Chandradiscovery of an X-ray jet and lobes in 3C 15

Abstract

We report the Chandra detection of an X-ray jet in 3C 15. The peak of the X-ray emission in the jet is 4.1 '' (a projected distance of 5.1 kpc) from the nucleus, and coincident with a component previously identified in the radio and optical jets. We construct the spectral energy distribution (SED) for this component, optical knot C, and find that X-ray flux is well below the extrapolation of the radio-to-optical continuum. We examine four models for the X-ray jet emission: (I) weak synchrotron cooling in equipartition, (II) moderate synchrotron cooling in equipartition, (III) weak synchrotron plus synchrotron self-Compton (SSC) cooling, and (IV) moderate synchrotron plus SSC cooling. Given weak evidence for a concave feature in the X-ray spectrum, we argue that case (II) can most reasonably explain the overall emission from knot C. Case (III) is also possible, but requires a large departure from equipartition and for the jet power to be comparable to that of the brightest quasars. In all models, (I)-(IV), electrons must be accelerated up to $\gamma_{\rm max} \ga 10^7$, suggesting that re-acceleration is necessary in knot C of the 3C 15 jet. Diffuse X-ray emission has also been detected, distributed widely over the full extent (63 kpc $\times$ 25 kpc) of the radio lobes. The X-ray spectrum of the diffuse emission is described by a two-component model, consisting of soft thermal plasma emission from the host galaxy halo and a hard nonthermal power-law component. The hard component can be ascribed to the inverse Comptonization of cosmic microwave background (CMB) photons by the synchrotron emitting electrons in the radio lobes. We compare the total energy contained in the lobes with the jet power estimated from knot C, and discuss the energetic link between the jet and the lobes. We argue that the fueling time ( t fuel ) and the source age ( t src ) are comparable for case (II), whereas t fuel ≪ t src is likely for case (III). The latter may imply that the jet has a very small filling factor, ~10 -3 . We consider the pressure balance between the thermal galaxy halo and non-thermal relativistic electrons in the radio lobes. Finally, we show that the X-ray emission from the nucleus is not adequately fitted by a simple absorbed power-law model, but needs an additional power-law with heavy absorption ($N_{\rm H} \simeq 10^{22-23}$ cm -2 ) intrinsic to the source. Such a high column density is consistent with the presence of a dense, dusty torus which obscures the quasar nucleus.