the x-ray line spectra of supernova remnants

written by John Kolena
august 2003, revised july 2015

Cas A

The spectra of the Supernova Remnant (SNR) Cas A below was produced by selecting  Quick Energy Spectrum  under Analysis for a region that contained virtually the entire supernova.
The first spectrum explores the energy range 1000 ev < E  < 10,000 ev.; the second spectrum, 200 ev < E < 1000 ev

On the vertical axis is plotted the log (number of counts/sec); on the horizontal axis is plotted the photon energy (in ev).  The above images are screen-shots of the actual spectrum made using Quick Energy Spectrum.

Although the bulk of the spectrum is continuous, a number of emission lines clearly stand out.  The major ones are listed in the table below.

The spectrum at energies less than 1 kev is not very strong in emission lines.  0102-72.3, a supernova remnant in the Small Magellanic Cloud, has a much richer spectrum in this energy region.
Cas A emission lines & continuum
(estimated) energy1
(estimated) wavelength 2
line identifications3
(wavelength, A; transition initial-->final levels)
gas temperature4
(millions of K)
Fe XVI (66.7;)
S XI (39.2; 39-->1)
S XII (34.5; 36-->1)
Ca XI (30.5; 27-->1)
Ca XI (25.4; 34-->1)
S XIV (24.3; 13-->3)
Ar XVI (23.5; 5/6-->1)
O VII (22.1; 2-->1)
Fe XVII (17.1; 2-->1)

Fe XVII (16.8; 5-->1)

 Fe XVIII (15.6; 9-->1)
Fe XVIII (14.5; 41-->1)
Ni XIX (14.1/14.0; 2/3-->1);
Fe XXI (14.0; 28-->7)

Ne X (12.1; 3,4-->1)  +  many FeXIX,XX,XXI
Mg XI (9.31-9.22; 2,5,6-->1)
Mg XII  (8.42; 3,4-->1)
Si XIII  (6.74; 2-->1)
Si XIV  (6.18; 4-->1; 6.19; 3-->1)
Si XIII (5.68; 13-->1)
S XV (5.10; 2-->1)
4.34    very broad
S XV (4.30; 13-->1)
4.13 very weak
S XV (4.09; 23-->1) ?
Ar XVII  (3.99; 2-->1);  S XVI (3.99; 6,7-->1)
8.2; 10
3.23   very broad
Ca XIX (3.21; 2-->1)
6.4 - 6.8
1.94 - 1.82   extremely broad
FeXXV (1.85-1.87; 2,5,6,7-->1) K shell
4.0 - 6.0 
3.10 - 2.07

1  energies estimated from spectral plot

2   wavelength calculated from   λ  =  h c/(energy)  =  12400. ev-A/(energy)  =   1240. ev-nm/(energy) 

3   notation: Ne X means Neon ionized 9 times, i.e., Neon with 9 electrons removed.

As a neutral neon atom has 10 electrons, this means that only 1 neon electron remains in Ne X and is responsible for producing the emission line in dropping from level 3 or 4 to level 1. 

Line identifications can be done interactively at ATOMDB version 1.3.1; alternatively here is a printable table of the strongest x-ray lines from 0.2 kev through 10.0 kev.  In selecting the most likely candidates for line identification, preference was given to lines with high emissivity (generally greater than 10-17).  These are generally also lives arising from excited levels near the ground state to the ground state.

The line identifications match those published in the Chandra Ed activity for Cas A, although those listed in the activity do not include the ionization stage.

4 temperature estimated from the emission lines

Temperature = 103 K/ev (energy required to produce the level of ionization/excitation observed);

Ionization energies per electron removed available here.  Each entry in the table is the amount of energy required to remove 1 electron.  To find the total energy required to remove, for example,  9 neon electrons,  the first 9 ionization energies in the neon row must be summed.
For example, to produce Ne X requires 21.6 ev (to remove the first electron) + 41.0 ev (to remove just the second electron) +  ...  + 1196 ev (to remove the 9th electron) = 2150 ev.  It also requires 1000 ev to excite the 10th electron to level 3/4 from which it emits the x-ray photon in dropping to the ground state.   Therefore, the total energy required to produce a Ne X electron in the excited state is roughly 3200 ev, equivalent to a temperature of 3.2 million degrees Kelvin.

The physics behind the formula    T = 103 K/ev (energy):

Maxwell's kinetic theory established a linear relation between temperature and the averge kinetic energy per particle for an ideal gas (E = 3/2 k T, where K = Boltzmann's constant).   [For blackbody radiation, the predominant source of energy in stars, there is a similar linear relationship between the average photon energy and the temperature of the ionized gas.]  The average energy per particle or energy per photon is also the average energy available for ionization and excitation of the atomic species present in the gas since energy is equally distributed between the various types of energy.  The effective proportionality constant between energy and temperature can be determined by looking at some specific astronomical examples:

a) in stars (blackbodies), the dark (absorption) lines of hydrogen in the optical part of the spectrum (the so-called Balmer series) are strongest in spectral class A stars, stars of (surface temperature) 10,000 K.  In order for a hydrogen electron to absorb visible light, the electron must be in the second energy level of the hydrogen atom, which requires an excitation of 10 ev.  It therefore appears that an energy of 10 ev is most common at temperatures of 10,000 K, thereby leading to the idea that the constant of proportionality between Temperature and energy is 103 Kelvin/ev .   In cooler stars, such as M stars, where the temperature is 3000 K, the average energy available for excitation or ionization is therefore onlyh 3 ev, insufficient to excite the electrons to the second level where they could absorb visible light.  Therefore, the dark lines of hydrogen in the optical part of the spectrum are absent or very very weak in M stars.  In the O stars, where the temperature is 30,000 K or higher, the averge energy available for excitation or ionization is 30 ev or higher, which is sufficient to ionize the hydrogen (this requires an energy of only 13.6 ev).  Therefore, the dark lines of hydrogen in the optical part of the spectrum are very weak in O stars.

b) in the solar chromosphere/corona (a non-blackbody gas), typical emission lines include include those of O V @ 630 A (2.5 x 105 K), Mg IX @ 368 A (106 K), and Fe XVI @ 331/354 A (2 x 106 K), where the temperatures given in () are estimates from the Encyclopedia of Astronomy and Astrophysics.  Using the ionization tables, the energies required to remove 4, 8, and 15 electrons, respectively, from O, Mg, and Fe and excite the emission line observed are 2 x 105 K, 1.1 x 106 K, and 2.9 x 106 K, respectively.

5 temperature determined from the continuous part of the spectrum

in general, there are 3 mechanisms that could contribute to a continuous spectrum at x-ray wavelengths:

a) blackbody (thermal) radiation (BBR)

BBR results from an opaque plasma (an ionized gas); BBR has a complicated energy dependence,

    flux   E3 (eE/kT - 1)-1

BBR is unlikely to be a contributing source to the SNR continuum since the SNR is transparent and not opaque

b) synchrotron radiation (SR)

This type of radiation results from electrons accelerated in a magnetic field that emit radiation as they move; SR has a power-law spectrum,

    flux   E-n

where E is the photon energy and n is a constant called the spectral index...
For this type of energy source, a plot of log flux vs log energy would be linear with slope  -n

SR could contribute to the continuous spectrum of SNRs if a pulsar is present in the SNR.  However, notice that the spectrum for Cas A is the wrong shape (log flux is not linear with log energy, but is linearly proportional to energy instead).

c) thermal bremsstrahlung (TB)

This type of radiation results from photons interacting with individual charged particles; TB also has a complicated energy dependence,

    flux   (kT)-1/2 e-E/kT

TB is the most likely source of the continuous spectrum in SNRs.  For TB, log flux should be linearly proportional to the energy (with negative slope), as it is in the Cas A spectrum.  The temperature can be determined from the slope of the log-flux vs E continuum.

Using the slope of the continuous spectrum between 4000 and 6000 ev gives a temperature of approximately 12 million degrees K.

Do the estimated line energies include sufficient information for a calculation of the approach/recession
speed of the remnant as a whole?

Curiously, my estimated wavelengths are all slightly longer than the equivalent wavelengths listed in the database.
Averaging the  Δλ/λ   for each of the lines gives (after throwing out the highest & lowest)   Δλ  =  0.0086  = v/c;
this is equivalent to a recession speed of v = 2700 km/s.  This seems unreasonably high (it is larger than the escape speed from the galaxy); perhaps there is some systematic effect causing the difference between the measured wavelengths and the database wavelengths?

I have seen no published information about the motion of the remnant.

Are the line widths determined by the expansion speed of the remnant?

The lines at 1.83 and 2.41 kev were used to estimate the half width at half maximum (HWHM) intensity = 100 kev.
  Δλ/λ  »   ΔE/E  =  100 kev/2000 kev  = 0.05
This would be equivalent to an expansion speed of 15,000 km/s, which is a bit high, but not totally unreasonable.

However, we need to compare this line width to the spectral energy resolution of the Chandra detectors to make sure that it is less than 100 kev.  The spectral resolution is  (coming soon)

how to make 3-color images of x-ray supernova remnants
(where the colors represent different element abundances/excitations)

There are probably many ways to do this; this method is the one that I figured out first.  It uses ds9 to create jpg images of energy cuts (each energy cut representing a particular element emission line).  Each energy cut is saved in a different color; I happened to use RBG colors, but others are available in ds9..  Then I used Photoshop Elements to combine the 3 images.  I'll lead you through an example of 3-color addition, then you're on your own.

1) Open Cas A via Virtual Observatory.  Do a Quick Energy Cut (under  Analysis, Chandra Ed Analysis Tools) for the energy range
6.4 - 6.8 kev which is where a strong line of Fe XXV resides.  (See the Cas A spectrum above.)  The image returned shows only photons in this particular range of  energy.  Change the color to, say, green, and then save this image as a separately-named jpg image.  However, be sure to delete the frame with the original Cas A image, or else that will also be saved as part of the jpg image.  Here's how the Fe XXV energy cut should look.

2) Delete this frame, go back to Virtual Observatory, and reload the original Cas A image.  Do another Quick Energy Cut, this time for the range of energy  1.65 - 1.95 kev, where a strong line of Si XIII exists.  Color this image blue, and save as another jpg image.
(Again, be sure to delete the original Cas A image frame before saving.)  Here's what my energy cut looked.

3) Finally, delete this frame, go back again to Virtual Observatory, and reload the original Cas A image. 
Do a third Quick Energy Cut, this time for the
energy range 2.3 - 2.5 kev, where a strong line of S XV exists. 
Color this image red, and save as another jpg image. 
(Again, delete the original Cas A image frame before saving.)  Here's how the final energy cut looked.

4) Now we combine the images. 
Open the Subaru image processor.  (it's free for educational use)
Open all three of the previously saved jpg images. 
Under Image, select Batch Processing
Make sure that the only the three jpg images you want are in the list.  I used Add under
Composite Method.  You can change the contrast and brightness in the usual ways (max, min, log, etc.)
Because you obtained each jpg for the same Cas A image, there is no need to align the images. 

Here is my best attempt at a combined image.

5) Notes.  The Si and the S seem to overlap in position, but the Fe seems to be missing from the blowout region at the upper left, at least compared to the Si and S.

6) Suggestions.  Another thing to try is use three images all from the same element, but in 3 different ionization stages; for example, do energy cuts at the energies corresponding to lines of Si XII, Si XIII, and Si XIV.

the spectrum of 0102-72.3 (a SNR in the SMC)

This spectrum shows the inner part of the SNR (composed of mostly supernova-processed material).  It is particularly rich in lines at energies less than 1 kev.

0102-72.3 emission line identifications
line energy (kev)
line wavelength (A)
line identification
N VII (20.9; 6/7-->1) 
N VII (19.8; 11/12 --> 1)
O VIII (19.0; 3/4-->1)
Fe XVIII (16.1; 4/5-->1) O VIII (16.0; 6/7-->1)
Fe XVIII (15.6; 9-->1)
Fe XVII (15.0; 27-->1)
Fe XVIII (14.4; 41/49/56-->1)
.91 - .96
13.6 - 12.9
Ne IX (13.4; 7-->1)
 Fe XXI (12.3; 40-->1) Fe XXII (12.3; 59-->1)
Ne X (12.1; 3/4-->1)

Kepler's supernova

This spectrum was produced from only a portion of the supernova remnant.

The spectrum of Kepler's SNR is a lot messier than that of Cas A because the count rate is about 10x lower.  However, at least four of the lines present in Cas A's spectrum also appear in the spectrum of Kepler's SNR.
Kepler SNR emission lines
(estimated) energy
(estimated) wavelength 
possible line identifications
(wavelength, A; transition initial-->final levels)
Mg XI (9.31-9.22; 2,5,6-->1)
Si XIII  (6.74; 2-->1)
S XV (5.10; 2-->1)
4.34  very broad
S XV (4.30; 13-->1)
Ar XVII  (3.99; 2-->1);
S XVI (3.99; 6,7-->1)

supernova 1987A

This image was produced from the entire supernova remnant, obviously a very young, but distant supernova.
Again, notice the much lower count rate for this snr, imaged on May 16, 2002.  (An earlier image, in 2000, showed virtually nothing.)

The Mg, Si, and S emission lines that appear in the two previous supernovas also appear in SN 1987A.
SN 1987A emission lines
(estimated) energy
(estimated) wavelength 
possible line identifications
(wavelength, A; 
transition initial-->final levels)
O VII (18.6; 13-->1)
Ni XIX (14.2; 2/3-->1)
Mg XI (9.31-9.22; 2,5,6-->1)
Si XIII  (6.74; 2-->1)
S XV (5.10; 2-->1)

supernova and supernova remnant pages

    supernova taxonomy (flow chart) by Montes
    kolena's supernova taxonomy

    Chandra catalog of x-ray SNRs in the Milky Way
    my catalog of Chandra-observed galactic SNR catalog

    Green's catalog of radio SNRs  2006 version [galactic coordinates, RA/Dec, angular size, type (Shell, Filled, Composite), 1-GHz flux, spectral index]
          documentation to Green's catalog
          detailed refs on SNR in Green's catalog

    SNRs in the LMC/SMC
    Chandra catalog of SNRs in the LMC/SMC
    List of x-ray supernovae  (NOT SNRs)

    the latest supernova discoveries
    spectra and more of recent supernova spectra

    supernova collapse/explosion simulation movies from LANL

    an exhaustive, but old, list of supernova and supernova remnant pages
    another exhaustive, but old, list of supernova and supernova remnant pages