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
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.
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(Angstroms) |
(wavelength, A; transition initial-->final levels) |
(millions of K) |
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Fe XXI (14.0; 28-->7) |
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(continuum) |
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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.
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/λ = 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.
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.
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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.
|
(kev) |
(Angstroms) |
(wavelength, A; transition initial-->final levels) |
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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.
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(kev) |
(Angstroms) |
(wavelength, A; transition initial-->final levels) |
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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
