An XMM–Newton survey of broad iron lines in Seyfert galaxies

By K. Nandra, P. O’Neill, I. George, J. Reeves. 2007

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We present an analysis of the X-ray spectra of a sample of 37 observations of 26 Seyfert galaxies observed by XMM–Newton in order to characterize their iron Kα emission. All objects show evidence for iron line emission in the 6–7 keV band. A narrow ‘core’ at 6.4 keV is seen almost universally in the spectra, and we model this using a neutral Compton reflection component, assumed to be associated with distant, optically thick material such as the molecular torus. Once this, and absorption by a zone of ionized gas in the line of sight is accounted for, less than half of the sample observations show an acceptable fit. Approximately two-thirds of the sample shows evidence for further, broadened emission in the iron K band. When modelled with a Gaussian, the inferred energy is close to that expected for neutral iron, with a slight redshift, and an average velocity width of ∼0.1c. The mean parameters are consistent with previous ASCA results and support the idea that the broad components can be associated with the accretion disc. Before proceeding to that conclusion, we test an alternative model comprising a blend of three to four narrow, unshifted emission lines (including the 6.4-keV core), together with one to two zones of highly ionized gas in the line of sight. Around one-third of the objects are not adequately fitted by this model, and in general better fits are obtained with a relativistic disc line model, which has fewer free parameters. None the less we find that absorption by ionized gas affects the spectrum above 2.5 keV in approximately half the sample. There is evidence for multiple ionized zones in at least three objects, but in all those cases a blurred reflector is required in addition to the complex absorption. We also identify a number of narrow emission and absorption features around the Fe complex, and the significance and interpretation of these lines is discussed. After accounting for these additional complexities, we determine the typical parameters for the broad reflection. The emission is found to come, on average, from a characteristic radius ∼15 rg and the average disc inclination is ∼ 40°. The broad reflection is on average significantly weaker, by a factor of ∼2, than that expected from a flat disc illuminated by a point source. Notwithstanding these average properties, the objects exhibit a significant and wide range of reflection parameters. We find that 30 per cent of the sample observations can be explained solely with narrow-line components, with no evidence for broadened emission at all. A further 25 per cent show evidence for significant broad emission, but at a characteristic radius relatively far from the black hole. The remaining ∼45 per cent are best fitted with a relativistically blurred reflection model. In 12/37 observations the characteristic emission radius is constrained to be <50 rg, where the gravitational redshift is measurable. For at least this subsample, our observations verify the potential for X-ray spectroscopy to diagnose the strong-gravity regime of supermassive black holes.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/ee6fe95d977f1818b99dfbb52426f6b65b372a22

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Citations in the corpus

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The Masses and Spins of Neutron Stars and Stellar-Mass Black Holes (2014, unknown): “[234,235]; also see [236]).”
An X-ray view of 82 LINERs with Chandra and XMM-Newton data (2009, background): “Nearby examples of both Seyfert 2 (Maiolino et al., 1998; Risaliti et al., 1999; Page et al., 2003; Cappi et al., 2006; Panessa et al., 2006), and Seyfert 1 (Nandra et al., 2007) or PG-QSOs (Piconcelli et al., 2005; Jimenez-Bailon et al., 2005) have been studied.”
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An XMM–Newton spectral survey of 12 μm selected galaxies – I. X-ray data (2010, methodology): “In reality, broad Fe Kα lines have been found with a lower energy than the energy we permit (e.g. Nandra et al. 2007), but we use this conservative range as broad Fe Kα lines are not the focus of this investigation.”
THE NUCLEAR SPECTROSCOPIC TELESCOPE ARRAY (NuSTAR) HIGH-ENERGY X-RAY MISSION (2013, background): “ASCA’s discovery of a prominent red tail in the iron line profile of the Seyfert 1 galaxy MCG-6-30-15 (Tanaka et al 1995) - later confirmed in this and other objects (e.g., Nandra et al. 2007; de la Calle Perez et al. 2010; Brenneman et al. 2011) - seem to validate this model.”
OBSCURATION-DEPENDENT EVOLUTION OF ACTIVE GALACTIC NUCLEI (2015, methodology): “…toroidal geometry simulated by the torus model of Brightman & Nandra (2011a), 2. a Compton-reflection component approximated by the pexmon model (Nandra et al. 2007) and 3. a soft scattering component, which is parameterised by an unabsorbed power law with the same spectral slope as the…”
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A physical model of the broad-band continuum of AGN and its implications for the UV/X relation and optical variability (2018, methodology): “There is a clear iron line in the X-ray spectrum, together with a Compton hump, so we include this in the model using gsmooth * pexmon (Nandra et al. 2007).”
New Spectral Model for Constraining Torus Covering Factors from Broadband X-Ray Spectra of Active Galactic Nuclei (2018, unknown): “2 In pexrav and pexmon (extension of pexrav, including fluorescent line emission; Nandra et al. 2007), the contribution of reprocessed continuum is parametrized with the spectral parameter R.”
The role of the reflection fraction in constraining black hole spin (2014, methodology): “However, some spectral surveys of local AGN have made estimates ofR by fitting the reflection signatures (e.g. Nandra et al. 2007; Brenneman & Reynolds 2009; Patrick et al. 2012).”
Suzaku observations of 'bare' active galactic nuclei (2012, background): “…galaxies do often display broad emission features that can be associated with relativistically broadened iron lines (see e.g. Tanaka et al. 1995; Nandra et al. 1999, 2007; Fabian et al. 2000, 2009), and C© 2012 The Authors Published by Oxford University Press on behalf of the Royal Astronomical…”
TESTING THE UNIFICATION MODEL FOR ACTIVE GALACTIC NUCLEI IN THE INFRARED: ARE THE OBSCURING TORI OF TYPE 1 AND 2 SEYFERTS DIFFERENT? (2011, methodology): “The only exception is NGC 7469, for which we use a gaussian prior of 85◦±2◦for the inclination angle of the torus, based on the value of the accretion disk viewing angle deduced from X-ray observations (Nandra et al. 2007), assuming that the disk and the torus are coplanar.”
An X-ray spectral model for Compton-thick toroidal reprocessors (2009, background): “A broad Fe K line (i.e., FWHM of the order of 30,000 km s−1 or more) is sometimes detected as well (e.g. Nandra et al. 2007).”
Irradiation of an accretion disc by a jet: general properties and implications for spin measurements of black holes (2013, background): “These relativistic lines are presentin a significant fraction of AGN spectra (Guainazzi, Bianchi & Dovčiak2006; Nandra et al. 2007; Longinotti et al. 2008; Patrick et al. 2011) and Galactic black hole binaries (Miller 2007; Duro et al. 2011; Fabian et al. 2012a).”
Broad iron L line and X-ray reverberation in 1H0707-495 (2009, background): “…broad iron K line seen in many AGN, is a further confirmation that the observed X-ray spectra from suppermassive black holes in radiatively efficient AGN are emitted from within few gravitational radii of the black hole (Tanaka et al. 1995; Nandra et al. 2007, see Miller 2007 for a review).”
The Suzaku View of Highly Ionized Outflows in AGN. 1; Statistical Detection and Global Absorber Properties (2012, background): “The strongest observational characteristics of such reflection include the Compton-reflection hump at ∼30−40 keV and the almost ubiquitous Fe Kα and Fe Kβ fluorescence lines at ∼6.4 and ∼7.06 keV (Nandra et al. 2007; Shu, Yaqoob & Wang 2010), respectively.”