We have obtained milliarcsecond-scale spectral index distributions for a sample of 190 extragalactic radio jets through the Monitoring of Jets in Active Galactic Nuclei with the VLBA Experiments ( MOJAVE ) project .
The sources were observed in 2006 at 8.1 , 8.4 , 12.1 , and 15.4 GHz , and we have determined spectral index maps between 8.1 and 15.4 GHz to study the four-frequency spectrum in individual jet features .
We have performed detailed simulations to study the effects of image alignment and ( u , v ) -plane coverage on the spectral index maps to verify our results .
We use the spectral index maps to study the spectral index evolution along the jet and determine the spectral distributions in different locations of the jets .
The core spectral indices are on average flat with mean value +0.22 \pm 0.03 for the sample , while the jet spectrum is in general steep with a mean index of -1.04 \pm 0.03 .
A simple power-law fit is often inadequate for the core regions , as expected if the cores are partially self-absorbed .
The overall jet spectrum steepens at a rate of about -0.001 to -0.004 per deprojected parsec when moving further out from the core with flat spectrum radio quasars having significantly steeper spectra ( mean -1.09 \pm 0.04 ) than the BL Lac objects ( mean -0.80 \pm 0.05 ) .
However , the spectrum in both types of objects flattens on average by \sim 0.2 at the locations of the jet components indicating particle acceleration or density enhancements along the jet .
The mean spectral index at the component locations of -0.81 \pm 0.02 corresponds to a power-law index of \sim 2.6 for the electron energy distribution .
We find a significant trend that jet components with linear polarization parallel to the jet ( magnetic field perpendicular to the jet ) have flatter spectra , as expected for transverse shocks .
Compared to quasars , BL Lacs have more jet components with perpendicular magnetic field alignment , which may explain their generally flatter spectra .
The overall steepening of the spectra with distance can be explained with radiative losses if the jets are collimating or with the evolution of the high-energy cutoff in the electron spectrum if the jets are conical .
This interpretation is supported by a significant correlation with the age of the component and the spectral index , with older components having steeper spectra .