Possible evidence for the disc origin for the powering of jets in Sgr A* and nearby elliptical galaxies

Abstract

Recent VLBA observation indicates the existence of an elongated (jet) structure in the compact radio source Sgr A*. This is hard to explain in the context of advection-dominated accretion flow (ADAF) model for this source. On the other hand, the mass accretion rate favoured by ADAF is 10–20 times smaller than that favoured by the hydrodynamical simulation based on Bondi capture. If the latter were adopted, the predicted radio flux would significantly exceed the observation. A similar situation exists in the case of nearby giant ellipticals, where the canonical ADAF model — the widely assumed standard model for these sources — also significantly overpredicts the radio flux. Based on these facts, in this paper we propose a truncated ADAF model for Sgr A* and three ellipticals M87, NGC 4649 and NGC 4636. We assume that the accretion disc is truncated at a certain radius Rtr within which the jet forms by extracting the energy of the disc. The radio flux is greatly suppressed owing to the radiative truncation of the disc and the fits to the observational data are excellent. For example, for Sgr A*, the model fits the observational spectrum very well from radio including the ‘excess’ below the break frequency to hard X-ray under a high accretion rate near the simulation value, and the predicted size-frequency relationship is also in excellent agreement with the observation; for M87, the predicted upper limit of the jet location is 24Rg, in excellent agreement with the observational result that the jet is formed on scales smaller than 30Rg, and the ≈20 per cent variability at ∼1 keV — which is hard to explain in another model that succeeded in explaining the low radio flux of M87 — is also marginally interpreted. The success of the model supplies possible evidence for the disc rather than the hole origin for the powering of jets.