Five years monitoring of extragalactic radio sources. II. Quiescent spectra and the evolution of outbursts.
Abstract
We combine data from our extensive monitoring of compact extragalactic radio sources at 12, 22, 37 and 77 GHz with other high-frequency radio observations to derive for the first time separate quiescent and flare spectra for 27 sources. The quiescent spectra outside periods of activity can be decomposed into contributions from knots and other steep-spectrum extended components, and from a compact core, which dominates at higher frequencies. The extent of flatness in compact source spectra has often been exaggerated due to the effects of high-frequency flares; we find that even the most compact cores start to become transparent around ν_0_=200 GHz. Below this last turnover the overall flatness of the spectra results from superposition of knot spectra; above the flatness (α_thin_~ -0.2 to -0.5) of the core spectra. The core spectra indicate tapered jet geometry. The observed spectral parameters of the cores are correlated. All the outburst spectra, altogether 17 flares in 15 variously classified sources, can be fitted with the same shape, that of a simple, homogeneous, self-absorbed source with α_thick_=+2.5 and α_thin_ ~ -0.2. This shape persists throughout the outbursts' development, the only exception being a possible late steepening of the thin spectra, indicative of energy losses. The evolution of flares (10 cases) is also similar, with a rapid rise, plateau and decay in luminosity, and a smooth (ν_m_ is proportional to t^-2^) descending of the turnover to lower frequencies. The simple shape and the overall evolution of the flares agree well with the shock model of Marscher and Gear (1985), but the lack of predicted flattening of the thin spectra with time, the too flat spectra and the final stages of evolution require modifications to the model. The universal electron energy exponent may be γ~1.4, as deduced from both quiescent and flare spectra; the usually observed α~-0.7 comes from electrons that have already suffered considerable energy losses after their acceleration. The radio spectra and the outbursts in variously classified compact sources - BL Lacs, blazar and nonblazar QSOs, galaxies - show no dramatic differences, indicating one common underlying radio structure and a simple and a single mechanism for flux variations: a jet with growing and decaying shocks.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- September 1988
- Bibcode:
- 1988A&A...203....1V
- Keywords:
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- Extragalactic Radio Sources;
- Quasars;
- Radio Observation;
- Stellar Spectra;
- Variable Stars;
- Centimeter Waves;
- Energy Dissipation;
- High Frequencies;
- Millimeter Waves;
- Stellar Flares;
- Astrophysics