The BeppoSAX view of bright Compton-thin Seyfert 2 galaxies

Abstract

Abstract. We present the analysis of 31 observations (17 of which are published here for the first time) of 20 bright Compton thin Seyfert 2 s, in the 0.1-200 keV band, performed with the BeppoSAX satellite. The sample consists of all Seyfert 2 s in the BeppoSAX public archive, with a 2-10 keV flux higher than 5 × 10 −12 erg cm −2 s −1 . The good statistics available and the broad energy band permit a detailed study of the main continuum components of these sources, i.e. the primary power-law, the reflected component, the soft emission and the high-energy cut-off. The main results of our analysis are: (1) the 3-200 keV intrinsic power-law has a mean photon index Γ = 1.79 ± 0.01, with a dispersion of σ = 0.23. (2) The high-energy exponential cut-off at E ∼ 100−300 keV is not an ubiquitous property of Seyfert galaxies: in ∼30% of the objects the continuum power-law does not drop up to energies of 300 keV or more. (3) A reflected component is present in almost all the sources (17 out of 21). The small variations of this component with respect to the intrinsic continuum, in objects with multiple observations, suggests that the reflector is not the accretion disk, but must be located much farther from the nucleus. (4) The range of ratios between the reflected and intrinsic components suggests that the circumnuclear medium is not homogeneous, and a significant fraction of the solid angle is covered by a gas thicker than that along the line of sight. (5) The iron Kα line is present is all but one the sources. The equivalent width is in the typical range of Seyfert 1 s (EW = 100−300 eV) in sources with low absorption (NH < 3 × 10 23 cm −2 ), and increases in more absorbed objects, as expected according to unified models. (6) The energy resolution of BeppoSAX is in general too low to measure the iron line width. However, in 6 cases we measured a significant line broadening.

X-ray spectral variability of Seyfert 2 galaxies

“…The same result was obtained by Risaliti (2002), who studied Seyfert 2s with BeppoSAX and found that the cold reflection component is compatible with being non-variable. They argue that if the reflection originates in the accretion disk, the reflection and the transmitted components must be closely related, but if the distance of the reflector to the SMBH is greater than the light crossing time of the intrinsic variations, the reflected component must remain constant.…”

“…They found short-term variations (hours) from the analysis of a light curve from 1996 and longterm variations when comparing the flux of these data with previous data from 1995, when it was about three times brighter. Risaliti (2002) studied two BeppoSAX observations taken in October 1996 and November 1997 and fit the data with an absorbed power law, a thermal component, a cold reflection, a warm reflection, and a narrow Gaussian line. They reported very similar spectral parameters for the two spectra.…”

“…Numerous studies have been made at X-ray frequencies to characterize the spectra of Seyfert galaxies (e.g., Turner et al 1997;Risaliti 2002;Guainazzi et al 2005b,a;Panessa et al 2006;Cappi et al 2006;Noguchi et al 2009;LaMassa et al 2011;Brightman & Nandra 2011a). The present work is focused on Seyfert 2 galaxies, which represent ∼80% of all AGN (Maiolino & Rieke 1995).…”

The X-ray view of giga-hertz peaked spectrum radio galaxies

“…In Compton-thick AGN, residual X-ray emission red-wards the photoelectric cutoff could be due to Compton-reflection of the otherwise invisible primary radiation off the obscuring matter 4 . Fe K α fluorescent emission with large EW ( ∼ > 600 eV, Risaliti 2002;Guainazzi et al 2005) is the unmistakable "smoking gun" of a Compton-thick AGN. We have therefore applied the baseline spectral model for Comptonthick AGN to the PKS1627+06 pn spectrum (the MOS spectra have very poor statistics, and do not provide any further constraints): a pure Compton-reflection continuum (model pexrav in Xspec, Magdziarz & Zdziarski 1995) plus a Gaussian unresolved emission line.…”

Prospects in space-based gamma-ray astronomy

“…A large region of this parameter space is virtually unexplored because we currently lack information on large AGN samples. Observations by BeppoSAX (Risaliti 2002;Perola et al 2002) of a handful of radio quiet sources, loosely locate this drop-off in the range 30- Figure 2. The 0.25-400 keV cosmic X-/γ−ray background spectrum fitted with synthesis models; measurements are from observations with different experiments as labelled.…”