5. Spectra of scintillating components in GPS sources
The spectral turnover of GPS sources at low radio frequencies could be either due to synchrotron self absorption or free-free absorption. In order to try and distinguish between these two principal processes, we attempted to determine better the spectra of the compact component using both IPS observations at low frequencies and VLBI observations at higher frequencies. Since all the emission with compact structures less than about 400 mas would contribute to the scintillating power, we have confined ourselves to those GPS sources in our IPS sample whose emission at a higher frequency is dominated by a single compact component. The peak brightness in any secondary component in the VLBI images is less by at least a factor of about 10, and it is reasonable to assume that the scintillations are almost entirely from the dominant component. Five of the objects in our sample satisfy this criterion.
We present the integrated spectra as well as that of the dominant component from the IPS and VLBI observations in Fig. 5; and the results are summarized in Table 6 which is arranged as follows. Column 1: source name; Column 2: optical identification; Column 3: the redshift; Column 4: frequency of the peak in the integrated spectrum, , in GHz; Column 5: frequency of the peak in the spectrum of the dominant component, in GHz; Column 6: the flux density of the dominant component at the peak frequency; Columns 7: the range in the VLBI sizes of the prominent components in mas, with each size being the geometric mean of the major and minor axes. Column 8: the size of the IPS component in mas; Column 9: the expected size of the dominant component if the turnover is due to synchrotron self absorption. The angular size of the self absorbed component can be estimated from (cf. Kellermann et al. 1981)
where is in mas, peak frequency in GHz, the flux density at the peak frequency, is in Jy and the magnetic field B in Gauss. We have assumed a redshift of 1 for the source 0742+103 Columns 10 and 11: the electron density of the absorbing medium, if the absorption is due to thermal free-free absorption. This is given by (cf. Osterbrock 1989)
where is in GHz, T is in units of 104 K and the size of the absorbing medium L in pc. We have assumed T=104 K and have listed the values of ne for L=10 and 50 pc.
Table 6. Estimated parameters of 5 GPS sources with one dominant component
The expected size of the components if the turnover is due to synchrotron self absorption ranges from about 1 to 5 mas assuming a magnetic field of 10-4 Gauss (e.g. Mutel et al. 1985). The sizes and spectra of the components are generally consistent with synchrotron self absorption, although models involving free-free absorption are viable (cf. Bicknell et al. 1997). In the radio source 0400+258 the total linear separation of the components extends to over 10 mas (Fey & Charlot 1997), and could extend to larger sizes with the extended structure appearing somewhat diffuse, although the size of the individual components range from 0.3 - 2.8 mas. If the low-frequency turnover is due to free-free absorption the required densities of an absorbing medium of thickness 10 and 50 pc at a temperature of 104 K, are listed in Table 6. These values are in the range of 200 to 1200 cm-3. Carvalho (1998) suggests that this dense gas might imply the frustration scenario where CSSs are as old as the larger FRII sources. However, estimates of this density could be reduced as in the model of Bicknell et al. (1997).
Recently Kuncic et al. (1998) considered the effect of induced compton scattering (ICS) by the external shell of thermal gas (Bicknell et al. 1997) to explain the low frequency turnover of the GPS sources. The Thomson optical depth ()of the ICS screen ranges from about 0.001 to 0.04 for the model parameters quoted in Kuncic et al. (1998). Using the peak frequency of the components (Table 6), we estimate the angular sizes of the components to be in range of about 1 to 8 mas for of 0.001 and 7 to 50 mas for of 0.04. These values are consistent with the component sizes listed in Table 6, making it difficult to distinguish unambiguously between the different processes responsible for the low-frequency turnover.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100