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Feb062012

A Slippery Supernova Remnant and its Compact Companion

 

by Sarah Scoles

Supernovae:
 
radiation-ridden, catastrophic events that form heavy elements and enrich space, in their destruction, with material that becomes part of new stars.
A supernova explosion ejects lots of energy and matter into space, producing a pretty picture for us to look at. But the supernova remnant doesn't look like a firework or a demon-mouth or a jellyfish right away: all that energy and material has to start at one spot (where the star was) and then expand outward. It takes time, and velocity. Luckily, time is good at moving relentlessly onward, and the supernova provides velocity by virtue of being, you know, a gigantic explosion.
All this "stuff" that is fast-and-furiously moving outward isn't just moving into a vacuum. There's other "stuff" out there in space. There are molecular clouds and stellar winds and temperature differences and alien motherships that affect how the supernova remnant moves differently through different parts of space.
After all, if all conditions were the same in all directions from the exploding star, all supernova remnants would be totally symmetric.
And that is just not true.  Source .
One SNR, G350.1–0.3, at first left astronomers wondering if it was, in fact, an SNR. G, as I'm going to call it from now on, did not have the characteristic shell structure that SNRs usually have, caused by interactions between the expanding material and gas it's running into. Some astronomers considered a shell a requirement of an SNR, and suggested G might be something else--an object outside our galaxy: a galaxy itself, or even a cluster of different galaxies. 
These are pictures of G at radio (a, top) and X-ray (b, bottom) wavelengths. The dashed line in (a) shows contour lines for X-ray data on top of the radio image. The dashed line in (b) shows a contour line for radio data on top of the X-ray image. The most interesting part, though, is the cross-hair in (a) that matches up with that little dot in (b). You'll have to keep reading to learn more about that (Gaensler, et al., 2008).
The first question astronomers asked was, "Hey, how fast is this stuff moving?" This question would be useful either way--whether G was a galactic supernova or an extragalactic galaxy.
By measuring the Doppler Effect, astronomers found that the wave of material from the supernova must be moving somewhere between 40 and 80 kilometers every second, away from us and with our own velocity subtracted. Things outside our galaxy move away from us faster than that, subject as they are to the expansion of the universe, and so G's place in our galaxy was secured.

 

 

In addition, though, to its wholesale movement relative to us, the gas zipping away from the origin of the explosion has some velocity relative to that point-of-explosion. Astronomers determined this velocity to be about 560 kilometers per second--more than 1,000,000 mph.
Since they knew how fast the gas was moving, and how wide the SNR was, they could wind the clock back and see how long ago the supernova happened.
How long ago did the supernova happen?
900 years. Duh.
But, wait--so we know this is a bright thing within our galaxy, made of gas moving really fast, and the event that we know produces objects with those characteristics is a supernova. But is that enough for us to say it's an SNR? If it's an SNR, why does this SNR have an unexpected shape?
The X-ray data suggest that the SNR is butting heads with dense, molecular gas. But this molecular cloud is believed to be only on one side of the SNR, which could cause its strangeness and asymettry.
When Gaensler, et al., looked at this area of the sky in data that was taken in order to find carbon monoxide emission, they found a cloud of carbon monoxide directly east of the SNR. It also had a velocity, relative to us, that matched the SNR's velocity relative to us, which means the cloud and the SNR are not just in the same spot 2-dimensionally (ie on our sky), but in actual 3-dimensional space. The astronomers are thus confident that they have found the cloud into which G is slamming.
But what about that cross-hair and its accompanying X-ray dot?
Well, when a star explodes as a supernova, it doesn't eject all its material. There's something left: a "central compact object" (or CCO), one that is typically as extreme and strange as the event that produced it, by virtue of having been compressed and having inherited things like magnetic field and angular momentum from its parent star.
So if you're talking about neutron stars, and saying that what's left of the original star is a neutron star, why are you calling it a "central compact object"? Are you being coy?
As you can see from the image above, radio waves were not detected from the CCO in this observation (which is why there's a so-not-astronomical-so-Photoshopped cross at its location), and no one has ever seen it in optical or infrared light, either. However, it looks pretty bright as an X-ray source!
This is also Photoshopped. Source.
We cannot, however, "resolve" it, or make a map of it. It is a point source, which means it's too small for our telescopes to see it as anything but smaller than their pixel size.
The point is, you can't just call something a neutron star because you think it's a neutron star because you think neutron stars are what should be in the middle of SNRs. That's not science. That's just making stuff up. You have to be able to observe effects of the neutron star's gravity, to prove it has sufficient mass/density, or you have to see pulsing radiation (observe it as a pulsar).
The problem with this CCO is that it's not in the middle of the SNR. And if the CCO is whence all this hot gas came, and from which it's fleeing, the CCO (it stands to reason) should be right at the center. Thus the first "C" in "CCO."
G, however, has more like an OCCO, which is a term I just made up for an off-center compact object, and which is much more pronouncable than CCO, anyway.
That means that the supernova gave the CCO a "kick" that sent it flying through space (at about 4,400,000 mph, or more than four times as fast as the SNR is expanding).
What kicked it so hard?
That's a matter up to great debate.
So how do we find out what kind of object this CCO is?
We try to see it in other wavelengths--radio, optical, infrared--by looking at it longer and harder. We try to look for pulsations, doing quickly sampled observations in those wavelengths, and in X-ray wavelengths.

 

To understand this object--and, by extension, all celestial objects--we need to combine forces. We need to think about things in space in terms of their total light output, and how they interact with their whole environment, even if we have to dig into other people's archival data to do that. After all, if I wanted to understand who you were and why you move so fast, I wouldn't just stare at you with night-vision goggles, would I?


ResearchBlogging.orgGaensler, B., Tanna, A., Slane, P., Brogan, C., Gelfand, J., McClure-Griffiths, N., Camilo, F., Ng, C., & Miller, J. (2008). The (Re-)Discovery of G350.1-0.3: A Young, Luminous Supernova Remnant and Its Neutron Star The Astrophysical Journal, 680 (1) DOI: 10.1086/589650

 

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