7 VLA Results: Evidence for a Jet?

The radio emission from PG 1211+143 is compact at all the frequencies and resolutions we have studied. In our deep K-band image, fitting a Gaussian to the source gives a best-fitting major axis of 22 mas, corresponding to 33 pc at the distance of the quasar assuming the cosmological parameters from the previous section. Given that residual phase uncertainties can induce blurring in a point-like source, this should probably be taken as an upper limit on the size of the source.

The flux densities measured from the various (non-simultaneous) radio data available to us are summarized in Table 6. In all cases these are the integrated flux densities of a Gaussian fitted to the data around the position of the quasar using the AIPS task JMFIT.

Kellermann et al. (1994) report a 5-GHz flux density of $0.80 \pm 0.1$ mJy for the source in observations taken in 1983, so, taking this with our two C-band flux densities, there is some modest evidence for a slow evolution of the apparent luminosity of the source at the 10% level on a timescale of decades. We have no information on whether the fast variability of the source seen in the X-rays is also present in the radio.

The radio SED of the source is interesting, because, assuming that variability can be neglected, we see a spectral index $\alpha$ (defined in the sense $S \propto \nu^{\alpha}$) of $-0.49 \pm 0.06$ between L and C-band, $-0.79 \pm 0.03$ between C and K-band, and $-1.0 \pm 0.4$ internal to the K-band (using the `lower' and `upper' halves of the band). Thus all the SED measurements are consistent with coming from optically thin synchrotron emission, with a steeper spectrum at higher frequencies presumably due to radiative losses. If we further assume that all the emission we see comes from a region of less than 30 pc in size, this gives a consistent picture in which the radio emission comes from an optically thin region: for example, assuming a prolate ellipsoidal emission region with major axis 22 mas and minor axis half the major axis, and taking $E_{\rm min}$ for the relativistic electrons to be 5 MeV, the minimum energy in the synchrotron-emitting particles and field would be around $5 \times 10^{53}$ erg, the equipartition field strength would be of order 700 $\mu$G, and self-absorption would be expected to become important at frequencies below a few hundred MHz. The source would need to be a factor of a few smaller before self-absorption would be expected to affect the lowest frequencies we observe. If the source is strongly projected, as might be expected for an object that appears as a quasar, then these constraints are relaxed, since the true physical size is $\lesssim 30/\sin \theta$ pc where $\theta$ is the angle to the line of sight.

Table: Radio flux densities for PG1211+143

Band	Central freq.	Date	Flux density
	(GHz)		(mJy)
L (FIRST$^{\rm a}$)	1.4	$1999$	$2.10 \pm 0.17$
C	4.9	1993 May 02	$0.95 \pm 0.07$
C	6	2015 Jun 18	$1.032 \pm 0.015$
K	22	2015 Jun 18	$0.370 \pm 0.011$
K (lower)	20	2015 Jun 18	$0.384 \pm 0.013$
K (upper)	24	2015 Jun 18	$0.320 \pm 0.017$

If we further assume that such a compact region is expanding at $\sim c$, then the minimum energy in the synchrotron-emitting plasma would imply jet kinetic powers $>3 \times 10^{44}$ erg s$^{-1}$, which, although high, is substantially less than the radiative luminosity of the quasar, and again this number would be reduced by projection, or by assuming slower expansion speeds. It should be noted, though, that the numerator of this calculation is the minimum energy, and departures from equipartition, which would imply higher energy densities in the synchrotron-emitting material, are very common in larger-scale radio sources. Given these uncertainties, it certainly does not seem impossible to imagine a scenario in which the shock driven by the jet/lobe system responsible for the radio emission gives rise to the fast bulk outflows detected in the X-ray and UV spectra. Although the shock driven by the jet provides the acceleration mechanism in this scenario, we would still expect the shocked gas to be photoionized by the central continuum source, consistent with our photoionization model for the absorption. Tombesi et al. (2014) have shown that both radio jets and UFOs can co-exist in AGN, and may even be related. Testing such a model would require very long baseline interferometric (VLBI) imaging sensitive enough to detect and resolve the radio emission at mas resolution.