<p>The image is a little weird because it's a composite involving two radio images, both from around 1.5 GHz: a lower-resolution image taken by the Canadian Galactic Plane Survey (~1.42 GHz) and a higher-resolution one taken by the Very Large Array (~1.5 GHz), along with infrared imaging from <a href="https://www.jpl.nasa.gov/missions/infrared-astronomical-satellite-iras" rel="nofollow noreferrer">IRAS</a> (in blue). There's <a href="https://public.nrao.edu/news/cannonball-pulsar/" rel="nofollow noreferrer">a nice gif showing the difference</a>; the CGPS image is held static and the VLA image fades and then disappears:</p>
<p><a href="https://i.sstatic.net/EWXeg.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/EWXeg.gif" alt="CGPS/VLA image of PSR J0002+6216 and the nebula CTB 1, at about 1.5 GHz. The CGPS image shows the curved shell of the supernova remnant and background objects, while the VLA image shows part of the remnant and the trail left by the pulsar." /></a>
<br>
<sup>Image credit: Composite by Jayanne English, University of Manitoba; F. Schinzel et al.; NRAO/AUI/NSF; DRAO/Canadian Galactic Plane Survey; and NASA/IRAS.</sup></p>
<p>Let's talk about what this shows:</p>
<ul>
<li>The crescent-shaped arc shows the edge of the expanding supernova remnant as it collides with the interstellar medium. As the ejecta moves through the ISM, the resulting shock waves heat it up, leading to emission. Since the pulsar has been kicked at a significant speed, there's probably asymmetry to the remnant (although inhomogeneities in the ISM can also lead to asymmetry). The darkness in the center is likely not a shadow but rather the result of how an observer's lines of site pass through the nebula — similar to <a href="https://astronomy.stackexchange.com/q/43767/2153">the appearance of planetary nebulae</a>.</li>
<li>The line does indeed trace the path of the pulsar through space. As discussed in the paper from the VLA observations (<a href="https://iopscience.iop.org/article/10.3847/2041-8213/ab18f7" rel="nofollow noreferrer">Schinzel et al. 2019</a>), the shape and spectrum of that region of radio emission point to (non-thermal) <a href="http://abyss.uoregon.edu/%7Ejs/glossary/synchrotron_radiation.html" rel="nofollow noreferrer">synchrotron emission</a> from a bow shock. Basically, as the pulsar moves through the interstellar medium, <a href="https://en.wikipedia.org/wiki/Pulsar_wind_nebula" rel="nofollow noreferrer">its wind</a> of energetic particles gets compressed and shocked from ram pressure — again, the result of something moving very quickly through the ISM.</li>
<li>I'm not positive what the other blobs are. Some are certainly stars; because of the low resolution of the CGPS image, though, I can't say for sure that all of them are.</li>
<li>All of the above features show up in the radio images, in orange. Although it's not explicitly stated anywhere, I believe the blue is infrared IRAS data. Much of it also traces the shape of remnant, hidden behind the CGPS data, although the blobs at bottom left and upper right are likely unrelated features, probably something ISM-ish.</li>
</ul>
<p>The bottom line is that it's all really just hot stuff glowing.</p>

CC BY-SA 4.0
2022-10-03T16:10:33.833Z
2022-10-03T16:10:33.833Z
HDE 226868
<p>This beautiful image (from <a href="https://apod.nasa.gov/apod/ap221002.html" rel="noreferrer">APOD</a>) looks like the trail of the ejected pulsar is illuminating a ball of gas and dust. My eye sees patches of light and shadow, but sometimes images can be deceptive. What looks like a shadow could just be a region that's not glowing. So is it really a case of illumination and reflection (albeit in IR), or is it somehow deceptive? And is the trail really interstellar medium that has been heated and ionized by the passing pulsar? And are the fuzzy blobs just stars imaged in the radio spectrum?</p>
<p>The single question that I might distill out of this is, how do I interpret this image?</p>
<p><a href="https://i.sstatic.net/zLw8Y.png" rel="noreferrer"><img src="https://i.sstatic.net/zLw8Y.png" alt="Supernova fragment with pulsar trail" /></a></p>

<radio-astronomy><supernova><nebula><pulsar><infrared>
How to interpret illumination in "pulsar cannonball" image
Mark Foskey
# How to interpret illumination in "pulsar cannonball" image

(Asked by: username_0 on 2022-10-03)

This beautiful image (from [APOD](https://apod.nasa.gov/apod/ap221002.html)) looks like the trail of the ejected pulsar is illuminating a ball of gas and dust. My eye sees patches of light and shadow, but sometimes images can be deceptive. What looks like a shadow could just be a region that's not glowing. So is it really a case of illumination and reflection (albeit in IR), or is it somehow deceptive? And is the trail really interstellar medium that has been heated and ionized by the passing pulsar? And are the fuzzy blobs just stars imaged in the radio spectrum?

The single question that I might distill out of this is, how do I interpret this image?

[![Supernova fragment with pulsar trail](https://i.sstatic.net/zLw8Y.png)](https://i.sstatic.net/zLw8Y.png)

**Tags:** radio-astronomy, supernova, nebula, pulsar, infrared

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## 1 answers

### Answer by username_1 on 2022-10-03. Score: 2 | Accepted answer

The image is a little weird because it's a composite involving two radio images, both from around 1.5 GHz: a lower-resolution image taken by the Canadian Galactic Plane Survey (~1.42 GHz) and a higher-resolution one taken by the Very Large Array (~1.5 GHz), along with infrared imaging from [IRAS](https://www.jpl.nasa.gov/missions/infrared-astronomical-satellite-iras) (in blue). There's [a nice gif showing the difference](https://public.nrao.edu/news/cannonball-pulsar/); the CGPS image is held static and the VLA image fades and then disappears:

[![CGPS/VLA image of PSR J0002+6216 and the nebula CTB 1, at about 1.5 GHz. The CGPS image shows the curved shell of the supernova remnant and background objects, while the VLA image shows part of the remnant and the trail left by the pulsar.](https://i.sstatic.net/EWXeg.gif)](https://i.sstatic.net/EWXeg.gif)
  
Image credit: Composite by Jayanne English, University of Manitoba; F. Schinzel et al.; NRAO/AUI/NSF; DRAO/Canadian Galactic Plane Survey; and NASA/IRAS.

Let's talk about what this shows:

* The crescent-shaped arc shows the edge of the expanding supernova remnant as it collides with the interstellar medium. As the ejecta moves through the ISM, the resulting shock waves heat it up, leading to emission. Since the pulsar has been kicked at a significant speed, there's probably asymmetry to the remnant (although inhomogeneities in the ISM can also lead to asymmetry). The darkness in the center is likely not a shadow but rather the result of how an observer's lines of site pass through the nebula — similar to [the appearance of planetary nebulae](https://astronomy.stackexchange.com/q/43767/2153).
* The line does indeed trace the path of the pulsar through space. As discussed in the paper from the VLA observations ([Schinzel et al. 2019](https://iopscience.iop.org/article/10.3847/2041-8213/ab18f7)), the shape and spectrum of that region of radio emission point to (non-thermal) [synchrotron emission](http://abyss.uoregon.edu/%7Ejs/glossary/synchrotron_radiation.html) from a bow shock. Basically, as the pulsar moves through the interstellar medium, [its wind](https://en.wikipedia.org/wiki/Pulsar_wind_nebula) of energetic particles gets compressed and shocked from ram pressure — again, the result of something moving very quickly through the ISM.
* I'm not positive what the other blobs are. Some are certainly stars; because of the low resolution of the CGPS image, though, I can't say for sure that all of them are.
* All of the above features show up in the radio images, in orange. Although it's not explicitly stated anywhere, I believe the blue is infrared IRAS data. Much of it also traces the shape of remnant, hidden behind the CGPS data, although the blobs at bottom left and upper right are likely unrelated features, probably something ISM-ish.

The bottom line is that it's all really just hot stuff glowing.


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