VARIABILITY PATTERN FROM X-RAY TO IR WAVELENGTHS IN THE ACTIVE NUCLEUS OF NGC-1566

This study presents the results of a multiwavelength monitoring of the active galactic nucleus in NGC 1566. Data collected in the X-rays, ul traviolet, visible and near-infrared domains have been combined togeth er, in order to disentangle the various components and to better under stand the patterns of variability in this active nucleus. At all wavel engths 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 w hich developed over 6 months, can be represented by a power law with i ndex beta approximately 1.5 from the near infrared to the ultraviolet and beta < 1 at higher frequencies. This does not imply however that w e 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 kinemati cs. Analysis of the near infrared variable component leads to the foll owing conclusions: (i) from its spectral shape, we derive that the var iable near infrared emission is consistent with thermal radiation from hot graphite particles approximately at their evaporation temperature T(d) approximately 1500 K; (ii) the mass of dust related to the varia ble near infrared emission is 7 10(-4) M.; (iii) assuming that the dus t particles are heated by the central ultraviolet source, the energy b alance in the close environment of the active nucleus tells us that th e dust covering factor is approximately 0.6; (iv) the time lag between the near infrared and ultraviolet high-curves, along an intense outbu rst, is found to be of 2 +/- 1 month while the bursts themselves are o f comparable width; and (v) given the ultraviolet luminosity of the ac tive nucleus, the dust particles would be located at the evaporation r adius r(e) approximately 7 light-weeks. Therefore, the emergent pictur e 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, interpre ted in terms of the black hole model, radiates at less than one hundre dth its Eddington luminosity and appears to be a scaled-down version o f the other active nucleus F9 in which, similarly, a hot dust componen t has been detected close to the BLR.