The broad-band X-ray spectrum of the Seyfert 1 galaxy, MCG+8-11-11

¹ Dipartimento di FisicaUniversità degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
e-mail: bianchi@fis.uniroma3.it
² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica, via U. La Malfa 153, 90146 Palermo, Italy
³ INAF/IASF-Roma, via del Fosso del Cavaliere, 00133 Roma, Italy
⁴ INAF-IASF Bologna, via Gobetti 101, 40129 Bologna, Italy
⁵ Instituto de Astronomía, Universidad Nacional Autónoma de México, Apartado Postal 70-264, 04510 Mexico DF, Mexico
⁶ Osservatorio Astronomico di Roma (INAF), via Frascati 33, 00040 Monte Porzio Catone, Italy

Received: 14 June 2010
Accepted: 9 July 2010

Abstract

Aims. Evidence is mounting that the main ingredients of the unification models of active galactic nuclei may behave differently from expectations, and be intimately related to fundamental physical parameters. The availability of high signal-to-noise broad-band X-ray spectra provides us with the opportunity to study in detail all the contributions from the materials invoked in these models, and infer their general properties, including whether their presence or absence is related to other quantities.

Methods. We present a long (100 ks) Suzaku observation of one of the X-ray brightest AGN, MCG+8-11-11. These data are complemented with the 54-month Swift BAT spectrum, allowing us to perform a broad-band fit to the 0.6–150 keV range.

Results. The fits performed in the 0.6–10 keV band provide results consistent with those of a a previous XMM-Newton observation, i.e. a lack of a soft excess, warm absorption along the line of sight, a large Compton reflection component (R ≃ 1), and an absence of a relativistic component of the neutral iron Kα emission line. However, when the PIN and Swift BAT data are included, the reflection amount drops significantly (R ≃ 0.2 − 0.3), and a relativistic iron line is required, the latter being confirmed by a phenomenological analysis in a restricted energy band (3–10 keV). When a self-consistent model is applied to the whole broad-band data, the observed reflection component appears to be entirely associated with the relativistic component of the iron Kα line.

Conclusions. The implied scenario, though strongly model-dependent, requires that all the reprocessing spectral components from Compton-thick material be associated with the accretion disc, and no evidence of a classical pc-scale torus is found. The narrow core of the neutral iron Kα line is therefore produced in Compton-thin material, such as the BLR, similarly to what is found in another Seyfert galaxy, NGC 7213, but with the notable difference that MCG+8-11-11 presents spectral signatures from an accretion disc. The very low accretion rate of NGC 7213 could explain the lack of relativistic signatures in its spectrum, but the absence of the torus in both sources is more difficult to explain, since their luminosities are comparable, and their accretion rates are completely different.