title: A kiloparsec-scale internal shock collision in the jet of a nearby radio galaxy.
abstract: Jets of highly energized plasma with relativistic velocities are associated with black holes ranging in mass from a few times that of the Sun to the billion-solar-mass black holes at the centres of galaxies. A popular but unconfirmed hypothesis to explain how the plasma is energized is the 'internal shock model', in which the relativistic flow is unsteady. Faster components in the jet catch up to and collide with slower ones, leading to internal shocks that accelerate particles and generate magnetic fields. This mechanism can explain the variable, high-energy emission from a diverse set of objects, with the best indirect evidence being the unseen fast relativistic flow inferred to energize slower components in X-ray binary jets. Mapping of the kinematic profiles in resolved jets has revealed precessing and helical patterns in X-ray binaries, apparent superluminal motions, and the ejection of knots (bright components) from standing shocks in the jets of active galaxies. Observations revealing the structure and evolution of an internal shock in action have, however, remained elusive, hindering measurement of the physical parameters and ultimate efficiency of the mechanism. Here we report observations of a collision between two knots in the jet of nearby radio galaxy 3C 264. A bright knot with an apparent speed of (7.0 ± 0.8)c, where c is the speed of light in a vacuum, is in the incipient stages of a collision with a slower-moving knot of speed (1.8 ± 0.5)c just downstream, resulting in brightening of both knots--as seen in the most recent epoch of imaging.
authors.affiliation: 1] Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA [2] University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA.
authors.forename: Eileen T
authors.initials: ET
authors.lastname: Meyer
authors.affiliation: 1] University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA [2] NASA Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, Maryland 20771, USA.
authors.forename: Markos
authors.initials: M
authors.lastname: Georganopoulos
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
authors.forename: William B
authors.initials: WB
authors.lastname: Sparks
authors.affiliation: Florida Institute of Technology, 150 West University Boulevard, Melbourne, Florida 32901, USA.
authors.forename: Eric
authors.initials: E
authors.lastname: Perlman
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
authors.forename: Roeland P
authors.initials: RP
authors.lastname: van der Marel
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
authors.forename: Jay
authors.initials: J
authors.lastname: Anderson
authors.affiliation: Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Sangmo Tony
authors.initials: ST
authors.lastname: Sohn
authors.affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA.
authors.forename: John
authors.initials: J
authors.lastname: Biretta
authors.affiliation: 1] Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA [2] Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Colin
authors.initials: C
authors.lastname: Norman
authors.affiliation: 1] Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, Maryland 21218, USA [2] Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA [3] Istituto Nazionale Astrofisica, Istituto di Radio Astronomia, Via Piero Gobetti 101, I-40129 Bologna, Italy.
authors.forename: Marco
authors.initials: M
authors.lastname: Chiaberge