[Paper Review] An X-ray view of absorbed INTEGRAL AGN

Alessandra De Rosa, Bassani, L.+5 more|Jan 30, 2008

Nuclear Physics and Applications4 citations

TL;DR

This study presents a broad-band X-ray analysis (0.2–200 keV) of seven absorbed Seyfert 2 AGN using INTEGRAL, XMM-Newton, Chandra, and ASCA data to investigate continuum components and absorption/reflection properties. It finds a high energy cut-off in all sources, Compton reflection components in five, and a soft excess in most—challenging the standard torus model and suggesting clumpy or geometrically thin obscuring media.

ABSTRACT

Aims. We present a 0.2--200 keV broad-band study of absorbed AGN observed with INTEGRAL, XMM-Newton, Chandra and ASCA to investigate the continuum shape and the absorbing/reflecting medium properties. Methods. The sources are selected in the INTEGRAL AGN sample to have a 20--100 keV flux below 8$ imes10^{-11}$ $\flux$ (5 mCrab), and are characterized by a 2--10 keV flux in the range (0.8--10)$ imes10^{-11}$ $\flux$. The good statistics allow us a detailed study of the intrinsic and reflected continuum components. In particular, the analysis performed on the combined broad-band spectra allow us to investigate the presence of Compton reflection features and high energy cut-off in these objects. Results. The column density of the absorbing gas establishes the Compton thin nature for three sources in which a measure of the absorption was still missing. The Compton thin nature of all the sources in this small sample is also confirmed by the diagnostic ratios F$x/F[OIII]. The Compton reflection components we measure, reflection continuum and iron line, are not immediately compatible with a scenario in which the absorbing and reflecting media are one and the same, i.e. the obscuring torus. A possible solution is that the absorption is more effective than reflection, e.g. under the hypothesis that the absorbing/reflecting medium is not uniform, like a clumpy torus, or that the source is observed through a torus with a very shallow opening angle. The high energy cut-off (a lower limit in two cases) is found in all sources of our sample and the range of values is in good agreement with that found in type 1 Seyfert galaxies. At lower energies there is clear evidence of a soft component (reproduced with a thermal and/or scattering model), in six objects.

Motivation & Objective

• To investigate the intrinsic continuum shape and absorption/reprocessing properties of absorbed AGN detected by INTEGRAL above 10 keV.
• To determine whether the observed X-ray emission in these sources is consistent with a single, uniform obscuring torus.
• To measure the high energy cut-off and reflection components (Compton hump and iron Kα line) in a sample of Compton-thin AGN.
• To assess the presence and nature of soft X-ray excesses below 2–3 keV and their implications for emission models.
• To validate the analysis method via cross-comparison with simultaneous Beppo-SAX data on ESO 103-G35.

Proposed method

• Combined broad-band spectral fitting using data from INTEGRAL (20–100 keV), XMM-Newton, Chandra, and ASCA (0.2–200 keV) to cover three decades of energy.
• Employed a physical model including a power-law continuum with high energy cut-off, Compton reflection (including iron Kα line), and soft excess components.
• Used a photoelectric absorption model with column density N_H to account for cold absorption, and tested for Compton-thick vs. Compton-thin regimes.
• Applied diagnostic ratios such as F_x / F[OIII]λ5007 to independently assess the Compton thin nature of the sources.
• Constrained reflection fraction R and cut-off energy E_c, and tested for degeneracies between parameters.
• Validated the analysis pipeline using simultaneous Beppo-SAX data on ESO 103-G35 for consistency checks.

Experimental results

Research questions

• RQ1 What is the intrinsic shape of the X-ray continuum in absorbed INTEGRAL AGN, particularly regarding the high energy cut-off?
• RQ2 Are the observed reflection components (Compton hump and iron Kα line) consistent with a single, uniform obscuring torus?
• RQ3 What is the nature and origin of the soft X-ray excess observed below 2–3 keV in most sources?
• RQ4 How do the measured reflection and absorption parameters correlate, and what do they imply about the geometry of the obscuring medium?
• RQ5 Is the Compton thin nature of the sources confirmed independently using diagnostic ratios such as F_x / F[OIII]?

Key findings

• The average absorbing column density is ĀN_H = (10.6 ± 0.2) × 10²² cm⁻², confirming all sources are Compton thin, including three previously unmeasured ones.
• A high energy cut-off is present in all sources, with upper limits below 300 keV, consistent with values observed in type 1 Seyfert galaxies.
• Compton reflection components (bump and iron Kα line) were detected in five sources; upper limits were obtained for two, indicating reflection is not dominant in all cases.
• The reflection fraction R and iron line equivalent width are too high to be explained by reflection in the same gas that produces the observed absorption, challenging the standard torus model.
• A soft excess is detected in six of seven sources, best modeled by a thermal black body (kT = 0.2–0.9 keV) or a scattered power-law, with IGR J12026-5349 showing an anomalously high soft excess (kT = 1.7 keV or A_IC/A_soft = 0.12).
• IGR J07565-4139 is the only source without a soft excess, but shows a narrow emission line at ~2.5 keV, suggesting a different local emission mechanism.