XMM-Newton observations reveal AGN in apparently normal galaxies

P. Severgnini, A. Caccianiga, V. Braito, R. Della Ceca, T. Maccacaro, A. Wolter, K. Sekiguchi, T. Sasaki, M. Yoshida, M. Akiyama, M. G. Watson, X. Barcons, F. J. Carrera, W. Pietsch, N. A. Webb

Research output: Contribution to journalArticlepeer-review

91 Citations (Scopus)


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 (NH > 1022 cm-2) and unobscured (NH < 1022 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 MR < M*, 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.

Original languageEnglish
Pages (from-to)483-492
Number of pages10
JournalAstronomy and Astrophysics
Issue number2
Publication statusPublished - 2003 Aug


  • Galaxies: active
  • X-rays: galaxies


Dive into the research topics of 'XMM-Newton observations reveal AGN in apparently normal galaxies'. Together they form a unique fingerprint.

Cite this