The discovery of extended thermal X-ray emission from PKS 2152-699: Evidence for a "jet-cloud" interaction

A Chandra ACIS-S observation of PKS 2152-699 reveals thermal emission from a diffuse region around the core and a hotspot located 10″ northeast from the core. This is the first detection of thermal X-ray radiation on kiloparsec scales from an extragalactic radio source. Two other hotspots located 47″ north-northeast and 26″ southwest from the core were also detected. Using a Raymond-Smith model, the first hotspot can be characterized with a thermal plasma temperature of 2.6 × 10⁶ K and an electron number density of 0.17 cm^-3. These values correspond to a cooling time of ∼1.6 × 10⁷ yr. In addition, an emission line from the hotspot, possibly Fe xxv, was detected at rest wavelength 10.04 Å. The thermal X-ray emission from the first hotspot is offset from the radio emission but is coincident with optical filaments detected with broadband filters of Hubble Space Telescope WFPC2. The best explanation for the X-ray, radio, and optical emission is that of a "jet-cloud" interaction. The diffuse emission around the nucleus of PKS 2152-699 can be modeled as a thermal plasma with a temperature of 1.2 × 10⁷ K and a luminosity of 1.8 × 10⁴¹ ergs s^-1. This emission appears to be asymmetric, with a small extension toward hotspot A, similar to a jet. An optical hotspot (extended emission-line region) is seen less than 1″ away from this extension in the direction of the core. This indicates that the extension may be caused by the jet interacting with an inner interstellar medium cloud, or that it is due to entrainment of hot gas. Future observations are discussed.