NGC 454: unveiling a new ‘changing look’ active galactic nucleus

Authors

  • E. Marchese,

    Corresponding author
    1. INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    2. Dipartimento di Fisica G. Occhialini, Università degli Studi di Milano-Bicocca, Piazza dell’Ateneo Nuovo 1, 20126 Milano, Italy
      E-mail: elena.marchese@brera.inaf.it (EM); valentina.braito@brera.inaf.it (VB)
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  • V. Braito,

    Corresponding author
    1. INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
    2. Department of Physics and Astronomy, X-Ray Astronomy Observational Group, Leicester University, Leicester LE1 7RH
      E-mail: elena.marchese@brera.inaf.it (EM); valentina.braito@brera.inaf.it (VB)
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  • R. Della Ceca,

    1. INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
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  • A. Caccianiga,

    1. INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
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  • P. Severgnini

    1. INAF – Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
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E-mail: elena.marchese@brera.inaf.it (EM); valentina.braito@brera.inaf.it (VB)

ABSTRACT

We present a detailed analysis of the X-ray spectrum of the Seyfert 2 galaxy NGC 454E, belonging to the interacting system NGC 454. Observations performed with Suzaku, XMM–Newton and Swift allowed us to detect a dramatic change in the curvature of the 2–10 keV spectrum, revealing a significant variation of the absorbing column density along the line of sight (from ∼ 1 × 1024 cm−2 to ∼ 1 × 1023 cm−2). Consequently, we propose this source as a new member of the class of ‘changing look’ active galactic nuclei (AGN), i.e. AGN that have been observed both in Compton thin (NH= 1023  cm−2) and reflection-dominated states (Compton thick, NH > 1024  cm−2). Due to the quite long time lag (six months) between the Suzaku and XMM–Newton observations, we cannot infer the possible location of the obscuring material causing the observed variability. In the 6–7 keV range, the XMM–Newton observation also shows a clear signature of the presence of an ionized absorber. Since this feature is not detected during the Suzaku observation (despite its detectability), the simplest interpretation is that the ionized absorber is also variable; its location is estimated to be within ∼ 10−3 pc from the central black hole, probably much closer in than the rather neutral absorber.

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