Monitoring of the optical and 2.5 - 11.7 µm spectrum and mid-IR imaging of the Seyfert 1 galaxy Mrk 279 with ISO
M. Santos-Lleó, J. Clavel, B. Schulz, B. Altieri, P. Barr, D. Alloin, P. Berlind, R. Bertram, D.M. Crenshaw, R.A. Edelson, U. Giveon, K. Horne, J.P. Huchra, S. Kaspi, G.A. Kriss, J.H. Krolik, M.A. Malkan, Yu.F. Malkov, H. Netzer, P.T. O'Brien, B.M. Peterson, R.W. Pogge, V.I. Pronik, B.-C. Qian, G.A. Reichert, P.M. Rodriguez-Pascual, S.G. Sergeev, J. Tao, S. Tokarz, R.M. Wagner, W. Wamsteker, and B.J. Wilkes, 2000, submitted to A&A

Mid-infrared images of the Seyfert 1 galaxy Mrk 279 obtained with the ISO satellite are presented. In addition, the results of a one-year monitoring campaign of the 2.5-11.7 µm spectrum of the active nucleus in Mrk 279 are also discussed, along with results of contemporaneous ground-based optical photometric and spectrophotometric monitoring. 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 shows a strong continuum, with a flux density per frequency unit that drops sharply toward increasing frequency and only very weak PAH features and broad Br alpha emission. The continuum flux is well approximated by a power-law of index alpha = -0.80 +/- 0.05 (F_nu, proportional to nu^{alpha}). The Mrk 279 spectral energy distribution shows a mid-TR, bump, which extends from roughly 2 to 15-20 µm. The mid-IR bump is consistent with thermal emission from dust grains that are reprocessing a variable UV source at a distance of greater than around 100 lt-d. No significant variations of the mid-IR flux have been detected during our observing campaign. In the context of the dust reprocessing model, this is consistent with the relatively low amplitude (about 10% rms) of the optical flux variability during the campaign. Despite the low amplitude of the optical variations, it was nevertheless possible to measure the time delay for the Hbeta emission-line response to the continuum variations, tau = 16.7^{+5.3)_{-5.6} days, consistent with previous measurements.