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The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes

Snios, Bradford; Nulsen, Paul E.J.; Wise, Michael W.; de Vries, Martijn; Birkinshaw, Mark; Worrall, Diana M.; Duffy, Ryan T.; Kraft, Ralph P.; McNamara, Brian R.; Carilli, Chris; Croston, Judith H.; Edge, Alastair C.; Godfrey, Leith E.H.; Hardcastle, Martin J.; Harris, Daniel E.; Laing, Robert A.; Mathews, William G.; McKean, John P.; Perley, Richard A.; Rafferty, David A.; Young, Andrew J.

The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes Thumbnail


Bradford Snios

Paul E.J. Nulsen

Michael W. Wise

Martijn de Vries

Mark Birkinshaw

Diana M. Worrall

Ryan T. Duffy

Ralph P. Kraft

Brian R. McNamara

Chris Carilli

Judith H. Croston

Leith E.H. Godfrey

Martin J. Hardcastle

Daniel E. Harris

Robert A. Laing

William G. Mathews

John P. McKean

Richard A. Perley

David A. Rafferty

Andrew J. Young


We use 2.0 Msec of Chandra observations to investigate the cocoon shocks of Cygnus A and some implications for its lobes and jet. Measured shock Mach numbers vary in the range 1.18–1.66 around the cocoon. We estimate a total outburst energy of $\simeq 4.7\times {10}^{60}\,\mathrm{erg}$, with an age of $\simeq 2\times {10}^{7}\,\mathrm{years}$. The average postshock pressure is found to be $8.6\pm 0.3\times {10}^{-10}\,\mathrm{erg}\,{\mathrm{cm}}^{-3}$, which agrees with the average pressure of the thin rim of compressed gas between the radio lobes and shocks, as determined from X-ray spectra. However, average rim pressures are found to be lower in the western lobe than in the eastern lobe by sime20%. Pressure estimates for hotspots A and D from synchrotron self-Compton models imply that each jet exerts a ram pressure gsim3 times its static pressure, consistent with the positions of the hotspots moving about on the cocoon shock over time. A steady, one-dimensional flow model is used to estimate jet properties, finding mildly relativistic flow speeds within the allowed parameter range. Models in which the jet carries a negligible flux of rest mass are consistent with the observed properties of the jets and hotspots. This favors the jets being light, implying that the kinetic power and momentum flux are carried primarily by the internal energy of the jet plasma rather than by its rest mass.


Snios, B., Nulsen, P. E., Wise, M. W., de Vries, M., Birkinshaw, M., Worrall, D. M., …Young, A. J. (2018). The Cocoon Shocks of Cygnus A: Pressures and Their Implications for the Jets and Lobes. Astrophysical Journal, 855(1), Article 71.

Journal Article Type Article
Acceptance Date Feb 9, 2018
Online Publication Date Mar 8, 2018
Publication Date Mar 8, 2018
Deposit Date Mar 29, 2018
Publicly Available Date Mar 29, 2018
Journal Astrophysical Journal
Print ISSN 0004-637X
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 855
Issue 1
Article Number 71
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Published Journal Article (2 Mb)

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© 2018. The American Astronomical Society. All rights reserved.

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