1.8 NGC 5252 0.0230 Sy 1.9 Centaurus A 0.0018 Sy 2 NGC 5506 0.0062 Sy 1.9 NGC 5674 0.0249 Sy 1.9 NGC 6300 0.0037 Sy 2 ESO 103-G35 0.0133 Sy 2 NGC 7172 0.0087 Sy 2 NGC 7314 0.0047 Sy 1.9 NGC 7582 0.0053 Sy 2 NGC 7679 0.01714 Sy 2
not have any evidence of such cut-off up to 300 keV. • A cold reflection component is detected in most spectra. Even if the errors on the normalization of this component are large (due to the degeneracy with the other continuum component, i.e. the power law with the high energy cut-off), it is possible to estimate the ratio R between the normalization of the reflection component and that of the intrinsic power law. We find that R has values between 0.5 and 2, i.e. close to the maximum expected in case of Compton-thick reflectors covering a large fraction of the solid angle as seen from the central source. This implies that the gas surrounding the active nuclei is not homogeneous, since we know that its column density along our line of sight is relatively small (according to reflection models, the reflection efficiency is extremely low for NH < 1024 cm−2 ).
• An iron line is detected in most sources. The equivalent width with respect to the intrinsic power law is in the range 100-300 eV, in agreement with what is observed in Seyfert 1 galaxies. As a further development, we obtained a stacked spectrum adding up the sources in our sample. Since multiple observations of the same sources revealed significant variability in almost all cases, we used all the available spectra. The only exceptions are the five spectra of Cen A: in order to prevent this source from having an excessive weight in the average spectrum, we used only the first spectrum. The result represents the average observed spectrum of Seyfert 2s. Since all sources have NH < 7 × 1023 cm−2 (and all but one have NH < 4 × 1023 ), the spectrum above 4 keV should be a good template for Seyfert 1s too. We fitted the spectrum above 4 keV with a power law with a high energy cut-off and an iron line. We find that the equivalent width of the line is EW= 200 ± 100 eV, in agreement with the results obtained in the analysis of single spectra. +0.03 and The best fit spectral index is Γ = 1.55−0.02 the cut off energy is EC = 121 ± 10 keV. We show the unfolded stacked spectrum in Fig. 1. Both the values of Γ and EC are lower than those found in the analysis of single spectra. This is due to two factors: 1) the observed continuum is the sum of the intrinsic power law and the reflection component. Indeed, if we fit the single spectra without a cold reflection component, the best fit value of Γ are significantly lower than 1.8; 2) adding up spectra with different slopes produces a spectrum with a higher curvature than the single original spectra. This curvature is best reproduced with a flatter power law with a lowenergy cut-off. 3. CONCLUSIONS WORK
AND
FUTURE
The above results provide the best determination so far of the average spectral parameters of bright Seyfert 2s. In agreement with previous results, they confirm that the X-ray intrinsic emission of Seyfert 2s is not different from that of Seyfert 1s. As a new finding, they suggest that
G. Risaliti / Nuclear Physics B (Proc. Suppl.) 132 (2004) 229–231
231
Figure 1. 4-200 keV stacked spectrum of our sample of Seyfert 2 Galaxies. the circumnuclear absorber is rather complex and not homogeneous. Much work can still be done with archival data of BeppoSAX observations of Seyfert 2s. In particular, we are performing a variability analysis of these sources, in order to study the temporal variations of the single components. Preliminary results of this work on the bright AGN NGC 4151 are shown by Puccetti et al. in this proceedings book. REFERENCES 1. P. Magdziarz & A.A. Zdziarski, MNRAS 273 (1995) 837 2. K. Nandra & K.A. Pounds, MNRAS 268 (1994) 405 3. S. Puccetti et al., these proceedings 4. G. Risaliti, A&A 386 (2002) 379
5. T.J. Turner & K.A. Pounds, MNRAS, 240 (1989) 833 6. T.J. Turner, I.M. George, K. Nandra & R.F. Mushotsky R.F., ApJS 113 (1997) 23
X-ray spectral properties of Seyfert 2s observed with BeppoSAX G. Risalitia a
∗
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
We present the results of a homogeneous analysis of BeppoSAX observations of 19 bright Compton-thin Seyfert 2s. Using the wide spectral coverage of BeppoSAX it is possible to determine the single spectral components with unprecedented precision. We find that the intrinsic emission of Seyfert 2s is well reproduced by a power law with a photon index Γ = 1.79 ± 0.01, and a dispersion σ = 0.23. A reflection component is present in most spectra. We discuss these results in the view of the unified model, and we show a stacked spectrum obtained adding up the 1-200 keV spectra of all the sources of the sample.
1. INTRODUCTION The hard X-ray spectrum of Seyfert 2 galaxies has been widely studied in the past years with the instruments on board EXOSAT, Ginga, ASCA [5,2,6] and are also being studied with the new generation satellites XMM-Newton and Chandra. However, a basic limitation in all these observatories is that the narrow energy range of the observations (typically 1-10 keV) prevents from removing the degeneracy between the spectral components. For example, the parameters of a model with an intrinsic power law, a cold reflection component and an iron Kα line, are hardly determined in the 4-10 keV range even in the cases with the best statistics available. For this reason, the wide spectral coverage of BeppoSAX (0.1-200 keV) provides a unique tool to study the complex spectra of Seyfert 2s, and disentangle the different components with unprecedented precision. Here we present the results of a spectral analysis of 30 BeppoSAX observations of 19 bright Seyfert 2 galaxies. The sample consists of all the sources in the BeppoSAX public archive with a 2-10 keV flux F2−10 > 10−11 erg s−1 cm−2 , absorbing column density NH < 1024 cm−2 and optically classified as Seyfert 1.8, 1.9 or 2. The same model has been used for all spectra, obtaining good fits in almost all cases. The model com∗ Also INAF - Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, I-50125 Firenze, Italy
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.04.040
ponents are an intrinsic power law with an exponential high-energy cut-off, and a low-energy photoelectric cut-off, a cold reflection component (we used the PEXRAV model in XSPEC, derived by the Compton-reflection model of Magdziarz & Zdziarski [1]), a warm reflection component (modeled with a power law with the same photon index as the intrinsic component) an iron Kα emission line, and thermal emission at low energy. More details on this analysis can be found in Risaliti et al. [4]. 2. RESULTS The main results of our spectral fitting can be summarized as follows: • All the spectra can be adequately fitted with the model described above (in 28 out of 30 we obtained a reduced χ2 lower than 1.10, while in the remaining two cases χ2 ∼ 1.25). The measured NH ranges between 1022 and 7 × 1023 cm−2 . This confirms that sources optically classified as type 2 Seyferts show absorption in the X-rays. In only one case (NGC 7679) do we not measure a column density in excess of the Galactic value. • The average photon index of the intrinsic power law is 1.79±0.01. The dispersion is σΓ = 0.23. • A high energy cut-off is present in 70% of the spectra. In the remaining 30% we do
230
G. Risaliti / Nuclear Physics B (Proc. Suppl.) 132 (2004) 229–231
Table 1 The sample of Compton thin, X-ray bright Seyfert 2 observed with BeppoSAX Name z Class. NGC 526a 0.0295 Sy 2 NGC 1365 0.0055 Sy 1.8 IRAS 05189-2524 0.0426 Sy 2 NGC 2110 0.0076 Sy 2 NGC 2992 0.0077 Sy 1.9 MCG-5-23-16 0.0083 Sy 2 NGC 4258 0.0015 Sy 1.9 NGC 4388 0.0084 Sy 2 NGC 4507 0.0118 Sy 2 IRAS 13197-1627 0.0172 Sy 1.8 NGC 5252 0.0230 Sy 1.9 Centaurus A 0.0018 Sy 2 NGC 5506 0.0062 Sy 1.9 NGC 5674 0.0249 Sy 1.9 NGC 6300 0.0037 Sy 2 ESO 103-G35 0.0133 Sy 2 NGC 7172 0.0087 Sy 2 NGC 7314 0.0047 Sy 1.9 NGC 7582 0.0053 Sy 2 NGC 7679 0.01714 Sy 2
not have any evidence of such cut-off up to 300 keV. • A cold reflection component is detected in most spectra. Even if the errors on the normalization of this component are large (due to the degeneracy with the other continuum component, i.e. the power law with the high energy cut-off), it is possible to estimate the ratio R between the normalization of the reflection component and that of the intrinsic power law. We find that R has values between 0.5 and 2, i.e. close to the maximum expected in case of Compton-thick reflectors covering a large fraction of the solid angle as seen from the central source. This implies that the gas surrounding the active nuclei is not homogeneous, since we know that its column density along our line of sight is relatively small (according to reflection models, the reflection efficiency is extremely low for NH < 1024 cm−2 ).
• An iron line is detected in most sources. The equivalent width with respect to the intrinsic power law is in the range 100-300 eV, in agreement with what is observed in Seyfert 1 galaxies. As a further development, we obtained a stacked spectrum adding up the sources in our sample. Since multiple observations of the same sources revealed significant variability in almost all cases, we used all the available spectra. The only exceptions are the five spectra of Cen A: in order to prevent this source from having an excessive weight in the average spectrum, we used only the first spectrum. The result represents the average observed spectrum of Seyfert 2s. Since all sources have NH < 7 × 1023 cm−2 (and all but one have NH < 4 × 1023 ), the spectrum above 4 keV should be a good template for Seyfert 1s too. We fitted the spectrum above 4 keV with a power law with a high energy cut-off and an iron line. We find that the equivalent width of the line is EW= 200 ± 100 eV, in agreement with the results obtained in the analysis of single spectra. +0.03 and The best fit spectral index is Γ = 1.55−0.02 the cut off energy is EC = 121 ± 10 keV. We show the unfolded stacked spectrum in Fig. 1. Both the values of Γ and EC are lower than those found in the analysis of single spectra. This is due to two factors: 1) the observed continuum is the sum of the intrinsic power law and the reflection component. Indeed, if we fit the single spectra without a cold reflection component, the best fit value of Γ are significantly lower than 1.8; 2) adding up spectra with different slopes produces a spectrum with a higher curvature than the single original spectra. This curvature is best reproduced with a flatter power law with a lowenergy cut-off. 3. CONCLUSIONS WORK
AND
FUTURE
The above results provide the best determination so far of the average spectral parameters of bright Seyfert 2s. In agreement with previous results, they confirm that the X-ray intrinsic emission of Seyfert 2s is not different from that of Seyfert 1s. As a new finding, they suggest that
G. Risaliti / Nuclear Physics B (Proc. Suppl.) 132 (2004) 229–231
231
Figure 1. 4-200 keV stacked spectrum of our sample of Seyfert 2 Galaxies. the circumnuclear absorber is rather complex and not homogeneous. Much work can still be done with archival data of BeppoSAX observations of Seyfert 2s. In particular, we are performing a variability analysis of these sources, in order to study the temporal variations of the single components. Preliminary results of this work on the bright AGN NGC 4151 are shown by Puccetti et al. in this proceedings book. REFERENCES 1. P. Magdziarz & A.A. Zdziarski, MNRAS 273 (1995) 837 2. K. Nandra & K.A. Pounds, MNRAS 268 (1994) 405 3. S. Puccetti et al., these proceedings 4. G. Risaliti, A&A 386 (2002) 379
5. T.J. Turner & K.A. Pounds, MNRAS, 240 (1989) 833 6. T.J. Turner, I.M. George, K. Nandra & R.F. Mushotsky R.F., ApJS 113 (1997) 23