<p><a href="https://i.sstatic.net/ZFbAn.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/ZFbAn.png" alt="Fig. 2: Image analysis of the final model" /></a></p>
<blockquote>
<p>Fig. 2: Image analysis of the final model: a, Model image corresponding to the image from Fig. 1 with a pixel size of 2 μas. Here, the tentative position of the jet apex is indicated with a circle. The size of the circle indicates the uncertainty in the apex location. b, Central brightness temperatures along the jet ridgelines from the model in four quadrants as a function of distance to the jet apex. The quadrants I, II, III and IV correspond to the similarly marked regions of the jet in a. Negative values for the distance to the jet apex are assigned for the counterjet region. Brightness temperatures of the fainter northwest (orange line) and brighter southeast (blue line) arms are shown in the top and bottom panels, respectively.</p>
</blockquote>
<p>From the body of the paper:</p>
<blockquote>
<p>The Cen A λ 1.3 mm jet exhibits three types of brightness asymmetry (<span class="math-container">$R$</span>): between the jet and counterjet, the sheath and spine, and the northwest versus southeast ridgelines (‘Brightness asymmetries’ in Methods). We take the two bright radiating streams of the approaching jet and counterjet as jet ‘arms’ and denote the maximum intensity region along each arm as ‘ridgeline’. The jet-to-counterjet intensity ratio <span class="math-container">$R_{j/cj}$</span>  can naturally be explained for a relativistic outflow with an inclination angle θ ≠ 90°, where jet emission will be Doppler boosted and counterjet emission de-boosted. We find <span class="math-container">$R_{j/cj}$</span>≳5, which is in agreement with centimetre-wave VLBI observations3 and suggests that the initial acceleration of the jet occurs within the inner collimation region imaged in this study.</p>
</blockquote>
<p>Above is from Nature Astronomy (Open Acces) <a href="https://www.nature.com/articles/s41550-021-01417-w" rel="nofollow noreferrer">Event Horizon Telescope observations of the jet launching and collimation in Centaurus A</a> found first in <a href="https://chat.stackexchange.com/transcript/message/58662963#58662963">The Observatory</a>.</p>
<p><strong>Question:</strong> Why does this Event Horizon Telescope image the Centaurus A radio jets show that both the brighter jet moving towards us and the dimmer (Doppler de-boosted) jet moving away from us have two arms? Are these two actually separate jet-like structures on each side, or an artifact of observation and processing?</p>
<hr />
<p>From Wikimedia Commons <a href="https://commons.wikimedia.org/wiki/File:RadioCenA-EN.png" rel="nofollow noreferrer">RadioCenA-EN.png</a>:</p>
<p><a href="https://i.sstatic.net/mW8EO.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/mW8EO.png" alt="Overview over the radio structure of Centaurus A.# First picture shows the overall radio emission of Centaurus A, extending over 2 million light years (assuming distance of 3.8 Mpc" /></a></p>
<blockquote>
<p>Overview over the radio structure of Centaurus A.# First picture shows the overall radio emission of Centaurus A, extending over 2 million light years (assuming distance of 3.8 Mpc</p>
</blockquote>
<blockquote>
<p>Radio structure of Centaurus A, redrawn with Powerpoint, exported to PNG. Basic Source: 'https://ned.ipac.caltech.edu/level5/March01/Israel/Israel2.html / Published in Astronomy and Astrophysics Review 1998, Vol. 8, pp. 237-278 Outer/Middle lobes: Parkes 5.0GHz Inner Lobes: VLA 4.9 GHz Jet: VLA 1.5 GHz Inner Jet: VLBI?</p>
</blockquote>

CC BY-SA 4.0
2021-07-22T10:47:49.750Z
The simple answer is that the two "arms" probably delineate the outer edges of the jet (the paper calls this the "sheath" of the jet). There is some discussion of this in the "Brightness asymmetries" section of the paper. The question then is a combination of "Why is the interior of the jet [called the 'spine' of the jet in the paper] so much fainter?" and "Why is the strongest sheath emission coming from the parts parallel to the line of sight?"; the answer seems to be that it's complicated and unclear..
Peter Erwin
CC BY-SA 4.0
2021-07-31T00:16:36.487Z
@PeterErwin yes, I've finally been able to steady myself sufficiently to read through the Methods section including the "Brightness asymmetries" subsection. I think that in this case your summary is a sufficient answer here. They suggest edge-brightening of a luminescent cylindrical tube along is insufficient, and they currently "put the presence of helical magnetic fields forward as the most likely, intrinsic explanation for edge-brightening" while noting some "more exotic scenarios... which might be tested through future observations". It's an open and shut case of "to be continued..." :-)
uhoh
<radio-astronomy><galactic-dynamics><active-galaxy><image-processing><astronomical-jet>
Why do the Centaurus A radio jets have two "arms" each? (Event Horizon Telescope image)
uhoh
# Why do the Centaurus A radio jets have two "arms" each? (Event Horizon Telescope image)

(Asked by: username_0 on 2021-07-22)

[![Fig. 2: Image analysis of the final model](https://i.sstatic.net/ZFbAn.png)](https://i.sstatic.net/ZFbAn.png)

> Fig. 2: Image analysis of the final model: a, Model image corresponding to the image from Fig. 1 with a pixel size of 2 μas. Here, the tentative position of the jet apex is indicated with a circle. The size of the circle indicates the uncertainty in the apex location. b, Central brightness temperatures along the jet ridgelines from the model in four quadrants as a function of distance to the jet apex. The quadrants I, II, III and IV correspond to the similarly marked regions of the jet in a. Negative values for the distance to the jet apex are assigned for the counterjet region. Brightness temperatures of the fainter northwest (orange line) and brighter southeast (blue line) arms are shown in the top and bottom panels, respectively.

From the body of the paper:

> The Cen A λ 1.3 mm jet exhibits three types of brightness asymmetry ($R$): between the jet and counterjet, the sheath and spine, and the northwest versus southeast ridgelines (‘Brightness asymmetries’ in Methods). We take the two bright radiating streams of the approaching jet and counterjet as jet ‘arms’ and denote the maximum intensity region along each arm as ‘ridgeline’. The jet-to-counterjet intensity ratio $R\_{j/cj}$ can naturally be explained for a relativistic outflow with an inclination angle θ ≠ 90°, where jet emission will be Doppler boosted and counterjet emission de-boosted. We find $R\_{j/cj}$≳5, which is in agreement with centimetre-wave VLBI observations3 and suggests that the initial acceleration of the jet occurs within the inner collimation region imaged in this study.

Above is from Nature Astronomy (Open Acces) [Event Horizon Telescope observations of the jet launching and collimation in Centaurus A](https://www.nature.com/articles/s41550-021-01417-w) found first in [The Observatory](https://chat.stackexchange.com/transcript/message/58662963#58662963).

**Question:** Why does this Event Horizon Telescope image the Centaurus A radio jets show that both the brighter jet moving towards us and the dimmer (Doppler de-boosted) jet moving away from us have two arms? Are these two actually separate jet-like structures on each side, or an artifact of observation and processing?

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From Wikimedia Commons [RadioCenA-EN.png](https://commons.wikimedia.org/wiki/File:RadioCenA-EN.png):

[![Overview over the radio structure of Centaurus A.# First picture shows the overall radio emission of Centaurus A, extending over 2 million light years (assuming distance of 3.8 Mpc](https://i.sstatic.net/mW8EO.png)](https://i.sstatic.net/mW8EO.png)

> Overview over the radio structure of Centaurus A.# First picture shows the overall radio emission of Centaurus A, extending over 2 million light years (assuming distance of 3.8 Mpc

> Radio structure of Centaurus A, redrawn with Powerpoint, exported to PNG. Basic Source: 'https://ned.ipac.caltech.edu/level5/March01/Israel/Israel2.html / Published in Astronomy and Astrophysics Review 1998, Vol. 8, pp. 237-278 Outer/Middle lobes: Parkes 5.0GHz Inner Lobes: VLA 4.9 GHz Jet: VLA 1.5 GHz Inner Jet: VLBI?

**Tags:** radio-astronomy, galactic-dynamics, active-galaxy, image-processing, astronomical-jet

> **Comments:**
> **username_1**: The simple answer is that the two "arms" probably delineate the outer edges of the jet (the paper calls this the "sheath" of the jet). There is some discussion of this in the "Brightness asymmetries" section of the paper. The question then is a combination of "Why is the interior of the jet [called the 'spine' of the jet in the paper] so much fainter?" and "Why is the strongest sheath emission coming from the parts parallel to the line of sight?"; the answer seems to be that it's complicated and unclear..  
> **username_0**: @PeterErwin yes, I've finally been able to steady myself sufficiently to read through the Methods section including the "Brightness asymmetries" subsection. I think that in this case your summary is a sufficient answer here. They suggest edge-brightening of a luminescent cylindrical tube along is insufficient, and they currently "put the presence of helical magnetic fields forward as the most likely, intrinsic explanation for edge-brightening" while noting some "more exotic scenarios... which might be tested through future observations". It's an open and shut case of "to be continued..." :-)  

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## 0 answers

