XMM-Newton observations reveal AGN in apparently normal galaxies

¹ Osservatorio Astronomico di Brera, via Brera 28, 20121, Milano, Italy e-mail: paola, caccia, braito, rdc, tommaso, anna@brera.mi.astro.it
² Dipartimento di Astronomia, Università di Padova, Vicolo dell'Osservatorio 2, 35122, Padova, Italy
³ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA e-mail: kaz, sasaki, akiyama@subaru.naoj.org
⁴ Okayama Astronomical Observatory, National Astronomical Observatory of Japan, Kamogata-cho, Asakuchi-gun, Okayama 719-0232, Japan e-mail: yoshida@oao.nao.ac.jp
⁵ X-ray Astronomy Group, Department of Physics and Astronomy, Leicester University, Leicester LE1 7RH, UK e-mail: mgw@star.le.ac.uk
⁶ Instituto de Fisica de Cantabria (CSIC-UC), Avenida de los Castros, 39005 Santander, Spain e-mail: barcons, carreraf@ifca.unican.es
⁷ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany e-mail: wnp@mpe.mpg.de
⁸ Centre d'Étude Spatiale des Rayonnements, 9 avenue du Colonel Roche, 31028 Toulouse Cedex 04, France e-mail: webb@cesr.fr

Corresponding author: P. Severgnini, paola@brera.mi.astro.it

Received: 12 February 2003
Accepted: 14 April 2003

Abstract

We have performed a detailed analysis of 3 optically normal galaxies extracted from the XMM Bright Serendipitous Source Sample. Thanks to the good statistics of the XMM-Newton data, we have unveiled the presence of an AGN in all of them. In particular, we detect both X-ray obscured ( cm^-2) and unobscured ( cm^-2) AGNs with intrinsic 2–10 keV luminosities in the range between 10⁴²–10⁴³ erg s^-1. We find that the X-ray and optical properties of the sources discussed here could be explained assuming a standard AGN hosted by galaxies with magnitudes , taking properly into account the absorption associated with the AGN, the optical faintness of the nuclear emission with respect to the host galaxy, and the inadequate set-up and atmospheric conditions during the optical spectroscopic observations. Our new spectroscopic observations have revealed the expected AGN features also in the optical band. These results clearly show that optical spectroscopy sometimes can be inefficient in revealing the presence of an AGN, which instead is clearly found from an X-ray spectroscopic investigation. This remarks the importance of being careful in proposing the identification of X-ray sources (especially at faint fluxes) when only low quality optical spectra are in hand. This is particularly important for faint surveys (such as those with XMM-Newton and Chandra), in which optically dull but X-ray active objects are being found in sizeable numbers.