Spectral and variability analysis of radio-loud and radio-quiet AGN observed with ISOPHOT.

M. Polletta ¹, & T. J.-L. Courvoisier ^1,2

¹ Geneva Observatory, Ch. des Mailettes 51, CH-1290, Sauverny, Switzerland

² INTEGRAL Science Data Centre, Ch. d' Ecogia 16, CH-1290, Versoix, Switzerland

The study of the far-infrared emission in radio-quiet and radio-loud quasars is one of the principal topics of the ISO project. Signatures of thermal dust as well as synchrotron emission are observed in both types of objects. However, several questions are still open about the origin of thermal dust emission (molecular torus, starforming region, host galaxy, ecc..), its interplay with the synchrotron component observed at radio frequencies and its contribution to the bolometric luminosity in radio-loud and radio-quiet objects.

Preliminary results of two ISOPHOT programs aimed to investigate these questions are presented. With the first program, photometric data at 9 wavelengths between 7.3 and 200 m have been obtained for 19 AGN (8 radio-loud quasars, 6 radio-quiet quasars, 4 narrow-line Seyfert 1 galaxies, and 1 radio galaxy) and secondly, ISOPHOT observations for a subset of other 9 AGN have been repeated in order to study variability.

Analysis of infrared spectra shows the presence of thermal dust radiation at several temperatures from 20 to 1600 K in all the objects, with total infrared luminosity ranging from 10¹¹ to 10¹⁴ L. Results of the variability analysis are presented only for 2 sources : a radio-loud quasar, for which an important variation (%) of the far-infrared flux has been observed and a radio-quiet quasar whose flux remains constant during the two epochs.

First infrared measurements are presented for eleven objects. Thirteen sources have also been observed by IRAS, only six of them have been detected by IRAS in at least two filters. Comparison between ISOPHOT and IRAS data shows a good agreement in all the cases, with the exception of two objects : 1) a radio-loud quasar for which ISOPHOT data at long wavelengths are 50% higher than the IRAS data and 2) a narrow-line Seyfert 1 galaxy, for which they are 50% lower. Several hypotheses have been evaluated for explaining the observed difference: variability, cirrus confusion, extended emission and calibration problems. Variability is suggested to be at the origin of the observed difference in the radio-loud quasar, whilst the presence of another source at 90 kpc from the nucleus and unresolved by IRAS is most likely to be the cause of the measured difference in the narrow-line Seyfert 1 galaxy.