Variability pattern from X-ray to IR wavelengths in the active nucleus ofNGC 1566.

Abstract

This study presents the results of a multiwavelength monitoring of the active galactic nucleus in NGC 1566. Data collected in the X-rays, ultraviolet, visible and near-infrared domains have been combined together, in order to disentangle the various components and to better understand the patterns of variability in this active nucleus. At all wavelengths covered by this study the continuous emission is found to vary. The soft X-rays vary by 40% on a time scale shorter than a month. The spectrum of the variable component, as it appears along an outburst which developed over 6 months, can be represented by a power law with index β~ 1.5 from the near infrared to the ultraviolet and β < 1 at higher frequencies. This does not imply however that we are dealing here with a single physical process. The BLR appears to be in an ionization-bounded situation. Profile study of lines arising from the NLR shows that this region has complex structure and kinematics. Analysis of the near infrared variable component leads to the following conclusions: (i) from its spectral shape, we derive that the variable near infrared emission is consistent with thermal radiation from hot graphite particles approximately at their evaporation temperature T_d_~1500 K; (ii) the mass of dust related to the variable near infrared emission is 7 10^-4^M_sun_; (iii) assuming that the dust particles are heated by the central ultraviolet source, the energy balance in the close environment of the active nucleus tells us that the dust covering factor is ~0.6: (iv) the time lag between the near infrared and ultraviolet light-curves, along an intense outburst, is found to be of 2+/-1 month while the bursts themselves are of comparable width; and (v) given the ultraviolet luminosity of the active nucleus, the dust particles would be located at the evaporation radius r_e_~ 7 light-weeks. Therefore, the emergent picture is that the BLR is associated with or just surrounded by an ensemble of graphite particles heated by the central ultraviolet source, at a radius around 2 light-months. The active nucleus of NGC 1566, interpreted in terms of the black hole model, radiates at less than one hundredth its Eddington luminosity and appears to be a scaled-down version of the other active nucleus F9 in which, similarly, a hot dust component has been detected close to the BLR.