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Дата изменения: Wed Apr 8 18:31:52 2015
Дата индексирования: Wed Apr 13 13:28:09 2016
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Cross-calibration investigations straylight (and others)
D Lumb

Motivation
· Following the last EPIC calibration meeting continued investigations into off-axis stray light · Aim is to understand reported gross discrepancies, and examine if a physical model also contributes to understanding on-axis cross calibration uncertainties

Method
· Looked into archive for observations with obvious single reflection arcs · Dominated by LMCX-1, Cyg X-3 and Galactic Bulge binaries · As the arcs are very extended across the focal plane have reanalysed all using the ESAS scripts · Made a comparison with SCISIM ray trace that replicates each observation

Main Analysis Steps
· Select regions for each camera and run esas · Extract spectra and background · Made an .arf that is a "guess" for typical off axis angle ( fn(E) knowing only a single reflection occurs ) · Assume source spectrum of a LMXRB e.g. const*phabs*(diskbb+comptt) probably the detailed spectrum doesn't matter to 1st order · Also require an in field background of gauss+gauss+apec+phabs*apec that has a different normalisation per camera

· Depending on epoch - find nearest RXTE ASM or MAXI observation (2-10keV flux or sometimes for latter even a spectrum ) · With MAXI data joint spectral fit with a constant multiplier between instruments · Or do EPIC spectral fit alone and just make a flux estimate comparison · Difference gives the straylight rejection factor · If there is a gross problem in any source, should at least be able to renormalise the 3 cameras to allow azimuthal trends to be tracked

Source flux

Ray Tracing Tasks
· Assume catalogued source position and the RA,DEC, PA PNT keywords to define a source input to SCISIM · Ray trace for all 3 telescopes · Especially for smaller off-axis angles, realised need a good estimate of focal plane offsets and geometries to determine fraction of single reflected arcs contained within observation selected regions (not in plain vanilla SCISIM) · Now realise that to get enough sources will need to use full range off-axis radii, where the "guess ARF" is NOT good enough ( more work )

Factors also considered
· By far biggest uncertainty is the source flux (RXTE 5% systematic on its Crab calibration, but sometimes are no simultaneous data) · Statistical errors on flux ~5-30% typically · Unknown effect of spectral model and the use of assumed .arf but probably a common factor · Compared average of observation with average of simulation to flag gross source flux problem · ~50 observations continued for final analysis

Radial Variation
· Nominal radial variation from SCISIM (circular focal plane FOV)

Radial Variation
· Radial variation for the pathology of observations i.e. when you add in the focal plane geometry

Azimuthal Variation
· Ray trace for a "typical" 0.8 degree off-axis source and uniform azimuthal distribution · Compared for both a 100 micron baffle shift or a 0.02° Rotation, onto a circular aperture in detector plane · Typical change +/- ~cosine like variation

Camera results - radial · Average of 3 cameras

Scaling for source flux error?
Average 3 cameras difference between measured and simulated

PN Camera results - azimuthal

PN Camera results ratio to simulation

PN Camera results - azimuthal
Scale for flux errors and divide by simulation

?

MOS1 Camera results - azimuthal

MOS1 Camera results ratio to simulation

MOS1 Camera results - azimuthal
Scale for flux errors and divide by simulation

?

MOS2 Camera results - azimuthal

MOS2 Camera results ratio to simulation

MOS2 Camera results - azimuthal
Scale for flux errors and divide by simulation

?

Conclusions still in progress
· Average straylight vs radius not grossly discrepant · After source flux adjustment, different azimuthal variation per camera · Not possible to say if due to baffle rotation or shift · Possible impact on Effective Area on axis (Fn of Energy) was ray traced only ~1% effect · Azimuthal variation like the Matteos effect · But also has similar effect to the telescope axis modification made XMM-CCF-REL-156 in 2004 ? · Is that a combination of two effects??????

Questions about ARF
· Work on clusters cross-calibrations and the XMM Catalogue point source comparisons show the similar effects · XMM gives lower temperature than ACIS, PN effective area returns progressively lower flux than MOS · Few % effect MG wondered if in the end we just use an aphysical fudge factor

ARF vs extraction radius
· Using SAS 13.0 ( as likely was done for pre 2014 IACHEC cross calibration exercises) · Generate an ARF for different spectral extraction radii up to ~10 arcmin · As expected compares quite well with the Encircled Energy output of calview when using ELLBETA accuracy · But seems very aphysical why doesn't mirror scattering get worse with increasing energy?

PN ARF

Mirror scattering should decrease ARF with energy

MOS ARF

Less obvious increase (except E>8keV)

Encircled energy differences
· 0.5 2 keV band and "typical" enclosed diameters 54 arcsec & 1.67 arcmin
PN 86.3 / 93.5% MOS1 83.0 / 90.9%

· 2 7 keV band and "typical" enclosed diameter 54 arcsec & 1.67 arcmin
PN 87.1 / 93.8% MOS1 82.8 / 90.6%

· PN would report 3- 5% lower flux yet actually its PSF is worse than MOS1 (admittedly core)

PN Encircled Energy Normalised to ~10 arcmin radius

PN ARF matches the ELLBETA description

Small (<1%) differences easily explained by chip gaps, bad pixels etc. ?

OOT Correction
· Seems to do nothing for the withootcorrection flag in arfgen ? · Exposure is supposed to be corrected for ~6% OOT effect, but comparison with GTIs and LIVETIMEs shows much larger reduction? · In any case with extended sources how do you bookkeep for increasing recovery of OOT events INTO spectral extraction? · PN has more complex time accounting any small errors potentially affect normalisation · And for clusters the cross-arf problem will surely blow up any incomplete ARF accounting as fn(E) !!

Example:
PN MOS1 76317

Observation Duration

69719

On Time Live Time Total GTI Image Exposure
Spectrum Exposure

64122
56489

68914
68012

64124
56359

68921
68002

Spectrum/Image Exposure

0.88

0.987

Flux and Spectral Differences?
· We are looking for few % differences in normalisation and also some trend with energy · Normalisation could be hostage to small errors in the LIVETIME/GTI calculations and OOT correction · Encircled energy function calculated by ELLBETA seems to go in wrong direction in energy AND between PN and MOS