Action #2342

Handle noisy CTA energy dispersion

Added by Knödlseder Jürgen about 6 years ago. Updated about 6 years ago.

Status:ClosedStart date:02/16/2018
Priority:HighDue date:
Assigned To:Knödlseder Jürgen% Done:

100%

Category:-
Target version:1.5.1
Duration:

Description

The energy dispersion histogram for CTA data are quite noisy, which turned out to pose problems in the fitting procedure when analysing the 1DC data. The issue can for example be seen in the following plot generated using csspecres of Cas A which shows a peak in the model spectrum near 2 TeV

specres_3D2_nosmooth.png (38.6 KB) Knödlseder Jürgen, 02/16/2018 04:29 PM

specres_3D2_etrue1.0.png (37.7 KB) Knödlseder Jürgen, 02/16/2018 04:31 PM

specres_3D2_etrue2.0.png (38.5 KB) Knödlseder Jürgen, 02/16/2018 04:32 PM

specres_3D2_etrue3.0.png (38.3 KB) Knödlseder Jürgen, 02/16/2018 04:32 PM

SED-etrue-smoothing.png (61.3 KB) Knödlseder Jürgen, 02/16/2018 04:33 PM

South_z20_50h.png (243 KB) Knödlseder Jürgen, 02/23/2018 11:29 AM

South_z20_50h_clip10.png (254 KB) Knödlseder Jürgen, 02/23/2018 11:29 AM

South_z40_50h.png (241 KB) Knödlseder Jürgen, 02/23/2018 11:29 AM

South_z40_50h_clip10.png (246 KB) Knödlseder Jürgen, 02/23/2018 11:29 AM

North_z20_50h.png (258 KB) Knödlseder Jürgen, 02/23/2018 11:30 AM

North_z20_50h_clip10.png (263 KB) Knödlseder Jürgen, 02/23/2018 11:30 AM

North_z40_50h.png (254 KB) Knödlseder Jürgen, 02/23/2018 11:30 AM

North_z40_50h_clip10.png (255 KB) Knödlseder Jürgen, 02/23/2018 11:30 AM

specres_3D2_onlyclipping.png (38.1 KB) Knödlseder Jürgen, 02/23/2018 11:31 AM

SED-onlyclipping.png (55.5 KB) Knödlseder Jürgen, 02/23/2018 11:31 AM

specres_3D.png (55.5 KB) Knödlseder Jürgen, 02/23/2018 01:06 PM

specres_3D_edisp.png (55.7 KB) Knödlseder Jürgen, 02/23/2018 01:06 PM

specres_3D_edisp_clipping.png (55.1 KB) Knödlseder Jürgen, 02/23/2018 01:06 PM

specres_3D_edisp_clipping_smoothing.png (55.7 KB) Knödlseder Jürgen, 02/23/2018 02:57 PM

North_z20_50h_clip15_smooth3.png (216 KB) Knödlseder Jürgen, 02/23/2018 03:11 PM

North_z40_50h_clip15_smooth3.png (203 KB) Knödlseder Jürgen, 02/23/2018 03:11 PM

South_z20_50h_clip15_smooth3.png (206 KB) Knödlseder Jürgen, 02/23/2018 03:11 PM

South_z40_50h_clip15_smooth3.png (198 KB) Knödlseder Jürgen, 02/23/2018 03:11 PM

hess_specres_3D_edisp_clipping_smoothing.png (48 KB) Knödlseder Jürgen, 02/23/2018 04:55 PM

hess_specres_3D_edisp.png (48.1 KB) Knödlseder Jürgen, 02/23/2018 04:55 PM

hess_specres_3D.png (47.2 KB) Knödlseder Jürgen, 02/23/2018 04:55 PM

energy-dispersion.pdf (1.31 MB) Preview Knödlseder Jürgen, 03/27/2018 02:53 PM

Specres_3d2_nosmooth Specres_3d2_etrue1.0 Specres_3d2_etrue2.0 Specres_3d2_etrue3.0 Sed-etrue-smoothing South_z20_50h South_z20_50h_clip10 South_z40_50h South_z40_50h_clip10 North_z20_50h North_z20_50h_clip10 North_z40_50h North_z40_50h_clip10 Specres_3d2_onlyclipping Sed-onlyclipping Specres_3d Specres_3d_edisp Specres_3d_edisp_clipping Specres_3d_edisp_clipping_smoothing North_z20_50h_clip15_smooth3 North_z40_50h_clip15_smooth3 South_z20_50h_clip15_smooth3 South_z40_50h_clip15_smooth3 Hess_specres_3d_edisp_clipping_smoothing Hess_specres_3d_edisp Hess_specres_3d

Recurrence

No recurrence.

History

#1 Updated by Knödlseder Jürgen about 6 years ago

It turned out that the problem is related to the noise in the energy dispersion histogram. Smoothing and clipping that histogram in the true energy direction using a Gaussian with sigma of 1, 2 and 3 pixels gives the following results:

The peak disappears, and from sigma=2 on the residuals are fine. A check of the fitted values with ctlike also shows that the true spectral values are well recovered with that smoothing. The resulting butterfly and spectral points, generated with ctbutterfly and csspec, are shown below:

#2 Updated by Knödlseder Jürgen about 6 years ago

I tried an alternative method which seems to be more robust, and which consists of clipping outlying pixels in the energy dispersion matrix. I implemented this algorithm as a kludge in GCTAEdisp2D. Below a comparison of the IRFs:

Original Clipped

The residual spectrum looks good:

And also the butterfly and SED are okay:

#3 Updated by Knödlseder Jürgen about 6 years ago

I tested the same procedure on G15.4+0.1 to check a source that is seen with the Southern array. Specifically, the South_z20_50h IRF is used for this source.

Below the residual spectra without energy dispersion, with energy dispersion, with a clipped energy dispersion and with a clipped and smoothed energy dispersion.

No Edisp Edisp
Clipped Edisp Clipped and smoothed Edisp

There is clearly some structure visible in the source and interstellar emission model at low energies when the energy dispersion is used, and this structure becomes weaker - though does not completely disappear - when clipped energy dispersion is used. I therefore added back some smoothing, which resulted in the last image. Still not perfect, but hopefully acceptable.

#5 Updated by Knödlseder Jürgen about 6 years ago

Here the IRF plots for the clipped and smooth energy dispersion:

Original Clipped and smoothed

#6 Updated by Knödlseder Jürgen about 6 years ago

Here the results for HESS J1943+213 which mixes observations with South_z40_50h and North_z20_50h.

No Edisp Edisp
Clipped and smoothed Edisp

Without energy dispersion there are substantial residuals, with the raw energy dispersion matrix there is an important feature near 1 TeV, and with the clipped and smooth energy dispersion everything looks good.

The only strange thing is that the Prefactor of the fitted source is too large:

Prefactor Index
True 5.6e-19 -3.100
No energy dispersion 10.0e-19 -3.139
Raw energy dispersion 10.39e-19 -3.135
Clipped and smoothed energy dispersion 11.09e-19 -3.117

Maybe the sky model is too simple and there are other sources in the field of view.

#7 Updated by Knödlseder Jürgen about 6 years ago

The reason for the discrepancy is that the source occurs twice in the GPS:

  <source name="HESS J1943+213" type="PointSource">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="56.000001727799" error="0" scale="1e-20" min="0" free="1" />
      <parameter name="Index" value="3.09999990463257" error="-0" scale="-1" min="-10" max="10" free="1" />
      <parameter name="PivotEnergy" value="1" scale="1000000" free="0" />
    </spectrum>
    <spatialModel type="PointSource">
      <parameter name="RA" value="295.979203700646" error="0" scale="1" free="1" />
      <parameter name="DEC" value="21.302224614766" error="0" scale="1" free="1" />
    </spatialModel>
  </source>
  <source name="TeV_J1943+2118" type="PointSource" tscalc="1">
    <spectrum type="PowerLaw">
      <parameter name="Prefactor" value="5.6" error="0" scale="1e-19" min="1e-07" max="1000" free="1" />
      <parameter name="Index" value="3.1" error="0" scale="-1" min="0" max="10" free="1" />
      <parameter name="Scale" value="1000000" scale="1" min="0" max="10000000" free="0" />
    </spectrum>
    <spatialModel type="SkyDirFunction">
      <parameter name="RA" value="295.979" error="0" scale="1" min="-360" max="360" free="1" />
      <parameter name="DEC" value="21.3022" error="0" scale="1" min="-90" max="90" free="1" />
    </spatialModel>
  </source>

The first source is in model_galactic_bright.xml, the second in model_agn.xml.

This means that the model parameter comparison should read as follows:

Prefactor Index
True 11.2e-19 -3.100
No energy dispersion 11.00e-19 ± 0.18e-19 -3.139 ± 0.011
Raw energy dispersion 10.39e-19 ± 0.17e-19 -3.135 ± 0.011
Clipped and smoothed energy dispersion 11.09e-19 ± 0.18e-19 -3.117 ± 0.010
Clipped and smoothed energy dispersion with IEM 11.13e-19 ± 0.18e-19 -3.112 ± 0.010

#8 Updated by Knödlseder Jürgen about 6 years ago

  • Status changed from In Progress to Closed
  • % Done changed from 50 to 100

Merged code in bugfix-1.5.1 and devel.

#9 Updated by Knödlseder Jürgen about 6 years ago

Here a presentation of the issue for the ctools call.

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