What Happened to the NGC 6251 Counterjet?

Abstract

We have used the Very Long Baseline Array to produce a high dynamic range image of the nucleus of NGC 6251 at 1.6 GHz and snapshot images at 5.0, 8.4, and 15.3 GHz to search for emission from a parsecscale counterjet. Previous VLBI images at 1.6 GHz have set a lower limit for the jet/counterjet brightness ratio near the core at about 80:1, which is larger than expected, given the evidence that the radio axis is fairly close to the plane of the sky. A possible explanation is that the inner few parsecs of the counterjet are hidden by free-free absorption by ionized gas associated with an accretion disk or torus. This would be consistent with the nearly edge-on appearance of the arcsecond-scale dust disk seen in the center of NGC 6251 by Hubble Space Telescope (HST). We detect counterjet emission close to the core at 1.6 GHz, but not at the higher frequencies. Given that the optical depth of free-free absorption falls off more rapidly with increasing frequency than the optically thin synchrotron emission from a typical radio jet, this result implies that the absence of a detectable parsec-scale counterjet at high frequencies is not due to free-free absorption unless the density of ionized gas is extremely high and we have misidentified the core at 1.6 GHz. The most likely alternative is a large jet/counterjet brightness ratio caused by relativistic beaming, which in turn requires the inner radio axis to be closer to our line of sight than the orientation of the HST dust disk would suggest.

Structural Transition in the NGC 6251 Jet: An Interplay With the Supermassive Black Hole and Its Host Galaxy

“…• (see also Table 1); a similar decrease is detected by Jones & Wehrle (2002). All jet cross sections are well represented with a single Gaussian component.…”

“…Its kiloparsec-scale radio emission has been investigated in great detail using the Very Large Array (VLA) (Perley et al 1984), and the parsec-scale jet emission using VLBI continuously extending to 50 mas (Jones et al 1986;Sudou et al 2000;Jones & Wehrle 2002). The mass of the SMBH is measured to be M = (6 ± 2) × 10 8 M (Ferrarese & Ford 1999).…”

Investigating the EGRET-radio galaxies link withINTEGRAL: The case of 3EG J1621+8203 and NGC 6251

“…2). On the other hand, Jones & Wehrle (2002) found θ < 47 • with the Very Long Baseline Interferometer (VLBI) measurement of the jet-counterjet ratio. Moreover, Chiaberge et al (2003) evaluated also the beaming factor δ = 3.2, that is lower than for blazars (8 ≤ δ ≤ 23), but greater than the case of Cen A (δ = 1.2).…”

Are Quasar Jets Dominated by Poynting Flux?

“…However, the reality of the counterjet detection in NGC 6251 is questioned by Jones & Wehrle (2002). In particular, they did not confirm the presence of the counterjet at 15 GHz, the frequency at which their observations had an angular resolution similar to those by Sudou et al (2000).…”