Radio Spectra

Radio spectra are usually displayed in the form of a logarithmic plot of flux density versus frequency. Sources with power law spectra are straight line in such a plot with slope equal to the spectral index, $-\alpha$. The spectral index can be defined at any frequency as the negative of the derivative $d(log S)/d(log \nu)$ or by the measurement of flux density at two arbitrarily selected frequencies.

Our measurements at 610 & 235 MHz give new information about the low frequency ($<$ 1 GHz) end of the spectra of the sources observed. However this is not adequate to study the complete radio spectrum of these sources. We have used the the high frequency flux densities of these sources as listed in the VLA calibrator manual and formed the radio spectra from 40 GHz to 235 MHz. It is to be noted that many flat spectra sources are variable and combining data of different epoch should be treated with caution.