I have started to explore calibration issues for ASTRO-E. At the same time, I am working on ASCA calibration issues. I have explored the effect of using the latest best estimate for the gold optical constants (derived from AXAF calibration). I have imported these constants into the raytracing code made by Nagoya University (adspf.f). I have also explored the effects of a contamination layer as done by the AXAF people. I find that both of these types of effects result in substantially different telescope areas as compared to the "official" ASCA response. In particular, both the change in the optical constants and the addition of a contamination layer individually give some of the residuals seen in ASCA data. Additionally, for ASTRO-E, this study suggests that we may purposely want to put down a layer of carbon to enhance the low energy area and wash out the irridium edges.
This is an "unoffical" page. Implementation of these effects in the offical response would require the attention of Nagoya University.
On inspection of the raytracing code used by the XRT team to produce the ASCA response, I discovered that the optical constants are mostly derived from 1982 Henke results. Dotani-san reports to me that:
AXAF mirror calibration suggests that the Henke 1982 values are not accurate. See below...
What is the effect of this?
I have taken the Nagoya raytrace code adspc.f and modified the reflectivity tables to include the latest Henke data. The Latest Henke data can be found at Henke Stuff . It includes the AXAF calibration results on the optical constants of gold as reported by Nelson et al (1994) in SPIE Volume 2280, Page 191. The above link to the Henke page also has many useful HTML pages to compute things such as reflectivities off arbitrary materials including multilayers...check it out. I used it to verify my reflectivity code. It turns out that Peter Serlimitsos also uses these newer Henke values as well.
When computing the reflectivity, one must use gold densities smaller than that of bulk (19.3 g/cm**3). This is due to gold sputtering effects. AXAF mirror work suggests denities 90% that of Bulk. Peter used 88% for BBXRT and for his own ASCA XRT code. While there are no comments in the code, I believe the Nagoya group uses a larger value here- but then again, they use different constants.
In my modified raytracing code, only the reflectivity portion is changed. The code asks for a gold density, then it does the usual raytracing.
I have computed new ASCA effective area curves for several gold densities. I have taken the new ASCA effective area curves for gold densities 88% and 90.7% that of bulk gold, and compared them with the effective areas computed with the Nagoya optical constants. The ratio of the New areas to the Old areas is in THIS POSTSCRIPT FILE
Note the bumps at 2.2 keV and 3.8 keV as well as the hard tails. This alone may explain many of the residuals seen in the ASCA data before people started fudging the response by adding 2.2 keV gaussian features. Compare this to figure 2 of Yaqoob et al in PASJ 46 L49, 1994. Currently, ASCAARF adds a gaussian at 2.2 keV for the SIS data and the GIS team adds a gaussian at 2.2 keV in their GIS response.
I need to see if these features in the ratio plots depend on off axis angle. The postscript file above is for on axis only.
More work to be done here.
My origional motivation in checking out the XRT mirror reflectivity code was to address contamination effects at low energies. We know that there is some "low energy calibration problem" for the ASCA SIS/XRT system. Currently, the belief is that the CCD deadlayer/ thermal+optical blocking filters are incorrectly modeled. This implies an absorption effect. An alternative solution to the "low energy calibration problem" would be that the telescope effective area is inaccurate. This would be a reflection effect.
AXAF mirror calibration has included a study on the effects of contamination on the effective area of grazing incidence optics. They have put out many papers including:
Elsner et al, SPIE Vol 1742 page 6.
Elsner et al, SPIE Vol 2279 page 332.
Graessle et al, SPIE Vol 2279 page 12.
1) a contamination layer increases the area in the 0.5 to 6 keV range. principly around 2.2 keV. The enhancement near 2.2 keV looks VERY similar to the "gold bump" in the asca data...even with as little as 75 angstroms of a carbon based contamination.
2) a contamination layer produces a downward goiing effective area going from 1 towards 0.277 keV which is the opposite of the expected mirror energy dependence. This may look like absorption similar to the effect of increasing the CCD dead layer- but with a somewhat different energy dependence. Infact in many cases, the contamination results in a HIGHER area at low energies.
3) the effect of adding a gaussian to the mirror effective area near 2.2 keV coupled with some of the enhance area due to the mirror contamination is to make a bump roughly where the XRT people have been working to "tune" the constants.
I have made an additional modification to the raytracng code which allows one to enter a chemical compound formula (eg. H2O, H8C20N4, and so on...), a density, and a film thickness. This film is then added on top of the gold and the reflectivity is computed for the resulting bilayer.
Cool plots
First, trying to add a uniform thickness of carbon with density 0.75 g/cm**3 ontop the gold for 3 carbon thickness, you get THIS POSTSCRIPT FILE Note the enhance area- even at low energies. Note that compared to a clean mirror, the slope of the effective area is different for contaminated mirrors. This may look like absorption in a model, but the area is infact bigger than one would expect.
Second, for a fixed thickness, I varied the carbon density from 0.25 to 1.75 g/cm**3. The resulting areas are in THIS POSTSCRIPT FILE . The dotted green line is the clean mirror effective area. The increasing density tends to increase the area. THIS POSTSCRIPT FILE shows the ratio of dirty to clean for the same contamination levels.
Third, I started looking a contamination composition effects. By this I mean, what if it was H2O or some other molecule instead of pure carbon. I made area curves for the same contamination thickness and unit density contaminated mirrors for several different contaminating compounds. You can see it in THIS POSTSCRIPT FILE . Notice that at edges for elements contained in the contaminate, there is a notch. Then just to the low energy side of the edge, the reflectivity is greatly enhance. For Oxygen, this may look like a bigger oygen edge in the CCD response- since we do not see this, I conclude that the contaminate must have very little oxygen (eg water is unlikely).
I have extended the studies to below the carbon edge and find that almost anything tends to increase the area below the carbon edge. For example see THIS POSTSCRIPT FILE . In ROSAT, there was evidence for a gold bump- which was "fudged" out. This could mean that the steeper spectral indices in ROSAT for AGN may be a result of the poorer energy resolution and enhanced area below 0.28 keV due to mirror contamination. I am exploring this with Steve Snowden now.
I should note, that the image quality does not change appreciable. But I need to look at off axis angles eventualy.
We may also need to study nonuniformity in the contamination thickness.
Please Help!
That is all for now (22 April 96). Keith Gendreau
5 May 1996
I have made some experimental XRT caldb files using the new optical constants. The files are all for "clean" mirrors but have a variety of gold densities around the AXAF/Peter best estimates. I am still making more....
The purpose behind these is to examine the high energy structure of the response. They should get rid of most of the gold bump as well- but some residuals due to contamination may remain.
To use these with ascaarf, do the following:
ascaarf xrtrsp=
and then follow along as usual...
I am interested in hearing about how the different gold densities compare
before continuing with contamination studies.
Note that the detector responses may already have mods to there response
due to the assumption that the telescope was correct before....etc...
so there is still some "antiphysics" floating around... you might get worse
fits with the new arfs- but atleast the telescope has physics in its response
now...
The files are:
clean_gold17_20.fits for a clean mirror with 17.2 g/cm**3 Au density.
clean_gold17_30.fits for a clean mirror with 17.3 g/cm**3 Au density.
clean_gold17_37.fits for a clean mirror with 17.37 g/cm**3 Au density.
clean_gold17_50.fits for a clean mirror with 17.5 g/cm**3 Au density.
More of these files will be on the way in the near future.
5 May 1996
While poking around, I noticed that the azimuthial dependence of the
effective area is rather strongly energy dependent. I do not mean the
PSF's azimuthal dependence- as has been known for some time.
I mean, for example:
For a point source at a fixed off axis angle (theta) the dependence of the
effective area for all photons landing in a 12 arcminute extraction circle
on the azumuthal direction (phi) of that point source is large at some
energies.
Imagine the crab at an off axis angle of 10 arcminutes. It could be at several
azimuthal angles (phis). The effective area you get for photons landing in a
12 arcminute extraction region on the focal plane depends on the azimuthal
angle. This is in no ways an intuitive result- and infact has not been
noticed before.
To see what I mean look at:
THIS POSTSCRIPT FILE .
where I compare the effective area at several energies and phi angles
for a fixed theta angle of 10 arcminutes.
The energy dependence is stronger with large theta and with energies above
about 6 keV. There are two things to consider here:
1) Is there some parameter in the ray tracing to adjust whic affects this?
ie. can it be adjusted out in a physical way?
2) How does "ASCAARF" do its phi interpolation? Currently telescope calibration
files are rather course in their phi information.
There may be some more to do here....
Another interesting thing to ask is: Given that the XRT boresight has an error
of about +/- 0.5 arcminutes, what is the effect on Spectral fitting?
I have computed ratios of the Effective Area vs Energy curves for adjacent
off axis angles at a fixed phi pf 45 degrees. Specifically, I look at the
effective area curve for phi=45 degrees and theta=XX arcminutes and
compare it to the effective area curve for phi=45 degrees and theta=XX+1
arcminutes. The ratio of these gives you some idea of what the effect of this
uncertainty is on your spectral fitting. Interesting off axis angles to
consider are: 0-1 arcminutes, 5-6 arcminutes, and 8-9 arcminutes. The
"standard" positions are 5-6 arcminutes for some detectors and 8-9 arcminutes
for others.
I have put these ratios in
THIS POSTSCRIPT FILE .
Notice that:
1) even 1 arcminute errors can give 8% uncertainties in normalization
2) 1 arcminute errors can EASILY give you the hard tail seen in the data.
But this effect competes/compliments the effects of using the new optical
constants.
Now consider the fact that the crab is actually a 3 arcminute
extended object- AND even in Makeshima-senseis thesis
there is evidence that the spectrum changes with position across
it.
I think it would be extremely foolish to continue with this arffilter
that the GIS team wants to release....
Other reasons the crab is bad:
1) it is much brighter than what we typically look at
2) there are big deadtime and gain corrections to be made
to the GIS
3) I would expect that the resolution also changes in the
GIS at high count rates...
The crab is a bad calibration source.
Since the Crab is actually about 3 arcminutes across and not a point source,
its use as a calibrator with conical mirrors is somewhat compromised.
The GIS team uses a point source XRT response to the crab to do its fitting.
They then make an energy dependent fudge called the "arffilter" to smooth
out the wiggles. I have simulated two possible crab distributions:
a uniform brighness 3 arcminute disk and a disk which falles like 1/r to
a cut off radius of 3 arcminutes. These simulations were done for the
crab at 5 arcminutes off axis. Next a standard point source response for
the central position was used to produce a ratio of the extendedn response
to the point source response. In both the extended and pointsource response,
the nerw optical constants were used. The results are in
THIS POSTSCRIPT FILE .
Note that htere are wiggles of the same order of size as the arffilter fudge.
The wiggles are at different energies, but recall that:
1) the arffilter still needs to correct for optical constant problems
2) The crab distribution is not quite round and may have an energy dependent
figure.
MORE to be done here!
A New Subject: Azimuthal Dependence of Effective Area
What is the effect of boresight uncertainty on Spectral Fitting?
15 May 1996
The effects of the finite crab size
Keiths Thesis .
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