Monitoring of the optical and 2.5-11.7 mu m spectrum and mid-IR imaging ofthe Seyfert 1 galaxy Mrk 279 with ISO

Mid-infrared images of the Seyfert 1 galaxy Mrk 279 obtained with the ISO s atellite are presented together with the results of a one-year monitoring c ampaign of the 2.5-11.7 mum spectrum. Contemporaneous optical photometric a nd spectrophotometric observations are also presented. The galaxy appears a s a point-like source at the resolution of the ISOCAM instrument (4-5 "). T he 2.5-11.7 mum average spectrum of the nucleus in Mrk 279 shows a strong p ower law continuum with alpha = -0.80 +/- 0.05 (F nu proportional to nu (al pha)) and weak PAK emission features. The Mrk 279 spectral energy distribut ion shows a mid-IR bump, which extends from 2 to 15-20 mum The mid-IR bump is consistent with thermal emission from dust grains at a distance of great er than or similar to 100 It-d. No significant variations of the mid-IR flu x have been detected during our observing campaign, consistent with the rel atively low amplitude (similar to 10% rms) of the optical variability durin g the campaign. The time delay for H beta line emission in response to the optical continuum variations is tau = 16.7(-5.6)(+5.3), days, consistent wi th previous measurements.