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

Mid-infrared images of the Seyfert 1 galaxy Mrk 279 obtained with the ISO satellite are presented together with the results of a one-year monitoring campaign of the 2.5-11.7 $\mu$m spectrum. Contemporaneous optical photometric and spectrophotometric observations are also presented. The galaxy appears as a point-like source at the resolution of the ISOCAM instrument (4-5\arcsec). The 2.5-11.7 $\mu$m average spectrum of the nucleus in Mrk 279 shows a strong power law continuum with $\alpha = -0.80\pm0.05$ ( ${ F_{\nu} \propto \nu^{\alpha}}$) and weak PAH emission features. The Mrk 279 spectral energy distribution shows a mid-IR bump, which extends from 2 to 15-20 $\mu$m. The mid-IR bump is consistent with thermal emission from dust grains at a distance of \ga 100 lt-d. No significant variations of the mid-IR flux have been detected during our observing campaign, consistent with the relatively low amplitude (~10% rms) of the optical variability during the campaign. The time delay for H$\beta$ line emission in response to the optical continuum variations is $\tau = 16.7^{+5.3}_{-5.6}$ days, consistent with previous measurements