ACIS time-dependent gain

ACIS Time-dependent Gain

New time-dependent gain corrections for the periods February-April 2006 (Epoch 25) and May-August 2006 (Epoch 26) were released in CALDB 3.2.4 (07 November 2006). Three existing TGAIN files were updated as well. The How CALDB 3.2.4 Affects Your Analysis section of the CIAO 3.3.0.1 release notes explains how they will affect your analysis.

Summary

Recent work has shown that the ACIS gain - the mapping between PHA channels and energy - is time dependent due to an increase in the CTI of the CCDs and evolution of the electronic gain of ACIS-I2.

For information, on how to correct your data, see the Apply the Time-Dependent ACIS Gain Correction thread.

Technical details can be found in the Corrections for Time-dependence of ACIS Gain calibration page.

What Analyses are Affected?

Users interested in CCD spectroscopy should apply the gain adjustment. Grating spectroscopy benefits as well, in the form of improved order sorting. It is not necessary to apply this adjustment if you are only doing timing or imaging analysis, but it will not have a negative effect, either.

For details on a specific release, refer to the CIAO release notes.

Determine the Size of the Adjustment

The show_tgain_corr script displays the size of the time-dependent gain correction - i.e. how much the energy or PHA value of an event changes - for a given source location. The most recent version of the show_tgain_corr script is v1.4 (21 Sept 2004):

unix% grep Id `which show_tgain_corr`
% $Id: show_tgain_corr,v 1.4 2004/09/21 21:12:17 dburke Exp dburke $

show_tgain_corr calls a helper script named show_tgain_corr.sl. The most recent version of that script is v1.5 (15 May 2006):

unix% grep Id $ASCDS_CONTRIB/share/slsh/local-packages/show_tgain_corr.sl
% $Id: show_tgain_corr.sl,v 1.5 2006/05/15 14:32:49 egalle Exp $

Note that $ASCDS_CONTRIB/share/slsh/local-packages/ is the default path in the standard CIAO scripts installation. If you do not have the scripts installed or need to update to a newer version, please refer to the Scripts page. To run show_tgain_corr, supply an ACIS chip number and the desired location in chip coordinates, e.g.

unix% show_tgain_corr 3 962 964

unix% show_tgain_corr 7 221 532

These command create a plot of the adjustment in energy for an on-axis source observed with chip I3 and chip S3, respectively. The plot compares the newest TGAIN files to the previous default versions.

The script includes options to plot in PHA and to store the output as a PS or GIF file; the help file for the script (ahelp show_tgain_corr) has more information.

Improvements in Calibration

CALDB 3.2.2 (15 May 2006)

New time-dependent gain corrections for the May 2005 - November 2005 time period (Epochs 23 and 24) were released in CALDB 3.2.2 (15 May 2006). Note that unless you are fitting a spectra with oxygen emission lines, the gain refinement is unlikely to have an effect on the spectrum larger than the uncertainties in determining the gain.

The How CALDB 3.2.2 Affects Your Analysis section of the CIAO 3.3.0.1 release notes explains how the new calibration will affect your analysis.
CALDB 3.2.4 (07 November 2006)

New time-dependent gain corrections for the periods February-April 2006 (Epoch 25) and May-August 2006 (Epoch 26) were released in CALDB 3.2.4 (07 November 2006). Three existing TGAIN files were updated as well. The How CALDB 3.2.4 Affects Your Analysis section of the CIAO 3.3.0.1 release notes explains how they will affect your analysis.