Chandra X-ray data exercise 2

• Finish your work on Tycho
• Get to know the G292.0+1.8 supernova remnant
• Emission lines from G292
• Comparing the remnants: Tycho vs. G292
• Two types of supernova
• Which type is each remnant?
• Extra credit

Today is the second of a two-part series of exercises in which you will analyze some X-ray data taken by the Chandra satellite. You can find the list of observations from which our targets will be taken at

• Chandra-Ed Observations at rutgers.edu
You can find a local copy of the FITS image we'll be examining today at
• G292.0+1.8 SNR, obsid 126
but just loading this into ds9 won't load the Chandra-Ed analysis tools.

We'll follow many of the steps and instructions given in X-ray Spectroscopy of Supernova Remnants - ds9 Version, which is a resource listed on Chandra X-ray Center's "Investigating Supernova Remnants" page.

Finish your analysis of the Tycho SNR

In other words, complete all the tasks listed on the previous day's webpage.

Get to know the G292.0+1.8 supernova remnant

Our second science target is the supernova remnant known as G292.0+1.8. Let's begin with a bit of an introduction: hello, you lovely X-ray dataset!

Part 6:

1. What does the name "G292.0+1.8" mean? (Hint: look up the name "SNR G292.0+1.8" on SIMBAD. , and read the page carefully)
2. What is the position of this object in (RA, Dec)?
3. Is there any optical emission at the location of this object? Explain your answer.
4. How long is the Chandra exposure of this object? How does that compare to the exposure time of Tycho?

Emission lines from G292

What sort of material makes up this supernova remnant? One way to find out is to look at its strong X-ray emission lines. You should already know how to do this, so ...

Part 7:

1. What are the 4-5 strongest emission lines from this supernova remnant? Make a table listing energy and element
2. Are there significant variations in the emission lines from place to place in the remnant? If so, describe them briefly.

Now, there is an important caveat to using these emission lines as markers of chemical composition. It's true that seeing a strong emission line associated with some element DOES mean that atoms of that element must be present in the gas; and, for strong emission lines, it probably means that there are lots of such atoms.

However, the absence of strong emission lines does NOT necessarily mean that an element is "missing" from the object. In order to produce emission lines due to a certain element, several criteria must be met:

• atoms of the element must be present, AND
• conditions in the object must be just right to excite many of these atoms into the proper energy level, from which they can radiatively de-excite by emitting a photon, AND
• no other process (such as collision) can de-excite the atoms more quickly

So, it is possible for the spectrum of an object to be dominated by lines of element X, even if X is not the most common element in the object. For example, look at this figure from "A Digital Spectral Classification Atlas" by R. O. Gray, showing spectra of main sequence stars in classes G0 through K5.

Figure 20 taken from "A Digital Spectral Classification Atlas" by R. O. Gray.

3. Use your basic astronomical knowledge to answer this: what are the two most common elements in the atmosphere of ordinary stars?
4. Look at the spectra in the figure above to answer this: what are the elements responsible for most of the lines visible in these spectra?
5. Do the most common elements in stars always produce the strongest lines?
6. (optional) Do you have any idea why a certain element's absorption lines might be completely missing from these spectra?

Comparing the remnants: Tycho vs. G292

Today, you've found some evidence for the chemical compostion of SNR G292.0+1.8. Last time, you investigated Tycho's SNR. It's time to compare them:

Part 8:

1. What are the 4-5 strongest emission lines from each remnant?
2. Would you call the two remnants very similar chemically, or rather different?

Two types of supernova

Supernovae come in many varieties, but there is one big division based on the progenitor of the explosion.

Core-collapse supernovae

begin as very massive stars, at least 8-10 times the mass of the Sun. As they age, they build up a series of layers of successively heavier elements around their cores:



When they run out of fuel in the core, the core collapses; the innermost core shrinks into a proto-neutron star, while the outer core falls onto this proto-neutron star and smashes into it. The collision triggers a complex set of nuclear reactions, which create a burst of neutrinos and sends a shock wave flying outward toward the surface. This shock wave heats and accelerates the intermediate and outer layers of the star, creating a hot, bright, expanding ball of gas.

These are sometimes called supernovae of types Ib, Ic, II, IIb, IIP, IIL, and a few other names. Their ejecta contain a mix of light and heavy elements; an example from one model of a very massive star is shown below.


Taken from Figure 8 of Ohkubo et al., ApJ 645, 1352 (2006)

Thermonuclear supernovae

start with white dwarf stars in close binary systems. If a single white dwarf accretes matter from a main-sequence companion, or if two white dwarves spiral into each other and merge, so that the resulting object exceeds the Chandrasekhar limit (about 1.4 solar masses), then the densities and temperatures in the object may reach such high values that a runaway thermonuclear reaction begins. (Almost) all the material in the object runs through a series of fusion reactions up to the iron-group elements; the resulting release of energy blows the entire object apart, creating a giant ball of hot, expanding gas.

These are sometimes called "type Ia" supernovae.

These nuclear reactions produce a somewhat different mix of elements in the ejecta of the explosion. Note that most of the nickel ("Ni") produced in these reactions is a radioactive isotope, Ni⁵⁶, which decays in a matter of weeks to iron (Fe).


Figure 8 of Fink, Hillebrandt and Ropke, A&A 476, 1133 (2007)

Which type is each remnant?

So, can you use your measurements of X-ray emission lines to assign a type to each of the two supernova remnants?

Part 9:

1. Tycho's supernova was ______________. Justify your answer in detail.
2. G292.0+1.8 was created by a supernova of type ______________. Justify your answer in detail.

Now, check to see if you are correct. Find (at least) one published paper which describes each event and assigns a type to it. Did the authors agree with you?

3. Tycho reference: ___________
4. G292.0+1.8 reference: ___________

Extra credit

The SN remnant G292.0+1.8 was observed a second time with Chandra some years later. You can read more about it at

• The Chandra press release on G292.0+1.8 (Oct 23, 2007)

Extra:

1. What is the Chandra Observation ID for this second, deeper study of G292.0+1.8?
2. What is the exposure time for this dataset? How does it compare to the exposure time for the earlier dataset?
3. Display this new dataset in one frame of ds9, and display the earlier one in a different frame. Do you notice any obvious difference in appearance? Describe, and, if you do, explain.
4. A pulsar formed in this explosion is said to be visible in this object; or, at least, a small nebula around the pulsar is said to be visible, if one looks at the X-ray photons with the highest energy. Can you find it? Try one or both of the following:
   • you could start with either dataset and display only photons with high energy values
   • you could search through the files associated with the long-exposure dataset for one which has a name like acisf06677N002_evt2.fits.gz and examine it in the ordinary way
Write down the approximate (RA, Dec) of the pulsar.

For more information

• Investigating Supernova Remnants has a number of instructional resources to help you start analyzing Chandra data
• SAOImage ds9 is a software package for displaying and interacting with astronomical data.
• If you want to know more about the SNR G292.0+1.8, you might read some of these papers:
  • Ghavamian and Williams, ApJ 831, 188 (2016)

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.