Suzaku observation of the LINER NGC 4102

¹ IESL, Foundation for Research and Technology, 711 10 Heraklion, Crete, Greece
e-mail: omaira@physics.uoc.gr; omaira@physics.uoc.gr
² Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece
³ Department of Physics and Astronomy, Leicester University, LE1 7RH, UK
⁴ Instituto de Astrofísica de Andalucía (CSIC), Granada, Spain
⁵ Instituto de Astrofísica de Canarias (IAC), C/Via Lactea, s/n, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
⁷ Grupo de Mecánica Espacial and Instituto Universitario de Matemática y Aplicaciones, Universidad de Zaragoza, 50009 Zaragoza, Spain
⁸ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, The Netherlands
⁹ NASA Goddard Space Flight Center, Astrophysics Science Division, Greenbelt, MD 20771, USA

Received: 8 November 2010
Accepted: 13 December 2010

Abstract

Context. Low-ionisation, nuclear emission-line region (LINER) nuclei are said to be different from other active galactic nuclei (AGN) due to the presence of complex absorbing structures along the line-of-sight and/or an inefficient mode of accretion onto the supermassive black hole. However, this is still open.

Aims. We investigate the broad band X-ray spectrum of NGC 4102, one of the most luminous LINERs in the Swift/BAT survey.

Methods. We studied a 80 ks Suzaku spectrum of NGC 4102, together with archival Chandra and Swift/BAT observations. We also studied the optical (3.5 m/TWIN at Calar Alto observatory) and near-infrared (WHT/LIRIS at Observatorio Roque los Muchachos) spectra that were taken at the same time as the Suzaku data.

Results. There is strong evidence that NGC 4102 is a Compton-thick AGN, as suggested by the Swift/BAT detected intrinsic continuum and the presence of a strong narrow, neutral FeKα emission line. We have also detected ionised Fe_XXV emission lines in the Suzaku spectrum of the source. NGC 4102 shows a variable soft excess found at a significantly higher flux state at the time of Suzaku observations when compared to Chandra observations. Finally, a complex structure of absorbers is seen with at least two absorbers besides the Compton-thick one, derived from the X-ray spectral analysis and the optical extinction.

Conclusions. All the signatures described in this paper strongly suggest that NGC 4102 is a Compton-thick Type-2 AGN from the X-ray point of view. The “soft excess”, the electron scattered continuum component, and the ionised iron emission line might arise from Compton-thin material photoionised by the AGN. From variability and geometrical arguments, this material should be located somewhere between 0.4 and 2 pc away from the nuclear source, inside the torus and perpendicular to the disc. The bolometric luminosity (L_bol = 1.4 × 10⁴³ erg s^-1) and accretion rate (ṁ_Edd = 5.4 × 10^-3) are consistent with other low-luminosity AGN. However, the optical and near infrared spectra correspond to that of a LINER source. We suggest that the LINER classification might be due a different spectral energy distribution according to its steeper spectral index.