Accretion Properties (accretion + property)

Distribution by Scientific Domains


Selected Abstracts


Radio imaging of the Subaru/XMM,Newton Deep Field , II.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008
The 37 brightest radio sources
ABSTRACT We study the 37 brightest radio sources in the Subaru/XMM,Newton Deep Field. We have spectroscopic redshifts for 24 of 37 objects and photometric redshifts for the remainder, yielding a median redshift zmed for the whole sample of zmed, 1.1 and a median radio luminosity close to the ,Fanaroff,Riley type I/type II (FR I/FR II)' luminosity divide. Using mid-infrared (mid-IR) (Spitzer MIPS 24 ,m) data we expect to trace nuclear accretion activity, even if it is obscured at optical wavelengths, unless the obscuring column is extreme. Our results suggest that above the FR I/FR II radio luminosity break most of the radio sources are associated with objects that have excess mid-IR emission, only some of which are broad-line objects, although there is one clear low-accretion-rate object with an FR I radio structure. For extended steep-spectrum radio sources, the fraction of objects with mid-IR excess drops dramatically below the FR I/FR II luminosity break, although there exists at least one high-accretion-rate ,radio-quiet' QSO. We have therefore shown that the strong link between radio luminosity (or radio structure) and accretion properties, well known at z, 0.1, persists to z, 1. Investigation of mid-IR and blue excesses shows that they are correlated as predicted by a model in which, when significant accretion exists, a torus of dust absorbs ,30 per cent of the light, and the dust above and below the torus scatters ,1 per cent of the light. [source]


A transition in the accretion properties of radio-loud active nuclei

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
Danilo Marchesini
ABSTRACT We present evidence for the presence of a transition in the accretion properties of radio-loud sources. For a sample of radio galaxies and radio-loud quasars, selected based on their extended radio properties, the accretion rate is estimated from the black hole mass and nuclear luminosity. The inferred distribution is bimodal, with a paucity of sources at accretion rates, in Eddington units, of the order of ,10,2, assuming a radiative efficiency of 10 per cent , and possibly spanning 1,2 orders of magnitude. Selection biases are unlikely to be responsible for such behaviour. We discuss possible physical explanations, including a fast transition to low accretion rates, a change in the accretion mode/actual accretion rate/radiative efficiency, the lack of stable disc solutions at intermediate accretion rates or the inefficiency of the jet formation processes in geometrically thin flows. This transition might be analogous to spectral states (and jet) transitions in black hole binary systems. [source]


Optical spectra of 15 Low Luminosity Compact Sources and the formation of jets

ASTRONOMISCHE NACHRICHTEN, Issue 2-3 2009
S. Buttiglione
Abstract We present the results of optical spectroscopy of 15 compact radio sources, selected to be among the lowest luminosity objects and expected to evolve into FR I radio galaxies. The aim of the work is to exploit the emission line characteristics as indicators of the nuclear activity , in terms of accretion properties , during the early stages of the production of jets. The emission line ratios identify most Low Luminosity Compact Sources (LLCSs) as low excitation galaxies, similar to evolved FR I. Also in terms of their optical , radio properties they appear to be indistinguishable from FR I. These findings support the view that jets could be produced by nuclei accreting both at high and low rates (and/or radiatively efficient and inefficient discs). They also argue against the idea that FRI represent a late stage after a powerful FR II phase ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]