Suzaku, Courtesy JAXA

X-ray Imaging Spectrometer (XIS)
Contamination Monitoring

Background

In late November 2005, it became apparent that the low-energy effective of each XIS instrument area was rapidly decreasing. We have since concluded that this is caused by a build-up of molecular contamination on the optical block filters (OBFs) which sit just above the CCD chips in the light path of each XIS. The amount and rate of contamination is different for each instrument, and it appears to be spatially non-uniform. Our best model suggests it is composed of primarily carbon and oxygen.

This page details monitoring of the on-axis contamination rate by the MIT XIS team. Included are presentation summaries as well as records of updates to the contamination model and trends. For current information applicable to GO data analysis, please refer to the Suzaku ABC Guide.


2015 January 29

XIS contamination update

Optical depth and total effective area at two energies. Dashed lines in the top panel are CALDB contami_20140825. Dashed lines in the bottom panel are extrapolations from recent data.

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A new CALDB file was constructed (20140825). XIS0 and XIS3 needed slight adjustments to the O column density at recent times, while XIS1 needs an adjustment to the N trend based on PKS2155 and RXJ1856, and then a re-fit of the E0102 data, which freezes the N column based on that trend.

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Line center offsets of the bright E0102 emission lines, showing how the gain correction is doing. Not well.

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2014 July 1

XIS contamination update

The latest E0102 observation, 2014-04-21, has been added to the contamination trend plot. In the plots of column density below, the red line is the current CALDB (contami_20130813) and the stars are spline points used to create that trend.

A new CALDB file will be constructed from these data this month. XIS0 and XIS3 need slight adjustments to the O column density at recent times, while XIS1 needs an adjustment to the N trend based on PKS2155 and RXJ1856, and then a re-fit of the E0102 data, which uses freezes the N column based on that trend.

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Optical depth and total effective area at two energies. Dashed lines in the top panel are CALDB contami_20130813. Dashed lines in the bottom panel are extrapolations from recent data.

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Line center offsets of the bright E0102 emission lines, showing how the gain correction is doing. Not well.

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2014 April 1

XIS contamination update

The latest E0102 observation, 2014-03-16, has been added to the contamination trend plot. In the plots of column density below, the black line is the current CALDB (contami_20130813). Spectral fits are also shown. This is the first E0102 observation taken after the Jan 2014 UVC problem, and there is no obvious change in the low-energy spectra or the trend of the contamination, which still appears to be decreasing.

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2013 December 5

XIS contamination update

The latest E0102 observation, 2013-09-29, has been added to the contamination trend plot. In the plots of column density below, the black line is the current CALDB (20130705).

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2013 July 5

XIS contamination update

The contamination trend since ~ 2012 has been updated. In the plots of column density below, the stars are cubic spline points and the solid lines are interpolations of the trends. Red is the current CALDB (20120719), black is the update (20130705). This should be implemented in the CALDB by the end of August. The plan is to update this every 6 months from here on: Tsujimoto-san and Wada-san will send me values for RXJ1856 and PKS2155, and I will add E0102 and fit the spline.

Note the divergence in C and O for XIS1 in 2012-2013. The reason is unclear.

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2013 May 24

XIS contamination by element (H, C, N, O) for each XIS

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XIS contamination by XIS

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E0102 spectral fits

(using EDM's nifty gain correction algorithm)
Download single 170-page PDF



2011 October 4

Using the same fits as 2011-09-27, here are the FWHM and gain trends, along with (for completeness) the contamination history with the spline fits.

PDF

PDF

PDF

Here are spectra from 2011-06-11, the most recent observation, showing the differences between the current CALDB contamination model (ae_xi*_contami_20091201.fits) and the spline fit detailed in 2011-09-27.

single PDF file of all 3 detectors

2011 September 27

All E0102 data up till 2011-06-29 are now included, using only normal full-window mode. All XIS1 SCI=2 keV and SCI=6 keV data are included. The best-fit trends are from proposed cubic spline interpolation, since simple analytic functions do a poor job of fitting the trends at all epochs. XIS1 and XIS2 are monotonically increasing. XIS0 also monotonically increases, but there are inflection points where the concavity changes (seen more easily in the C column history, below). XIS3 as fit is not monotonically increasing; this appears warranted in the C column history below, but it should be discussed, as should the use of by-eye cubic spline fitting.

Attached below are CALDB contamination files, in the same format as ae_xi*_contami_20091201.fits, with updated C,O,H columns reflecting these trends. The O/C and H/C ratios remain as in the previous HCO model. The cubic spline has been extrapolated to 2015. Also attached are data files of the cubic spline points for each XIS.

O column history -- PDF

C column history -- PDF

Updated contamination files:
ae_xi0_contami_edm_20110927.fits
ae_xi1_contami_edm_20110927.fits
ae_xi2_contami_edm_20110927.fits
ae_xi3_contami_edm_20110927.fits

Spline points (columns are MJD and C column density in 1018 cm-2):
splinepoints_xis0.dat
splinepoints_xis1.dat
splinepoints_xis2.dat
splinepoints_xis3.dat


2011 August 22

All E0102 data up till 2011-06-29 are now included, using only normal full-window mode. All XIS1 SCI=2 keV and SCI=6 keV data are included. The updated CALDB contamination trends from ae_xi*_contami_20091201.fits are plotted; these use the time-variable HCO model.

Also changed from previous plots: the O column density is now being plotted instead of C; and the errorbars are 1-sigma instead of 90%.

PDF


2010 October 06

New E0102 data from 2010 August 29, in normal full-window mode for XIS0 and XIS1. XIS3 was in P-Sum mode, and is excluded. First, the best-fit contamination assuming C/O = 6 and with trends from the current CALDB files:
ae_xi0_contami_20090128.fits
ae_xi[123]_contami_20081023.fits
ae_xi[0123]_makepi_20091202.fits

PDF

Here are the gain offsets and excess linewidth for the same CALDB fit. "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

PDF

PDF


2010 August 13

New E0102 data from 2010 June 19, in normal full-window mode for all three chips. First, the best-fit contamination assuming C/O = 6 and with trends from the current CALDB files:
ae_xi0_contami_20090128.fits
ae_xi[123]_contami_20081023.fits
ae_xi[0123]_makepi_20091202.fits

PDF

Here are the gain offsets and excess linewidth for the same CALDB fit. "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

PDF

PDF

Finally a spectrum from XIS1 of the latest observation (2010 June 19) showing an excess of counts below 0.5 keV compared to the E0102 emission model plus C+O contamination model:

PDF


2010 July 22

No contamination update, but some investigation into gain errors at low energy. Below are the best-fit line centers for the OVII, OVIII, NeIX, and NeX from all E0102 observations on XIS0 and XIS1, both SCI off and on, using the most current calibration products to date. A single Gaussian was fit to each line along with a Bremsstrahlung continuum and contamination fixed to the best-fit value for that date. The plotted lines are simple unweighted least-squares fits to all the data points, and sigma_y is the scatter about this fit. Errors are 1-sigma.

There is a clear trend of increasing line center in XIS1 for all four emission lines, and slightly less in XIS0, perhaps due to over-correction of CTI. The first two data points for XIS1 in 2005 are about 5 eV higher than the trend would predict. This is probably not a difference due to SCI, as more than half of the datasets in 2006 are also SCI-off.

PDF (all figures in one file)



2010 May 25

New E0102 data from 2010-Feb-05 and 2010-Apr-05, in normal full-window mode for all three chips. First, the best-fit contamination assuming C/O = 6 and with trends from the current CALDB files:
ae_xi0_contami_20090128.fits
ae_xi[123]_contami_20081023.fits
ae_xi[0123]_makepi_20091202.fits

PDF

Here are the gain offsets and excess linewidth for the same CALDB fit. "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

PDF

PDF


2010 Jan 16

New E0102 data from 2009-Dec-12 for XIS1 only; XIS0,3 were observed in P-Sum mode and so are not included.
First, the best-fit contamination assuming C/O = 6 and with trends from the current CALDB files:
ae_xi0_contami_20090128.fits
ae_xi[123]_contami_20081023.fits

PDF

Here are the gain offsets and excess linewidth for the same CALDB fit. "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

PDF

PDF

Next, the best-fit contamination assuming the empirical HCO model and with trends from 2009-Oct-11:

PDF

Finally, optical depth and transmittance. The top plot shows the evolution of the optical depth at two energies, for each detector. The bottom plot shows the total effective area for all detectors summed. Each trend has been extrapolated (dashed lines) at its current rate through the end of 2012.

PDF


2009 Dec 21

New E0102 data from 2009-Oct-11/12 and 2009-Oct-27, with fits using the HCO empirical contamination model.

Attachments:

Contamination rate plot PDF


2009 Oct 11

Analysis of HCO empirical contamination model with E0102. Please see the attached PDF file.

Attachments:

PDF

2009 July 21

Updated with the 2009 April 23 and 2009 June 26 observations. The processing and trend fits are the same as described in 20090402. This still assumes C/O = 6 and does not yet use the new E0102 emission model from IACHEC.

The last datapoint (2009 June 26) occurred after the XIS0 charge leakage problem (on 2009 June 23). There are no measurable effects on the FWHM or gain after this event, indicating that the calibration is likely unaffected for quads B and C, where the source is detected.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

Linewidth trend plot PDF

Gain trend plot PDF


2009 May 18

Updated with the 2009 March 9 observation. The processing and trend fits are the same as described in 20090402. This still assumes C/O = 6 and does not yet use the new E0102 emission model from IACHEC.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

Linewidth trend plot PDF

Gain trend plot PDF


2009 April 02

Updated processing (gain and FWHM calibration) including all data through the 2008 Dec 14 observation. A new functional form for the contamination rate has been adopted:

     N_C =  A (1+Ct) [1-exp(t/B)]

where the parameters are:

XIS A (C atoms/cm2) B (days) C (days-1)
0 1.5787 65.870 2.3527e-3
1 4.5424 199.74 6.2105e-7
2 4.8792 131.13 -3.1809e-4
3 5.7999 89.809 -2.2643e-5

Full details of the processing and an anlysis of the temporal variations is included in this memo:

edm_contam_memo_20090402.pdf

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data.

Linewidth trend plot PDF

Gain trend plot PDF


2009 Jan 20

Updated with the 2008 Dec 14 observation. The processing and trend fits are the same as described in 20081201. This still assumes C/O = 6 and does not yet use the new E0102 emission model from IACHEC.

The contamination has held steady for XIS1 and XIS3, but continues to increase for XIS0. The solid black line for XIS0 shows the trend currently implemented in CALDB, while the dotted black line shows the 20081201 fit to data after April 2007.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data. Latest four data points use RMFs generated with CALDB version rmfparam_20080311. Prior data use rmfparam_20080121.

Linewidth trend plot PDF

Gain trend plot PDF


2008 Dec 01

Updated with the 2008 Oct 22 observation. The processing and trend fits are the same as described in 20080404. This still assumes C/O = 6 and does not yet use the new E0102 emission model from IACHEC.

The contamination has held steady for XIS1 and XIS3, but continues to increase for XIS0. (The last two XIS0 points are right on the XIS3 points.) I note there seems to be a correlation between the XIS0 contamination increase after April 2007 and the gain shift increase during the same period; however the latest point has a much smaller gain shift. I have not re-processed the previous data, which all use PROCVER <= 2.2.8.20. The latest point uses PROCVER 2.2.11.32.

The solid black line for XIS0 shows the trend currently implemented in CALDB, while the dotted black line shows a new fit to data after April 2007:

     N_C =  a*(1-exp((mjd0-mjdbr)/b))
          + c*(1-exp((mjdbr-MJD)/d))   for XIS0 (MJD > mjdbr)
     # XIS0
     # broken exponential

     current CALDB (solid)         new fit (dotted)
     a              3.939          a              3.939
     b            270.069          b            270.069
     c              1.827          c              4.007
     d            238.417          d            728.928
     mjd0       53595.408          mjd0       53595.408
     mjdbr      54200.6            mjdbr      54200.6

The new XIS0 contamination trend has a small effect on count rate calculations for AO4. For an observation planned in Sep 2009, here are the differences in total (XIS0+1+3) effective area and count rates for the assumed CALDB trend and the new trend:

energy Aeff(t=20090901)/Aeff(t=0) AO4 count rate overestimation
  CALDB new trend  
0.5 keV 0.245 0.234 4%
0.6 keV 0.294 0.286 3%
1.0 keV 0.716 0.703 2%

The second column shows the ratio of the total effective area on 2009 Sep 01 to the contamination-free effective area (i.e. at launch), assuming the trends in the current CALDB (solid lines on the plot). The third column is that ratio assuming XIS0 increases according to the dotted line. The count rate at 0.5 keV will be 4% lower than calculated for the current trends. Of course, if XIS0 contamination levels off like XIS3, then the discrepancy will be smaller than this.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data. Latest four data points use RMFs generated with CALDB version rmfparam_20080311. Prior data use rmfparam_20080121.

Linewidth trend plot PDF

Gain trend plot PDF


2008 Sep 04

Updated with the 2008 Aug 13 observation. The processing and trend fits are the same as described in 20080404. This still assumes C/O = 6 and does not yet use the new E0102 emission model from IACHEC.

The contamination has held steady for XIS1 and XIS3, but increased again for XIS0. (The last XIS0 point is right on the XIS3 point.) I note there seems to be a correlation between the XIS0 contamination increase after April 2007 and the gain shift increase during the same period. I don't know what that mean yet.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data. Latest data (20080605) uses RMFs generated with CALDB version rmfparam_20080311.

Linewidth trend plot PDF

Gain trend plot PDF


2008 Jun 25

Updated with the 2008 June 05 observation. The processing and trend fits are the same as described in 20080404. This still assumes C/O = 6 and does not yet use the new E0102 emission model from IACHEC.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data. Latest data (20080605) uses RMFs generated with CALDB version rmfparam_20080311.

Linewidth trend plot PDF

Gain trend plot PDF


2008 Apr 29

Updated with the 2008 April 08 observation. The processing and trend fits are the same as described in 20080404.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth and gain offset vs. date. Linewidth measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). "Gain offset" is an additive energy scale offset from the XSPEC "gain fit" command, positive gain shift means that the PI values are higher than the model, so the model line energies must be shifted up. Open points indicate SCI-on data, crosses indicate SCI-off data. Data from Jan 2006 and later use RMFs generated with CALDB version rmfparam_20080121.

Linewidth trend plot PDF

Gain trend plot PDF


2008 Apr 4

Several updates to the contamination fitting:

  1. Data from 20080215 and 20080315 (last two points) are now from the archive, with v2.2.7.18 processing.
    This has increased the contamination by a few percent, because the ARF I had been using lacked attitude information. With a proper ARF, the effective area is 3.5% larger, so the contamination increases to account for the now lower flux. (I only allow the contamination to vary in the fit, the E0102 normalization is fixed.)
  2. All RMFs have been re-made with CALDB file rmfparam_20080121.
    This produces much better modeling of emission lines in SCI-on data. Compare the linewidth trend plot below with that from 2008 Mar 27.
  3. All data have been reprocessed with makepi_20080131.
    These data were already PROCVER >= 2.0.6.13, and the SCI-on data had been reprocessed with makepi_20071031 (see 2007 Dec 19), so this should make little difference in the PI values.

In general, the contamination is slightly lower than in previous fits, but by much less than the systematic error and barely noticeable in the plot. I suspect the change is due to details of fitting the bright lines with the improved FWHM vs. the "intrinsic" linewidth hack that I use in XSPEC, but I have not confirmed this.

For the contamination trend, I have not changed the exponential fits for XIS1,2,3 and XIS0 before April 2007. After April 2007, there is an altered version of the broken exponential compared to 2007 Dec 19 and 2008 Mar 3. For completeness, here are the full fitting parameters:

     N_C =  a*(1-exp((mjd0-MJD)/b))    for XIS1,2,3 and for XIS0 (MJD <= mjdbr)

     N_C =  a*(1-exp((mjd0-mjdbr)/b))
          + c*(1-exp((mjdbr-MJD)/d))   for XIS0 (MJD > mjdbr)
     # XIS0
     # broken exponential
     a              3.939
     b            270.069
     c              1.827
     d            238.417
     mjd0       53595.408
     mjdbr      54200.6
     
     # XIS1
     # single exponential
     a             4.678
     b           208.834
     mjd0      53595.379
     
     # XIS2
     # single exponential
     a             4.299
     b           117.472
     mjd0      53595.349
     
     # XIS3
     # single exponential
     a             5.795
     b            90.843
     mjd0      53595.348

with N_C = C column density in 1018 cm-2, C/O = 6 by number, and MJD is the Mean Julian Day. mjd0 is about 53595.4, or 2007-08-13, when the XIS doors were opened.

Attachments:

Contamination rate plot PDF

Best-fit "extra" linewidth vs. date. This measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). Open points indicate SCI-on data, crosses indicate SCI-off data. Data from Jan 2006 and later use RMFs generated with CALDB version rmfparam_20080121.

Linewidth trend plot PDF


2008 Mar 27

The 2008 March 15 data have been reprocessed with the updated FWHM parameters (makepi_20080131). The best-fit contamination for XIS0 for this date has dropped by ~ 3% as a result of the new processing (from 4.67 to 4.51 x 1018 C atoms cm-2). The 90% error bar on this data point is about 1.5%, so this is a significant change. XIS1 and XIS3 contamination values also change a bit with the new processing, but not as significantly.

Attached are the new rate plot (left) and the old rate plot (right, from 2008 Mar 26). Only the last data point has been reprocessed.

Attachments:

with new processing (last data point) (PDF) without new processing (PDF )

Best-fit "extra" linewidth vs. date. This measures how accurately the RMF reflects the "true" linewidth (as produced by the instrument and processing). Open points indicate SCI-on data, crosses indicate SCI-off data. The latest data point has been reprocessed with makepi_20080131.

Linewidth trend plot PDF


2008 Mar 26

Updated contamination history with 2008 March 15 observation of E0102. The data for the March and Feb 2008 points were processed by EDM and are missing some of the normal processing (attitude correction, for example), as described in 2008 Feb 20 entry. This processing uses gain parameters from ae_xi*_makepi_20071031.fits, not the latest version (20080131) which has updated FWHM parameters.

Also attached are a PDF file comparing spectra between Feb/Dec and Mar/Feb, including the ratio of each spectrum; and a PDF file checking for correlations between contamination and various fitting parameters (gain offset, intrinsic or "extra" linewidth, and RXJ0103 normalization).

Attachments:

Contamination rate plot PDF

Comparison spectra for Dec 2007, Feb 2008, and Mar 2008 (PDF only)
Correlation plots (PDF only)

2008 Mar 3

Updated fit to the recent XIS0 increase. This is the same as the 2008 Feb 20 plot, except the XIS0 post-April 2007 contamination trend is re-fit:

XIS0 is fit with a sort of broken exponential with a break around April 2007 (MJD 54200.6). The form is:

     N_C =   a*(1-exp((mjd0-MJD)/b))    for MJD <= mjdbr

     N_C =   a*(1-exp((mjd0-mjdbr)/b))
           + (c-a)*(1-exp((mjdbr-MJD)/d))   for MJD > mjdbr

with N_C = C column density in 1018 cm-2, C/O = 6 by number, and MJD is the Mean Julian Day. mjd0 is 53595.4, or 2007-08-13, when the XIS doors were opened. This is slightly different from the form used before for the broken power law, but only in the particulars of the parameter definition, not the functional form.

The parameters are:

# XIS0
# broken exponential
a              3.939
b            270.069
c              5.7
d            180.
mjd0       53595.408
mjdbr      54200.6

The other chip fits and that for XIS0 before April 2007 have not been changed.

Attachments:

Contamination rate plot PDF


2008 Feb 20

The attached plot (one of them) shows the XIS contamination history including the latest E0102 observation from 2008-02-15, after the CCD warm-up. The right-most points are from that observation, and the solid lines show the previous best-fit trends. There's clearly less contamination in XIS1 and XIS3, and XIS0 is stable from the last point (or lower than it would have been if it kept on its increasing trend).

The data used here were processed by me and are missing some of the normal processing (attitude correction, for example), but I don't think that's affecting things much. I've also attached a plot showing E0102 spectra from 2007-12-02 and 2008-02-15, and there is very little difference from one to the other. The gain and spectral resolution have not changed noticeably at these energies beyond the trends I've been seeing over the last year.

Attachments:

Contamination rate plot PDF

Comparison spectra PDF


2007 Dec 19

I've attached the latest rate plot. The dotted lines show the fit through April 2007 (the current CALDB version). Solid lines show the fits listed here.

Here are the new parameters for the XIS central contamination rate, assuming C/O = 6, as measured by E0102 through the 2007-12-02 observation. Most of the data are rev2.0.6.13, and I have reprocessed them to update the PI values as per the instructions on the web page. The model is the same as before, however I allow a variable gain offset and extra linewidth linked among the 25+ Gaussian emission lines. This drastically improves the fit for SCI-on data taken after early 2007, as I believe you are aware. The typical "extra" Gaussian sigma is 15-20 eV in all chips, and typical gain shift is +5 eV in XIS0 and less in the other chips. (So the energy of the data are too high compared to the model in XIS0.) I will compile this information and send it on, but probably not before the holidays.

XIS1,2,3 are fit with a single exponential function, as before:

     N_C = a*(1-exp((mjd0-MJD)/b))

with N_C = C column density in 1018 cm-2, C/O = 6 by number, and MJD is the Mean Julian Day. mjd0 is about 53595.4, or 2007-08-13, when the XIS doors were opened.

XIS0 is fit with a sort of broken exponential with a break around April 2007 (MJD 54200.6). The form is:

     N_C =   a*(1-exp((mjd0-MJD)/b))    for MJD <= mjdbr

     N_C =   a*(1-exp((mjd0-mjdbr)/b))
           + c*(1-exp((mjdbr-MJD)/d))   for MJD > mjdbr

The parameters are:

# XIS0
# broken exponential
a              3.939
b            270.069
c              5.562
d            762.720
mjd0       53595.408
mjdbr      54200.6

# XIS1
# single exponential
a             4.678
b           208.834
mjd0      53595.379

# XIS2
# single exponential
a             4.299
b           117.472
mjd0      53595.349

# XIS3
# single exponential
a             5.795
b            90.843
mjd0      53595.348

Attachments:

Contamination rate plot PDF


2007 Dec 12

Here is an updated contamination history, including the E0102 observation on 2007 Dec 02. The XIS0 contamination has not increased since October. It is consistent with no change, within the uncertainty, and in fact looks a bit lower.

I've included a freely-varying gain shift in the new analysis, so the other data points have changed a bit compared to one another, but not overall. The average gain shift (for the SCI-on data) ranges from +5 to +10 eV for XIS0 and XIS1, but much lower (+1 to +3) for XIS3. This likely causes a systematic error similar to the one I've shown on the plot, but not enough to explain the XIS0 contamination increase.

Attachments:

Contamination rate plot PDF


2007 Dec 5

I've analyzed the rev2 E0102 data through 2007-10-25. This is all rev2.1.6.15 or earlier (mostly rev2.0), so I followed the instructions on this web page to correct the PI values for the SCI-on data:

http://heasarc.gsfc.nasa.gov/docs/suzaku/analysis/sci_gain_update.html

I'm also using the latest CALDB and FTOOLS versions (as of today, at least).

I enclose the updated contamination rate plot. The lines are the same as the plot from 2007-10-01; the dotted lines are the trends from the original fit, and the solid lines are the trends fit to the 2007-10-01 update. I have not included any fits to these points yet.

The XIS0 contamination is increasing very quickly, with the latest sample almost near the XIS3 value. The enclosed spectrum (e0102_20061022_spec.pdf) verifies this:

green = XIS0 2006-10-22 black = XIS0 2007-10-25

blue = XIS3 2006-10-22 red = XIS3 2007-10-25

(Sorry for the poorly-annotated plot.) The effective area has clearly decreased for XIS0 over the last year, while it has stayed roughly constant for XIS3.

Contamination on XIS1 continues to increase slowly.

The data points prior to April 2007 don't match the lines very well (especially for XIS0). In the prior fitting, I used a single RMF for the SCI-on data (what you get from running xisrmfgen with a date of 2005-08-13). Now I am using xisrmfgen with the full SCI CTI parameters, and the results are not very good, with poor reproduction of the redistribution function at low energies (see attached figure e0102_20061022_xis0_spec.pdf). The line centers are not bad (off by less than 10 eV), but the wings are not reproduced well. This probably accounts for much of the XIS0 discrepancy from Oct 2006 to April 2007.

Attachments:

Contamination rate plot PDF

Comparison spectra PDF

XIS0 spectrum from 20061022 showing CTI effects PDF


2007 Oct 1

Attachments:

Contamination rate plot PDF


2007 May 7

Here are the new parameters for the central contamination rate, as measured by E0102 from rev1.2 data, assuming C/O = 6, and where

N_C = a*(1-exp(c-MJD/b))

with N_C = C column density in 10^18 cm^-2. Note that I'm using MJD now instead of "days since 2005-08-13". However I still assume zero contamination on 2005-08-13, so (c-MJD) = "days since 2005-08-13" from the old form. Note that c is not a fitted value, it's just the mean observing time of the 2005-08-13 data.

########################################################################
# as of Tue Apr 24 13:34:07 EDT 2007
# params are fitted through 2007-04-10, using rev1.2, new model
# new model = include HMXB RXJ0103, wilm abund, vern xsect

# XIS0
define a 3.76558987299718
define b 282.597273120363
define c 53595.4083
# XIS1
define a 4.77791776540853
define b 197.744159380601
define c 53595.379
# XIS2
define a 4.07951865562546
define b 105.966875089484
define c 53595.3503
# XIS3
define a 5.99819756132834
define b 86.5701015652393
define c 53595.3542

Here are the old parameters which are used in the current xissimarfgen, just FYI:

########################################################################
# as of 2006-08-25, using rev0.7, old model
# these params are used in xissimarfgen contami file
# ae_xi*_contami_20061016.fits

# XIS0
define a 2.88939156841589
define b 170.437176167262
define c 53595.4083
# XIS1
define a 4.57935004724336
define b 176.836204404981
define c 53595.379
# XIS2
define a 3.95873438224372
define b 102.612617255168
define c 53595.3503
# XIS3
define a 5.88313373917308
define b 87.1269963157196
define c 53595.3542

I've also attached the latest figure, although I think maybe you already have it. The dotted line is the old fit, solid line is the new fit.

Attachments:

Contamination rate plot PDF


2007 Apr 24

Here is the latest contamination plot from the E0102 data. This includes all data through the 2007-04-10 observation. Observations with SCI on have been included (and noted); these have been processed as per the web page instructions to remove the CTI correction. The processing is similar to the previous version of the plot.

The solid lines are exponential fits to all the E0102 data points. The dotted lines show the exponential fits used in xissimarfgen (specifically the ae_xi*_contami_20061016.fits CALDB files which observers are currently using). This was based on rev0.7 processing and a slightly different spectral model. XIS1 and XIS3 are still pretty close, while XIS0 has deviated quite a bit from the projected model.

I've included the PKS2155 cross-calibration results on the plot, but not in the fit. The XIS1 value is from the recent Sersic 159-03 paper (Werner et al. 2007, astro-ph/0704.0475) while the others are from the Ishida et al. memo. I'm currently processing RXJ1856 rev1.x to include that, though if anyone has done this I'd be happy to use their data.

The bottom panel shows the baseplate temperatures of each sensor, with the same color coding. XIS2 and XIS3 show a large excursion between MJD 54050 and 54100 (Dec 2006), presumably due to orbital effects. Intriguingly, the XIS3 contamination drops right after this. I'm not sure what to make of it.

Attachments:

Contamination rate plot PDF


2007 Feb 27

There are some differences in analysis between this plot and the previous one.

  • I have changed the emission model to include a second source which falls in the E0102 extraction region. It's a variable HMXB that dominates the spectrum above 2 keV, but contributes little to the flux below this. It does change the shape of the Bremsstrahlung continuum, which might account for the systematically higher contamination values I'm getting now.
  • all the data except the very first and last epochs are rev1.2; the first (2005-08-13) is rev0.7 and the last (2007-01-15) is rev1.3
  • the last data point is 2007-01-15, second to last is 2006-10-21, the several before that are spaced by about a month. (I've changed to MJD on the x axis, so this is probably less obvious.) There was an SCI-on observation in 2006 Dec which I haven't included.
  • the solid lines show the fit using the new emission model, rev1.2, all epochs
  • I haven't included RXJ1856 on this plot since I haven't re-fit the rev1.2 data
  • epochs 10,11,12 suffer from the CTI error for Aug-Nov 2006 data, and I haven't corrected this yet; I think it will only reduce the error bars

Attachments:

Contamination rate plot PDF

Spectral comparison PDF


2006 Oct 2

Here is an EPS version of the contamination plot. I've extended and refit the exponential function to include the 2006-08-25 data. Here are the new fit parameters, FYI:

[N_C/1018 cm-2] = a*[1-exp(-day/b)]

# fit parameters
# a units are 1018 cm-2
# b units are days
# day is number of days since 2005-08-13
chip         a         b
XIS0  2.889392 170.437176
XIS1  4.579350 176.836204
XIS2  3.958734 102.612617
XIS3  5.883134 87.126996

Attachments:

Contamination rate plot:
PDF


2006 Aug 4

Here is a quick update on the contamination with the latest E0102 observation, dated 2006-07-17. The FI chip values appear to be leveling off. I don't really understand the BI value, which is higher than before. I believe there are effects other than contamination that are driving this, and I'll try to summarize what I know for my talk on Tuesday at GSFC. The exponential fit for the BI ignores the 2005-08-31 data point, otherwise the best fit is basically a straight line, which fits quite poorly. The error bars are 90%, and this still assumes C/O = 6.

I'm in the process of estimating the A*Omega values for the latest points and some points in the future. If anyone else would like to do this as well for their favorite non-uniformity model, here are the central contamination fitting parameters:

chip  a     t0
-----------------
XIS0  3.01  183.
XIS1  5.30  222.
XIS2  3.99  105.
XIS3  5.83   85.1

where:

N_C = a*(1-exp(-t/t0))

with N_C = C column density in 1018 cm-2
     t   = time in days 

Attachments:

Contamination rate plot (PDF)
Comparison of rev0.6 and rev0.7 (PDF)
Contamination rate plot:


2006 Jul 12

Here are some contamination updates. First, I've analyzed the latest (2006-06-26) observation of E0102, and attached an updated contamination rate plot (first plot). The latest point is day 318, and it has not been included in the fit shown on the plot (dotted lines). The BI low-energy effective area has decreased since the previous (2005-05-21) observation, apparent from both the spectrum and the fitted contamination value. The FI effective area hasn't changed much, and is fairly consistent with extrapolation of the exponential function.

Second, here are some numbers to use for the contamination uncertainty. I've run four additional models on the 2006-04-16 data, and the table below shows the contaminant carbon column density that results. Two of the models (B & C below) bracket the assumed C/O ratio. As one would expect from the pure-C results, the contamination carbon column increases with higher C/O ratio and vice versa. The other two models (D & E) adjust for possible energy-independent changes in the effective area, as perhaps seen in the Crab data. D assumes a 5% reduction, E 10%, and the fitted carbon column decreases.

The A model is the "default" C/O = 6, fixed normalization version. The second and third tables list the difference and ratio (respectively) between each result and the default model.

Some additional sources of error that you've probably thought of:

  • using rev0.7 data with the current model (e.g. in xissimarfgen), since those parameters are based on the rev0.6 data. The fitted contamination is lower for rev0.7 than rev0.6 by about 0.4 x 10^18 cm^-2 for the FIs, 0.2 x 10^18 cm^-2 for the BI. (See the second attached plot, which shows the difference vs. time.)
  • uncertainties in the E0102 model, especially the SMC absorbing column. I think I've reduce this a bit by assuming zero contamination in August 2005 and fitting the model to that data, but I estimate the systematic error to be ~ +/- 0.2 x 10^18 cm^-2.
  • uncertainties in the fitted time evolution function. The broken line model is a poor fit, depending on when the science observations are taken.
# E0102 - contaminant N_C
# DATE 2006-04-16
# DAY 246.088
# rev0.7 processing
# units are 10^18 C atoms cm^-2
# A:  C/O = 6, norm = 1 (default fit)
# B:  C/O = 3, norm = 1
# C:  C/O = 12, norm = 1
# D:  C/O = 6, norm = 0.95
# E:  C/O = 6, norm = 0.90

    XIS0 err    XIS1 err    XIS2 err    XIS3 err
------------------------------------------------
A  2.40 0.08   3.59 0.05   3.56 0.09   5.48 0.11
B  1.87 0.07   2.81 0.04   2.77 0.07   4.26 0.08
C  2.79 0.10   4.17 0.06   4.15 0.11   6.42 0.13
D  2.02 0.08   3.28 0.05   3.16 0.09   5.04 0.11
E  1.63 0.07   2.96 0.05   2.76 0.09   4.60 0.10

# residuals compared to default N_C
# N_C(X) - N_C(A)
     XIS0   XIS1   XIS2   XIS3
-----------------------------
B  -0.53  -0.78  -0.79  -1.22
C   0.39   0.59   0.59   0.94
D  -0.38  -0.31  -0.40  -0.44
E  -0.77  -0.62  -0.80  -0.88

# ratio compared to default N_C
# N_C(X) / N_C(X)
     XIS0   XIS1   XIS2   XIS3
-----------------------------
B   0.78   0.78   0.78   0.78
C   1.16   1.16   1.17   1.17
D   0.84   0.91   0.89   0.92
E   0.68   0.83   0.77   0.84

Attachments:

Contamination rate plot (PDF)
Comparison of rev0.6 and rev0.7 (PDF)
Contamination rate plot:


2006 May 24

I've constructed a new empirical model for the on-axis contamination evolution, this time assuming DEHP (C24H38O4, or C/O = 6 by number). Attached please find an updated history plot, as well as all fitted spectra for the two sources I used, E0102 and RXJ1856. Errorbars on the history plot are 90% confidence.

For the fitting, I fixed C/O = 6, and only fit above 0.3 keV (0.3-1 keV for RXJ1856, 0.3-2 keV for E0102). I initially included a variable, energy-independent normalization factor in each fit, since the 1-2 keV region of E0102 shows a slight (~5-10%) countrate decrease which is difficult to explain from a simple C24O4 contaminant. The fitted normalization and contamination turned out to be highly correlated, however, so I abandoned this and froze the normalization for all chips and epochs for each object.

The evolution model is a broken line, with the break set to the 20051216 observation. This seems to be the simplest (artificial) model given the sparse time samplings. Here are the parameters, first in column density of C and then in the xspec varabs C parameter (I'm no longer using "depth" in microns):

# C column density
# slope = 10^18 cm^-2/day
# inter = 10^18 cm^-2
# break @ day 126.3
chip    slope1    inter1    slope2    inter2
XIS0  0.012611  0.351729  0.008149  0.912850
XIS1  0.020215 -0.009724  0.013369  0.794435
XIS2  0.025753  0.022866  0.007130  2.363714
XIS3  0.030836  1.029650  0.009726  3.682819

# C varabs parameter, assuming angr
# slope = varabsC/day
# inter = varabsC
# break @ day 126.3
chip    slope1    inter1    slope2    inter2 
XIS0  0.003474  0.096895  0.002245  0.251474
XIS1  0.005569 -0.002679  0.003683  0.218852
XIS2  0.007094  0.006299  0.001964  0.651161
XIS3  0.008495  0.283650  0.002679  1.014551

Note that varabsO = 0.07109*varabsC when using angr abundances.

Some notes:

  • E0102 and RXJ1856 agree when fitting for contamination above 0.3 keV, but there remains a very large discrepancy in the C-band below 0.3 keV. In the attached RXJ1856 spectra (rxj1856_comparison.pdf), you can see that the C2404 model fits the Oct C-band data much better than March. From Oct to March the C-band count rate dropped by 50%, and a C+O absorber can't do this. E0102 possibly shows the same trend, although the count rate is very low. So perhaps the contaminant is more complex, containing heavier elements than C+O, or there's some other instrumental problem. Observers using the C-band should not rely on this contamination model.
  • I have made no correction for energy-independent changes. The model normalizations are fixed for all detectors and epochs, which is probably incorrect given the recent Crab results etc., but there's not much contamination-free bandpass to provide leverage in these sources. This is a source of systematic error.
  • For these reasons, it would be useful to get observer feedback and include other sources in the fit.

The XIS team is preparing an update to the spatial-nonuniformity model, and Keith Arnaud will soon release a new xspec model akin to xisabs incorporating this model. Also, another E0102 observation has been completed and will be analyzed soon.

Attachments:

Contamination rate plot (PDF)
E0102 spectra (PDF)
RXJ1856 spectra (PDF)
RXJ1856 C-band comparison (PDF)
Contamination rate plot:


2006 May 10

I'm posting the results of the most recent observation of E0102 (2006-04-17). The XIS team has been discussing this via email, which is why this has taken a while, and discussions continue. I've attached a number of figures. Thanks to Hayashida-san, Mark, and Hamaguchi-san for their help with this.

Figure 1 shows the results of my initial fit to the new E0102 data, compared to the recent RXJ1856 and previous E0102 results and assuming pure carbon contamination. There is a clear discrepancy between E0102 and RXJ1856 at recent times. The errorbars for RXJ1856 reflect some discrepancies in the fits, based on (mostly) which energy range is included. This is still being discussed within the team.

Figure 2 shows the equivalent C depth if DEHP is the contaminant (C24H38O4, so O/C = 1/6 by numer). Adding O has greatly reduced the necessary C in the E0102 fit, but hardly changed the C in the RXJ1856 fit. Note that this is only the C column density, the O also produces absorption, but is not represented directly on the plot. There is still some discrepancy between the columns derived from the two sources, but it is within the systematic error.

It seems that the "C depth" inferred from E0102 is much more sensitive to the amount of O in the contaminant than the "C depth" inferred from RXJ1856. The reason for this can be seen in Figures 3 and 4. Figure 3 shows the count rate spectra of RXJ1856 (black) and E0102 (red) for the BI. Most of the flux of E0102 is above the O edge near 530 eV. Most of the flux of RXJ1856 is below this. Figure 4 shows the transmission of the two different contamination models used to fit the most recent E0102 point for XIS3 (the last blue point in Figures 1 and 2). The black line assumes C only (for the point in Figure 1), and the red line assumes O/C = 1/6 (for the point in Figure 2). The tranmission is identical above the O edge at 530 eV; therefore both of these contaminant models fit the E0102 data above 530 eV. Below this there is a discrepancy, but the count rate from E0102 is very low in this region. So in this source, absorption due to O is easily mimicked by pure C absorption, except when there are sufficient counts below 500 eV. (In the few obs with sufficient counts, O/C is constrained to be 10-20%. In most of the obs, only a 90% upper limit of 20% is obtained.)

RXJ1856 samples lower energies and the C edge well, and the transmission from O and C is obviously different here, so including O in the contaminant will not greatly change the amount of C.

The last four figures show normalized count rates for three bright emission lines. I've also included values for RXJ1856 and Eta Carina, which have been scaled to the best-fit count rate for the initial observation. This is a model-free way of showing the effective area changes, and a more consistent picture emerges. One might believe that the degradation is leveling off, especially for the worst-affected sensors XIS2 and XIS3. (Note that RXJ1856 and Eta Car count rates are for similar energy bands as the E0102 emission lines; they don't exhibit the lines themselves.)

In summary, the discrepancy between E0102 and RXJ1856 (and probably other sources) might be due to the spectral regions being fitted and the assumed contamination composition. Incomplete knowledge of the response at these energies also complicates things, especially for the BI below 300 eV.

The XIS team is working to produce the best contaminant model for users to employ in their spectral analysis. In the meantime, I (personally) have these caveats, which are open for discussion:

  • using the current C-only empirical model (xisabs) is incorrect for energies between the C and O edges (about 280 to 530 eV)
  • using xisabs for energies only above 530 eV may be okay for times before 2006-01-01
  • users should be careful about using a particular contaminant model to fit a broad range at low energies; in particular, the C-band, the 300-500 eV range, and above 500 eV are very sensitive to the details of the contaminant composition (as one would expect)

Attachments:

PDF figures (8 pages)
Figure 2 (rate plot):


2006 Feb 1

Here is an analysis of the XIS contamination time dependence, as measured with four observations of 1E0102-72:

obs. date    exposure (ksec)
2005-08-13   1.5-4.0
2005-08-31   24
2005-12-16   64
2006-01-17   9.0

The contamination was measured as described in my previous XOOPS posting. This has been emailed to the SWG, but I include it here for completeness.

A more thorough description of the analysis is forthcoming. The attached figure shows the best-fit contamination values for each epoch and detector (with errorbars), as well as linear fits for each detector. The t=0 data (2005-08-13) were assumed to have no contamination; they were included in the fits anyway. For comparison, I've included the RXJ1856 results of Hayashida-san (filled circles). They line up reasonably well. The rates for XIS0,1,2 are consistent with being linear. XIS3 shows a fast increase during August, and a possible turnover at later times.

Only carbon was included in this model. Including oxygen does not improve the fit, and amounts are consistent (within large errors) with zero. The oxygen contamination is estimated to be 10 % (+-5%) of the carbon contamination based on the analysis of RXJ1856. However, this result is affected by the gain shift and needs further investigation for modeling.

Note that these results and the empirical formula below are only valid for point sources observed at the XIS nominal aimpoint. The extraction region was the standard 6mm point source aperture assumed by the ARFs.

CONTAMINATION TIME DEPENDENCE

The best fit lines have the following parameters plus errors, with zero time defined as 2005-08-13 (177242000 seconds since 2000-01-01, MJD 53595.412):

change in effective carbon column density
                          XIS0       XIS1       XIS2       XIS3
slope (1e16 cm^-2/day)    1.6 +0.1   2.7 +0.1   3.1 +0.1   4.1 +0.5
intercept (1e16 cm^-2)    4.4 +4.0  -9.6 +15   -3.2 +14    54. +50.

The easiest way to include this absorption component within XSPEC is to use the "varabs" model. This enables one to set the H column to zero, which cannot be done completely with the "vphabs" model, and also have only one parameter for each absorbing element. See the XSPEC help for more information. The following time models show the change in the varabs(3) parameter, assuming the "angr" abundance table (C/H = 3.63e-4) and the same zero time as above:

change in varabs carbon parameter
                       XIS0    XIS1    XIS2    XIS3
slope (varabsC/day)    0.0043  0.0073  0.0085  0.0114
intercept (varabsC)    0.012  -0.026  -0.009   0.148

The time2day and time2sec ftools (and friends) are helpful in determining the time offset of a particular observation.

Attachments:

PDF file


2006 Jan 24

Here are preliminary results for the latest E0102 observation. I've fitted spectra for XIS1 and XIS3 (see attached). Here are the numbers for the contamination, which has increased (in fitted absorption depth) by about 15%, a difference of about 2 sigma (fit parameter error).

changes from 2005-12-16 to 2006-01-17:
               XIS1   XIS3
C depth:       +17%   +13%
gain shift:    -4.3ev -1.4ev

I've included my whole original table, with the XIS1 and XIS3 numbers updated. The spectra show a pretty obvious decline in the O line intensities, anyway, as well as the low energy continuum. This is more obvious in the BI than the FI.

---------------------------------------------------------------
surface density of carbon (10^-5 grams/cm2)

date       XIS0          XIS1          XIS2          XIS3
20050813   0             0             0             0
20050831   0.9 +-0.2     0.2 +-0.2     0.5 +-0.3     1.9 +-0.2
20051216   2.6 +-0.3     6.4 +-0.5     7.6 +-0.4    10.4 +-0.6
20061217                 7.4 +-0.4                  11.6 +-0.6

---------------------------------------------------------------
thickness (microns; assumes density of 2.2 g/cm3)

date       XIS0          XIS1          XIS2          XIS3
20050813   0             0             0             0
20050831   0.04 +-0.01   0.01 +-0.01   0.02 +-0.01   0.08 +-0.01
20051216   0.12 +-0.01   0.29 +-0.02   0.35 +-0.02   0.47 +-0.03
20061217                 0.34 +-0.02                 0.53 +-0.03

---------------------------------------------------------------
gain shift (eV)

date       XIS0          XIS1          XIS2          XIS3
20050813   -6.3 +-1.0     1.7 +-0.7    -6.8 +-0.8    -5.1 +-0.9
20050831  -11.4 +-0.4   -11.5 +-0.3   -11.2 +-0.4   -10.9 +-0.4
20051216  -12.7 +-0.4   -15.4 +-0.2   -14.4 +-0.4   -13.4 +-0.4
20061217                -19.7 +-0.4                 -14.8 +-0.7

Attachments:

PDF file of all spectra
 
 

2006 Jan 18

Here is a first analysis of the three observations of 1E0102-72.3, in an attempt to characterize the contamination. I have attached spectra showing the model fit for the 2005-08-13, 2005-08-31, and 2005-12-16 observations for each detector. Here are some measures of the contamination and gain "shift", with caveats as mentioned below. Fitting errors are all in the 10% range, although systematic errors are likely larger and less well-determined.

---------------------------------------------------------------
surface density of carbon (10^-5 grams/cm^2)

date       XIS0          XIS1          XIS2          XIS3
20050813   0             0             0             0
20050831   0.9 +-0.2     0.2 +-0.2     0.5 +-0.3     1.9 +-0.2
20051216   2.6 +-0.3     6.4 +-0.5     7.6 +-0.4    10.4 +-0.6

---------------------------------------------------------------
thickness (microns; assumes density of 2.2 g/cm^3)

date       XIS0          XIS1          XIS2          XIS3
20050813   0             0             0             0
20050831   0.04 +-0.01   0.01 +-0.01   0.02 +-0.01   0.08 +-0.01
20051216   0.12 +-0.01   0.29 +-0.02   0.35 +-0.02   0.47 +-0.03

---------------------------------------------------------------
gain shift (eV)

date       XIS0          XIS1          XIS2          XIS3
20050813   -6.3 +-1.0     1.7 +-0.7    -6.8 +-0.8    -5.1 +-0.9
20050831  -11.4 +-0.4   -11.5 +-0.3   -11.2 +-0.4   -10.9 +-0.4
20051216  -12.7 +-0.4   -15.4 +-0.2   -14.4 +-0.4   -13.4 +-0.4

These numbers and the attached spectra seem to show the contamination is getting worse for most of the chips, compared to the RXJ1856 observations in October. XIS0 seems to have changed little. Also, there's evidence for contamination as far back as August, akin to what Takei-san sees in his first post.

The model, provided by Paul Plucinsky of CfA, is a bremsstrahlung continuum with 24 Gaussian emission lines culled from Chandra and XMM grating results and adapted (by Paul) to fit the ACIS data and track changes in the response. There are two absorption components, one well-constrained Galactic component and one poorly-constrained SMC component. The SMC column is probably the major systematic error. I have also included a varabs component with only carbon for the contamination.

For each XIS detector, I have fit the model to all three observations simultaneously. The contamination absorption column of the 0813 spectrum was fixed at zero, the other two were allowed to float independently. I fixed the emission line centers, but needed a width (sigma) of about 15 eV to help the spectral resolution come out right. There appear to be some residual problems with the line widths in the spectra, in any event. Finally, I allowed the line and continuum normalizations to vary (tied between the observations), as the fit was poor using the straight ACIS values. The fitted values are generally within 10% of the ACIS model numbers.

Thus I have effectively used the (short) 0813 observations as a zero-contamination baseline for the later two observations. The chip temperature was a bit higher for this dataset (-80 vs. -90 C), but ground tests show this should have little impact on the QE of the FI chips (2% or less). The BI chip is a different story, but probably the QE at -80 C is within 20% of the -90 C value near 0.5 keV (although there's very little information to back up that statement).

Attachments:

PDF file of all spectra
 
 

Presentations

Click on a preview image to download the full PDF presentation file.


Poster from SWG Meeting, ISAS
2006 Feb 17

Talk from US SWG Meeting, GSFC
2006 Aug 08

Poster from AAS Meeting, Honolulu
2007 May


MKI     
MIT
Last updated: Thu Jan 29 10:21:49 EST 2015
email: milleric@mit.edu