HETG/E0102/Cal - Pollock-Shock
Back to E0102 for Calibration
Discrete lines (Pollock) and Shock models - 15 October 2006
The E0102 spectrum as observed with the Chandra
HETG (Flanagan et al. 2004) and the XMM-Newton RGS (Rasmussen et al. 2001)
clearly show the bright lines from H-like and He-like ions of
elements O, Ne, Mg, Si and H-like C. Some hints of Fe ion lines
were indicated in the RGS spectrum.
It should be noted that neither of these papers undertook to
create a model to fit the complete spectrum of E0102.
For calibration purposes a continuum-plus-lines model
has been created by Andy Pollock based on re-analysis of RGS
data; that model expressed in XSPEC format (.xcm) and converted to
format (which uses the XSPEC model library) as well
(.par) is given here in these files:
An early approximation to the global E0102 spectrum was
made using wabs times a simple shock model - the model and
it's parameters are given by the ISIS .par file:
Because it is
sometimes tricky for fitting algorithms to minimize complex
(and table-lookup) models like vnpshock, the temp and tau
parameters were randomly selected and then the norm and
abundances fit. After thousands of instances the
best overall temp-taus were then selected. Fe was then added
and other elements adjusted.
Plots of the Pollock model and the simple shock model (calculated
using the XSPEC NEIVERS of 1.1) are given here:
- e0102_pollock_shock_ICWG .pdf
- The three plots on this page are made in roughly similar format to the
plots on pages 6,7,8 in Paul Plucinky's (2nd) presentation to the
"ICWG" in June 2006, "E0102 as a Standard Candle for X-Ray Astronomy".
Specifically, the Pollock model is shown in Red and the Shock model in Green.
To emphasise where Fe lines are in the shock model, the shock model
without Fe included is shown as the Blue line. The y-axis shows
counts per bin and the same ARF and RMF have been applied to both models.
(For reference the ARF and RMF are appropriate to the LETG as it was
during an SNR 1987A observation. This has the property of good low-E response
and a resolution that is not too good but not too blurry either - similar
to the velocity blur Pollock's model includes...)
- e0102_pollock_shock_hires .pdf
The three plots on this page are shown at the model resolution - narrow lines.
The Pollock model (Red) has its lines 0.15 times narrower than the xcm model
specifies whereas the Shock model (Blue) has bin-narrow lines - this helps to
distinquish them. The Grey lines are from the Fe in the shock model and
clearly show the "forest" these ions produce.
The middle panel is zoomed in a bit and plotted in wavelength.
The bottom panel shows the low-Energy end in the same color coding as
above - note the Blue (from Silicon, I believe) lines in the shock model
at ~ 0.37 and 0.40 keV.
In addition, this bottom panel includes, for reference, Carbon (orange) and Nitrogen
Comparing the two models
"Why compare with some random shock model?"
The idea is that this shock model is based on a range of temp and
ionization times and so gives a good idea of all the lines that might
be present - it can also be recomputed using improved line databasess
and shock model physics. Comparing the shock model with the Pollock
model then gives us some guidance for what to look for in the real
data, e.g., to decide if Fe is present, etc. That will be the subject
of further work.
There are similarities and some clear
differences between these two models:
- There is clear agreement on most of the bright lines and
their locations; the
flux agreement is good as well (given that the simple shock was meant
to be an approximation at the 20% level and has only 9 adjusted parameters
in addition to the norm and N_H.)
- Looking at the 0.5-0.8 keV and 1.4-1.5 keV ranges,
the continuum level for the two models is very similar, including a
similar edge at ~ 0.53 keV.
- Fe emission/lines are (claimed to be) not explicitly included
in the Pollock model,
however comparing these spectra it is clear that the main
two places where Fe lines show up in the shock model is also
where there are "extra" lines in the Pollock model.
So, the Pollock model
has the degrees of freedom to "cover" for the Fe emission lines.
the regions: 0.69 to 0.75 keV and 0.80 to 0.89 keV contain
5 lines each in the Pollock model (red), whereas in the shock model
these same regions show 4 and 7 peaks (green) --- of these, 3 and 4
peaks, respectively, are due to Fe emission (not seen in blue).
- The presence of Fe in the shock model also shows
up as a peak-like bit of extra flux at ~ 0.965 keV
and an enhanced "continuum level" in the 1.08 to 1.3 keV range.
It is interesting that the Pollock model continuum in the 0.9 to
1.3 keV range (red) is at about the level of this "pseudo-continuum" due
to the many (blurred) Fe lines (green):
In the Pollock model the first
redge component at 0.87 keV adds the equivalent of the Fe
pseudo-continuum; this edge does show up at a lower level in the shock model,
and is seen more clearly in the high-resolution plots:
Other, easily corrected, differences:
- The Shock model does not have a line at ~ 1.15 keV that is in the
Pollock model - this looks to be the Ne IX "Lyman delta" 5-1 transition and
probably should be in the shock model
(perhaps the line is not in the XSPEC NEIVERS=1.1 line list? )
- The Pollock model probably needs to add lines that show up
at ~ 1.58 and 1.65 keV (from 3-1, 4-1 transitions of Mg XI/XII?)
- The N_H(in 10^22) value used in the wabs component is different for the
two models: 0.0536 for Pollock and 0.08 for the simple shock.
For both models, however, N_H is likely degenerate with other parameters --- the
agreement of the resulting continuum level, e.g., around 0.55 keV, is
more relevant to calibration issues.
- The models diverge at the energy extremes: below 0.45 keV and above 1.5 keV.
This is not surprising since most of the data-overlap which constrains the models
is in the 0.5 to 1.5 keV range. Note that at higher energies, the Pollock
model only has a brems and redge components with kT = 0.36 keV,
whereas the shock model has a range of kT's up to 0.6 keV ---
better at including
the Hughes et al. (2000, Table 2) estimated blastwave
NEI-kT of ~ 0.48 keV and the Flanagan et al. (2004, Table 6)
Ne emission temp of kT ~ 0.58 keV.
Velocity broadening of the lines:
- The Pollock model includes line width values, sigmas, to take
into account the high velocities that are present in E0102. This
aspect of the spectrum is not part of the simple shock model
given here and is better examined using a spatial-spectral model
of E0102 that takes into account the various components (ejecta,
blastwave) and their motions. Such a model, similar to the
E0102_Jun05 modeling, will be used in future work
in this series.