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Title:
Free-free absorption in the nucleus of Cen A: evidences from 7-mm emission variability
Authors:
Abraham, Z.; Barres de Almeida, U.; Dominici, T. P.; Caproni, A.
Affiliation:
AA(Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária 05508-090, São Paulo, Brazil; ), AB(Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária 05508-090, São Paulo, Brazil; ), AC(Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária 05508-090, São Paulo, Brazil; ), AD(Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão 1226, Cidade Universitária 05508-090, São Paulo, Brazil; )
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 375, Issue 1, pp. 171-176. (MNRAS Homepage)
Publication Date:
02/2007
Origin:
MNRAS
MNRAS Keywords:
radiation mechanisms: thermal , galaxies: active , galaxies: individual: Centaurus A , galaxies: jets , radio continuum: galaxies , X-rays: galaxies
DOI:
10.1111/j.1365-2966.2006.11278.x
Bibliographic Code:
2007MNRAS.375..171A

Abstract

The existence of free-free absorption in the nucleus of Cen A was suggested by Tingay & Murphy in 2001 as an explanation for the large value of the spectral index between 2.2 and 5 GHz. In this paper we present further evidence for this absorption, based on the observations of short time-scale variability at mm wavelengths and their lack of correlation with X-ray emission, as expected if they were both originated in the same physical process. To explain the short-term variability, we assumed that the mm-wave emission is produced near the base of the inhomogeneous parsec-scale jet that has a bulk velocity of about 0.5c. The presence of an ionized media between the jet and the observer will produce the observed 7-mm flux density variability, as different jet features move behind the absorbing material. We estimated that a region with 5 × 106 electrons cm-3 and 1.5 × 1015 cm radius can explain the intensity and the duration of the mm-flux variations, but the corresponding column density would not be enough to absorb significantly the X-ray flux, explaining the lack of correlation between the radio and X-ray variability. The upper limit for the cloud size inferred for the absorbing region shows that the media surrounding the core of Cen A must be clumpy.
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