XRT response function modification

XRT response function modification
----- correction for light leak and X-ray filter contamination

(Aki Takeda, 19 March 2021, initial version. )
(29 March 2022, modified for the XRT calibration meeting 2022.)

0. Introduction

Reviewing the project status as of the previous meeting

XRT irradiance (5-60A), filter ratio Te and EM

Te and EM derived from the XRT signals collected within 1.1 R_⊙ using the filter ratio method.
Al_mesh and Ti_poly filter ratio for Feb-2008 through May-2015.
Al_mesh and Al_poly filter ratio after Jun-2015.
Isothermal coronal spectrum calculated for Te and EM integrated over the wavelength range, 5-60Å, etc.

Stray light image subtraction done.
No correction of effective area functions.

Stray light image subtraction done.
Effective area modification done.
pre-filter opening ratio below are assumed:
- phase 1 (after 9-May-2012 12:00) : 0.001
- phase 2 (after 14-Jun-2015 12:30) : 0.033
- phase 3 (after 27-May-2017 11:00) : 0.075
- phase 4 (after 29-may-2018 00:00) : 0.099

Despite our current best effort of correction, the above plots have serious problems, which need to be further corrected.

- significant difference in Te in two solar minimums(cycle 23 & 24).

- significant difference in EM in two solar minimums(cycle 23 & 24).

These results indicate that there is still a missing piece of puzzle to fully (quantitatively) correct the visible stray light contamination included in our X-ray images. [status as of Dec-2020]

1. How to better correct effective area functions

approach 1: Assume additional contamination on Focal Plane Analysis Filters(FPAFs)

Applying the similar corrections as those performed in Narukage et al.(2011), in which the temperature around the FPAFs and the frequency of the filter use are key factors.

Fig. 7 from Narukage etal 2011	Fig.37 from Narukage etal (2011)

Variation of temperature around FPAF after the thermal phase 3

Frequency of filter use

Monthly frequency	Monthly exposure time

Attempt 1 : full stray light correction + strong filter contam.

Assumed FPAF contamination thickness:

factor relative to the last value of curent DB	converted to the thickness

Derived Te, EM and irradiance (5-60A)

multi-plot (irrad, Te & EM)
* Ti_poly/Al_mesh filter ratio curve problem with large amount of contamination: examples 1 and 2 . We could barely determine the pairs of filter contamination thickness that yield reasonable curves of Te, EM, and Irrad.
[status as of the 2021/10/26 meeting]

approach 2: Stray light might not affect XRT effective area

Possibility 1: It is possible that entrance filter holes cause higher apparent visible light open area than X-ray open area, because of differences in how white light and X-rays scatter in the telescope.
(a comment from Engineering team, via K. Reeves on 29-Sep-2021)

However, we should apply the stray light correction to some extent , otherwise results in unrealistic EM variation (the lowest values in 2017 are lower than those during the solar minimum, 2019-2020, as shown in the above left plot).

[Follow-up of the 6-Oct meeting]
Variation of the full-Sun signals(DNs) with different filters : Al_mesh singals are prone to attenuate by contamination (after 2017).
* Al_mesh, Ti_poly and Al_poly plots were made from the current irradiance study.
* Thin_Be signals were measured by my 2021 REU student, Stefanie Davis.
* Al_poly data points (2008-2015) can be increased by additional work.

Al_mesh	Ti_poly

Al_poly	Thin_Be

Attempt 2 : mild stray light correction + mild filter contam.

Chianti 9.0 version NOT tried.

Attempt 3 : mild stray light correction + mild filter contam. with Chianti ver.10.0

Reduced pre-filter open fraction applied:

 
phase 0: ofrac(i)=0
phase 1: ofrac(i)=0.001 --> 0
phase 2: ofrac(i)=0.033 --> 0.033*0.50
phase 3: ofrac(i)=0.075 --> 0.075*0.70
phase 4: ofrac(i)=0.099 --> 0.099*0.80

factor relative to the last value of curent DB	converted to the thickness

Fig. IrTeEM_3

Attempt 4 : Consideration of different filter pairs used before/after phase 2

So far it has implicitly been assumed that the filter pairs Ti_poly/Al_mesh (used for phase 0 and 1) and Al_poly/Al_mesh (phase 2 to present) will give consistent Te and EM without any correction. But recently a comparison of Te and EM obtained from Ti_poly/Al_mesh and Al_poly/Al_mesh filter using the data all obtained +/- 7 minutes was made.

2008 (phase 0)	2011 (phase 0)	2014 (phase 1)


Calc. with Chianti ver.9 Te and EM	Calc. with Chianti ver.9 Te and EM	Calc. with Chianti ver.9 Te and EM

An attempt to get the mission-long Al_poly/Al_mesh equivalent Te and EM .

phase 0
- [new Te] = [Te_tpam] * 1.15 (→ 1.10 modified in the plot below)
- [new EM] = [EM_tpam] * 0.96 (→ 0.98 modified in the plot below)
phase 1 : no change
phase 2-4 : no change (of course)

The new irradiance for the phase 0 is to be calculated from the new Te and EM.
It turns out that this attempt fails: Te and irradiacne become too high (even with the reduced version of the factor).

Fig. IrTeEM_4

Attempt 5: Back again to full stray light correction.

get highest Te but EM too low (ver. b)	get low Te and highest EM (ver. a)

Fig. IrTeEM_5b	Fig. IrTeEM_5a

Both versions (a and b) turned out not favorable for the idea of "Al_poly/Al_mesh equivalent irradiance". But ver.(a) gives the same level of Te, EM, and irradiance for the past two minimum periods (around 2008 and around 2019), so a good candiate of our final irradiance products.

Summary

In order to obtain the "reasonable" set of Irradiance, Te and EM curves, under Chianti ver.10, the candidates of our final irradiance products are

mild stray light corr. + mild filter contam.(Fig_IrTeEM_3) or
full stray light corr. + full filter contam.(Fig_IrTeEM_5a)

The latter is simpler with less assumption, so would probably be the better candidate.

Data requests received so far:

Adithya H.N. (YIESPL, India)
Sherry Chhabra (NRL, GEU)
Shin Toriumi (ISAS, Japan)