EDP Sciences
Free access
Issue
A&A
Volume 404, Number 1, June II 2003
Page(s) 133 - 144
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030435


A&A 404, 133-144 (2003)
DOI: 10.1051/0004-6361:20030435

The spectral-curvature parameter: An alternative tool for the analysis of synchrotron spectra

B. W. Sohn1, 2, U. Klein2 and K.-H. Mack2, 3, 4

1  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2  Radioastronomisches Institut der Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
3  ASTRON/NFRA, Postbus 2, 7990 AA Dwingeloo, The Netherlands
4  Istituto di Radioastronomia del CNR, via P. Gobetti 101, 40129 Bologna, Italy

(Received 23 August 2001 / Accepted 17 March 2003)

Abstract
A new intuitive tool for the analysis of synchrotron spectra is presented. The so-called Spectral Curvature Parameter (SCP), when plotted versus the high-frequency spectral index ( $\alpha_{\rm high}$) of synchrotron sources, provides crucial parameters on the continuum spectrum of synchrotron radiation without the more complex modeling of spectral ageing scenarios. An important merit of the SCP- $\alpha$ diagram, in respect to the conventional colour-colour diagram (i.e. $\alpha$- $\alpha$ diagram), is the enhanced reliability of extracting multiple injection spectra, $\alpha_{\rm inj}$. Different from the colour-colour diagram, tracks of different $\alpha_{\rm inj}$s, especially when the synchrotron particles are young, exhibit less overlap and less smearing in the SCP- $\alpha$ diagram. Three giant radio galaxies (GRGs) and a sample of Compact steep spectrum (CSS) souces, which are parti cularly suitable for this kind of analysis, are presented. GRGs exhibit asymmetries of their injection spectral indices  $\alpha_{\rm inj}$ in the SCP- $\alpha_{\rm high}$ diagram. The obtained $\alpha_{\rm inj}$s and the trends in the sources are cross-checke  with the literature and show remarkable confidence. Besides the spectral steepening which is well understood in the framework of synchrotron ageing models, spectral flattening is prominent in the radio lobes. The spectral flattening is a clue to efficient re-acceleration processes in the lobes. This implies that interaction with the surrounding intergalactic or intra-cluster medium is an important characteristic of GRGs. In the SW lobe of DA 240, there is a clear sign of CI and KP/JP bifurcation at the source extremity. This indicates a highly relativistic energy transportation from the core or in situ acceleration in this typical FR I lobe. Our analysis proves, if exists, KP spectra imply the existence of strong $B_{\rm sync}$ field with $B_{\rm sync} > B_{\rm CMB}$. In the CSS sources, our result confirms the CI model and  $B_{\rm sync} \gg B_{\rm CMB}$. The synchrotron self-absorption is significant in the CSS sample.


Key words: galaxies: jets -- radio continuum: galaxies -- methods: data analysis

Offprint request: B. W. Sohn, bwsohn@mpifr-bonn.mpg.de

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