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

Abstract

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 μm 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″). The 2.5-11.7 μm average spectrum of the nucleus in Mrk 279 shows a strong power law continuum with α = -0.80 ± 0.05 (F v ∝ v α) and weak PAH emission features. The Mrk 279 spectral energy distribution shows a mid-IR bump, which extends from 2 to 15-20 μm. The mid-IR bump is consistent with thermal emission from dust grains at a distance of ≳ 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 Hβ line emission in response to the optical continuum variations is τ = 16.7 -5.6 +5.3 days, consistent with previous measurements.