An XMM-Newton spectral survey of 12 μm selected galaxies - I. X-ray data

Abstract

We present an X‐ray spectral analysis of 126 galaxies of the 12 μm galaxy sample. By studying this sample at X‐ray wavelengths, we aim to determine the intrinsic power, continuum shape and obscuration level in these sources. We improve upon previous works by the use of superior data in the form of higher signal‐to‐noise ratio spectra, finer spectral resolution and a broader bandpass from XMM–Newton. We pay particular attention to Compton thick active galactic nucleus (AGN) with the help of new spectral fitting models that we have produced, which are based on Monte Carlo simulations of X‐ray radiative transfer, using both a spherical and torus geometry, and taking into account Compton scattering and iron fluorescence. We use this data to show that with a torus geometry, unobscured sightlines can achieve a maximum equivalent width of the Fe Kα line of ∼150 eV, originally shown by Ghisellini et al. In order for this to be exceeded, the line of sight must be obscured with NH > 1023 cm−2, as we show for one case, NGC 3690. We also calculate flux suppression factors from the simulated data, the main conclusion from which is that for NH≥ 1025 cm−2, the X‐ray flux is suppressed by a factor of at least 10 in all X‐ray bands and at all redshifts, revealing the biases present against these extremely heavily obscured systems inherent in all X‐ray surveys. Furthermore, we confirm previous results from Murphy & Yaqoob that show that the reflection fraction determined from slab geometries is underestimated with respect to toroidal geometries. For the 12 μm selected galaxies, we investigate the distribution of X‐ray power‐law indices, finding that the mean (〈Γ〉= 1.90+0.05−0.07 and σΓ= 0.31+0.05−0.05) is consistent with previous works, and that the distribution of Γ for obscured and unobscured sources is consistent with the source populations being the same, in general support of unification schemes. We determine a Compton thick fraction for the X‐ray AGN in our sample to be 18 ± 5 per cent which is higher than the hard X‐ray (>10 keV) selected samples. Finally we find that the obscured fraction for our sample is a strong function of X‐ray luminosity, peaking at a luminosity of ∼1042‐43 erg s−1.

NuSTAR UNVEILS A HEAVILY OBSCURED LOW-LUMINOSITY ACTIVE GALACTIC NUCLEUS IN THE LUMINOUS INFRARED GALAXY NGC 6286

“…While the model used by Brightman & Nandra (2011) can reproduce well the XMM-Newton and the spatially integrated Chandra spectra, it severely underpredicts the hard X-ray flux inferred by NuSTAR as illustrated in Figure 4. This might be related either to heavy obscuration of the X-ray source or to flux variability between XMM-Newton and NuSTAR observations, although variability would not be able to explain the very flat hard X-ray spectrum.…”

“…The only previous X-ray study of this source, carried out using XMM-Newton observations, did not find any evidence of an AGN (Brightman & Nandra 2011). The XMMNewton can in fact be well represented by a model taking into account only a collisionally ionized plasma and an unabsorbed power-law component, representing thermal emission from the starburst and X-ray radiation produced by X-ray binaries, respectively.…”

“…The XMM-Newton EPIC spectrum of NGC 6286 was analyzed by Brightman & Nandra (2011) who found that it could be well represented by an unabsorbed power-law continuum plus a thermal plasma 16 with the photon index fixed to Γ=1.9. This is in disagreement with the hard ( 0.17 1.03…”

“…To further study the absorbing material we used the TORUS model developed by Brightman & Nandra (2011), which considers reprocessed and absorbed X-ray emission from a spherical-toroidal structure. In this model the line of sight column density is independent of the inclination angle, which we fixed to the maximum value permitted (θ i = 87°.1).…”

The weak Fe fluorescence line and long-term X-ray evolution of the Compton-thick active galactic nucleus in NGC 7674

“…Murphy & Yaqoob (2009) provide tabulated results of Monte Carlo simulations of an AGN illuminating a doughnut-shaped torus with a fixed opening angle and covering factor of 0.5 (the MYTORUS model). Brightman & Nandra (2011) assume, instead, a torus defined as a conical section of a sphere with variable opening angle and hence variable covering factor (the TORUS model). Both models assume Solar abundances and treat absorption, reflection and Fe Kα fluorescence self-consistently.…”