The X-ray variability and the near-IR to X-ray spectral energy distribution of four low luminosity Seyfert 1 galaxies

¹ Physics Department, University of Crete, PO Box 2208, 710 03 Heraklion, Crete, Greece e-mail: jhep@physics.uoc.gr
² IESL, Foundation for Research and Technology, 711 10 Heraklion, Crete, Greece
³ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, 85740 Garching, Germany
⁴ Institute of Astronomy and Astrophysics, National Observatory of Athens, I. Metaxa & V. Pavlou, 152 36 P. Penteli, Athens, Greece

Received: 31 May 2008
Accepted: 26 August 2008

Abstract

Context. We present the results from a study of the X-ray variability and the near-IR to X-ray spectral energy distribution of four low-luminosity, Seyfert 1 galaxies.

Aims. We compared their variability amplitude and broad band spectrum with those of more luminous AGN in order to investigate whether accretion in low-luminosity AGN operates as in their luminous counterparts.

Methods. We used archival XMM-Newton and, in two cases, ASCA data to estimate their X-ray variability amplitude and determine their X-ray spectral shape and luminosity. We also used archival HST data to measure their optical nuclear luminosity, and near-IR measurements from the literature, in order to construct their near-IR to X-ray spectra.

Results. The X-ray variability amplitude of the four Seyferts is what one would expect, given their black hole masses. Their near-IR to X-ray spectrum has the same shape as the spectrum of quasars that are 10²–10⁵ times more luminous.

Conclusions. The objects in our sample are optically classified as Seyfert 1–1.5. This implies that they host a relatively unobscured AGN-like nucleus. They are also of low luminosity and accrete at a low rate. They are therefore good candidates to detect radiation from an inefficient accretion process. However, our results suggest that they are similar to AGN that are 10²–10⁵ times more luminous. The combination of a “radiative efficient accretion disc plus an X-ray producing hot corona” may persist at low accretion rates as well.