The nuclear accretion in the FRI radio galaxy IC4296 from CHANDRA and VLBI observations

A high angular resolution study of the nucleus of the FR I galaxy IC 4296 using Chandra ACIS-S and VLBA observations is presented, with the aim of studying the nature of the accretion process. Pointlike and hard X-ray emission is found, well described by a moderately absorbed power law of ’¦£=1.48+0.42-0.34 no iron fluorescence line from cold material is detected. The 0.3-10 keV luminosity is 2.4×1041 ergs s-1, which is ~400 times lower than the accretion luminosity resulting from the estimated Bondi mass accretion rate and a radiative efficiency of 10%. On the parsec scale, a jet and a counterjet extend out from a central unresolved ``core'' in the 8.4 GHz image. Their orientation is in good agreement with that of the large-scale jets, and their bulk speed is relativistic. The parsec-scale spectrum is convex over 2-22 GHz. The observed nuclear luminosity is not likely to be reconciled with the accretion luminosity by assuming that Compton thick material surrounds the nucleus. Low radiative efficiency accretion flow models (ADAF and its variants) cannot account for the observed emission and spectral shape of the nucleus in the radio band. The power in the jets accounts for a sizable fraction (>~10%) of the accretion luminosity; therefore, the mass accretion rate need not be much different from the estimate obtained using Bondi's theory. A jet-dominated origin also for the observed radiative losses of the nucleus is suggested. This could also explain the finding that a nuclear luminosity orders of magnitude lower than that of ``normal'' radio-loud AGNs is accompanied by a relatively higher radio emission.