Free-free absorption in the nucleus of Cen A: evidences from 7-mm emission variability

Details Free-free absorption in the nucleus of Cen A: evidences from 7-mm emission variability Free-free absorption in the nucleus of Cen A: evidences from 7-mm emission variability

Feb 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007mnras.375..171a&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 375, Issue 1, pp. 171-176.

Astronomy and Astrophysics

Astronomy

2

Radiation Mechanisms: Thermal, Galaxies: Active, Galaxies: Individual: Centaurus A, Galaxies: Jets, Radio Continuum: Galaxies, X-Rays: Galaxies

Scientific paper

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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