Other articles

In reverse chronological order (most recent ones first)

1. GIS gain calibration of recent observations This supersedes earlier (2001 Apr) reprocessing and affects more sequences (June 26, 2002)
2. Notice on the recent GIS gain correction(November 3, 1999)
3. New GIS blank sky event lists released(October 7, 1999)
4. Status of the GIS Gain Calibration (November 21, 1997)
5. Correction of the Long Term Gain Variation (July 16, 1997)
6. GIS Background and Data Selection Criteria (original announcement on Feb. 10 1996, updated on Feb. 20, 1997)
7. GIS spectral calibration with the Crab Nebula in 1994 (original announcement in Jan. 1995, updated on July 31, 1996)
8. GIS3 gain new correction method (July 10,1996)
9. GIS3 gain mis-correction after August 1995 (April 12, 1996)
10. New GIS RMFs released (March 7, 1995)
11. Warning about the GIS background files (original announcement on December 14, 1994, updated on October 7, 1999)

1. The radial gain correction formalism is changed from 2nd order to 4th order polynomial function (GIS-S2).

   The raw pulse-height channel (PHA) for each GIS event is converted into energy channel (PI) through the following processes (1)-(3) (see also the Status of the GIS calibration page):
   (1) Correct temperature dependence of the PMT gain
   (2) Correct position dependence of the PMT gain
   (3) Compensation of secular changes of the gain map
   

   The gain map correction factors are provided through tables contained in the "temp2gain.fits" file. The correction factor of (3) had been described with a 2nd order polynomial function of radius, so far. (The 3rd and 4th order factor has been installed in the software but has not been used.)

   In figure 1, we show the center energy (PI) of the copper-K line versus the radius for GIS-S2 since the launch. Circles indicates the PI channels calculated using the old 2nd order polynomial correction functions. The solid line in the panels indicate the nominal energy of the line, and the each panel corresponds to half year terms, as April to September 1993 (1), October 1993 to March 1994 (2),..., October 1998 to March 1999 (12). We see in the last three terms a systematic discrepancy. It becomes 0.8 % at most in the outer region ( 60 ch < r < 70 ch) of the field of view for this half year, although the discrepancy is less than 0.5 % at 1CCD nominal position. Although we have to note that we have systematic uncertainty of the GIS gain of +/- 1 % (region between dashed lines), the residual structure indicates that the 2nd order polynomial formulation no longer describes the radial trends well. Therefore we adopt 4th order polynomial descriptions and succeeded to compensate them in the accuracy of < 0.5 % (figure 1 asterisks).

   Please note again that the discrepancy of the Cu line energies for the recent data expected by using the old (2nd order) gain map is at most ~0.8 % (figure 1), which is smaller than the +/- 1 % systematic error. Therefore, Guest Observers will not have to reprocess their data for most practical purposes. In the newly released table, we provide the 4th order coefficients for the data since 1997 October. The GSFC/ASCA GOF will reprocess the data after that occasion to revise the archival data as well as the newly processed data but will not be re-distribute to observers. The old and new correction factors are shown in figure 2.

2. Azimuthal gain fluctuation at the rim region of GIS-S3

   The GIS team warns that gradual degradation at the rim region (r > 23 arcmin or r > 90 channel) of GIS-S3 is causing an azimuthal gain fluctuation. The discrepancy is observed in the region outside 90 channel from the detector center and could be up to 0.5 % nominally though we see double peak feature from the azimuthally integrated copper-K emission line obtained after 1998 July 1 (figure 3).

   However, please note that as a default events from the detector region r > 88 channel are not assigned the sky coordinates by "ascalin" and thus truncated in the standard pipe-processing and "ascascreen". Therefore this warning would apply only to the Guest Observers who have reprocessed their data by running 'ascalin' by themselves in order to revive the event from the outermost region.

Here is the short description of how the new correction method works (see the ASCA Newsletter No.5 article by Idesawa et al. [ html or postscript] for details). At the beginning of the mission when the gain maps were correct, no additional correction was needed. As time has passed, the positional gain variation has become gradually significant, such that the central part needs higher correction factor than the outer parts. The position dependent gain correction factor is described by parabola function of the distance from the detector center. The coefficients of the parabola are determined for each sensor every six months, and written in the "gis_temp2gain.fits" file as well as the parameters to determine the temperature dependence of the gain. "Temp2gain" interpolates those polynomial coefficients and determine the instantaneous coefficients for a particular observation sequence. The coefficients thus determined are written in the header of the gain history file (*.ghf), and "ascalin" uses these values to carry out the positional gain correction.

For Guest Observers who have received data which were processed before May 1997, there are three ways to cope with the GIS gain problem when they found the problem is significant. First, you may wait for your data to be reprocessed with the latest and most reliable processing system. We are continuously reprocessing all the ASCA data with a new processing system named REV2. Second, if you are concerned with the overall gain shift, but not with the positional dependence, you may redetermine the PI values applying new gain and off-set values (click here to see how to do that). Third, you may process the data by yourself using ascalin v0.9t (or newer) and the new gain history file in which positional gain correction factors are calculated. Temp2gain, the processing program to produce the gain history file is not released to public, but we can run temp2gain for particular sequences on requests and create the new gain history files having the positional gain correction factors. If you are interested in this option, please contact the ASCA GOF: visit the Feedback form.

NOTE: GIS gain map correction

E.Idesawa, H.Kubo, Y.Ishisaki, K.Ebisawa, Y.Fukazawa, M. Ishida, K.Makishima, M.Tashiro, Y.Ueda and the GIS team

As we reported on April 13, 1996, the GIS-S3 gain apparently becomes over-estimated towards later epochs of the mission. This was first noticed through the calibration using ionized iron lines from the SNR Cas A. We further investigated this issue over the 25 sectors (azimuthal 8 sectors times radial 3 sectors and center) of the GIS-S3 detector plane, using the instrumental copper line which can be detected over the entire detector plane after sufficiently long integration times.

We confirmed that the overestimation is more pronounced in the central region than in the rim region of the GIS-S3 field of view, and that the effect increases with time. These effects are thought to originate from slight gain decreases in the rim region of GIS-S3, where the calibration isotope is attached. This in turn may arise either from some decrease in the ultraviolet transmission of the quartz window of the gas cell, or to a slight performance drift in the imaging phototube. The effects are anyway very small, and corrections can be made with a sufficient accuracy to the data from both past and future observations. We find no evidence of azimuthal deviation of the gain from the original positional gain correction map (gain map).

The deviation from the original gain map is well described with a parabolic function with respect to the radius from the detector center, and with a linear function of the ascatime. The coefficients for the parabolic function have been determined for each year, using the averaged day- and night-earth data; thus we can correct the gain map by interpolating these coefficients.

A new version of the 'ascalin' to create the CORRECTED PI is under development by ASCA GOF in collaboration with the GIS team. For users' convenience we indicate below the analytic form of the gain map correction function to be implemented in the new ascalin.

We have applied the same method to the GIS-S2 data as well, and found no noticeable deviation from the original gain map within 1%. However for consistency, we have also applied the same investigation to the GIS-S2 data and derived the nominal correction coefficients (which are consistent with "no correction" within the errors).

---------------------------------------------------------------------

The gain map compensation function is,

	new-PI = old-PI / (correction factor),
	correction factor = (A2 * r**2 + A0)*ascatime
	                                      + (B2 * r**2 + B0),

where

	r**2 = (DETX - DET_XCEN)**2 + (DETY - DET_YCEN)**2 ,
	DET_XCEN= 128.5 (PIXELS), DET_YCEN= 128.5 (PIXELS).

Coefficients A0, A2, B0 and B2 for GIS-S2 and GIS-S3 are shown in table below. Note that A2=A0=B2=0 and B0=1 corresponds to "no change to the original gain map".

------------------------------------------------------------------------
sensor	A2		A0		B2		B0
------------------------------------------------------------------------
S2	-3.945E-15	+1.090E-11	+2.851E-08	+1.004E+00
------------------------------------------------------------------------
S3	-3.871E-14	+2.974E-10	+6.128E-07	+9.970E-01
-----------------------------------------------------------------------

We have confirmed that this correction compensates the gain change to an accuracy of 1%, within 90 GIS-pixels from the detector center.

NOTE: GIS-S3 gain mis-correction

by the GIS team (M. Tashiro)

Our calibration with iron lines from the SNR Cas A reveals that the absolute gain of GIS-S3 is apparently overestimated by 1 - 2 % since around August of 1995. The deviation is gradually increasing, and iron-K line center energy exceeds those obtained by GIS-S2 and SISs by about 1 % since the second half of 1995. There is no problem with GIS-S2.

This gain drift of GIS-S3 is not real, but likely to originate from the current gain correction algorithm. The GIS team investigated the phenomenon and found that the deviation is likely to occur in the process determining the long-term gain history with pulse-height peak of the calibration isotope (55Fe) attached to the GIS-S3 sensor. It is suspected that the long-term gain decrease of GIS-S3 is slightly more pronounced at the fov rim than at the fov center, so that the gain correction factor calculated using the calibration isotope (attached at the fov rim) tends to be slightly overestimated.

We are working to calibrate the newly discovered effect, using not the GIS-S3 calibration source but the intrinsic copper-K line. When it is established, we will release the function which will describe the long-term gain drift due to this effect and the corrected version of gis_temp2gain.fits in which the corrected absolute gain will be written.

Guest observers who found the GIS-S3 gain problem can take the following methods to cope with the problem:

1. Wait release of the new gis_temp2gain.fits, and run 'ascalin' on GIS-S3 event files using this file. This will be the best way, but not available soon.
2. In the 'xspec' session, use 'gain' command to see the effect of the gain change by changing the energy scale a posteriori. This is the easiest, and the gold-M edge feature at 2.2 keV, which originates in the XRT effective area, may be a fiducial mark to estimate the correct gain. The data of long-term gain drift due to the effect, which will be released by the GIS team, will also be a guideline for this 'manual' correction. However, please be careful that this method is not precisely correct, since spectral features produced by the non-linearity of the energy-channel relation (e.g. GIS xenon-L feature at 4.8 keV) cannot be corrected in this manner.
3. GOF is going to release a new ftool, 'gispi', with which users can redetermine the PI values of the input GIS event file using a user-input gain value (which may be fixed, e.g., using the xspec gain command). Using the corrected event file, users can re-extract the spectral file having a correct energy scale.

The GIS team has just released version 4.0 of the GIS RMF. The two new response matrices are called gis2v4_0.rmf and gis3v4_0.rmf, and are available from the heasarc.gsfc.nasa.gov anonymous FTP account in the directory /caldb/data/asca/gis/cpf/95mar06. They supersede the response matrices released on 1994 April 20 (gis2v3_1.rmf and gis3v3_1.rmf).

In these previous versions, the low-energy tail of the pulse-height distribution was underestimated, causing an artificial soft-excess to appear in spectra with high absorption and low-energy photons. In the new RMFs, the low-energy response has been modified by taking account of ground calibration results and new theoretical calculations. The artificial soft-excesses disappear with the new RMFs.

However, the response at energies above 4 keV has not been changed. This means that the high energy response problems noted in the Calibration Uncertainties (systematic residuals around 6 keV and above 8 keV) remain unsolved. But note that these residuals are about 5 per cent, and are dominated by statistical errors for sources of several counts per second or less.

For the new RMFs, in common with previous versions:

• The GIS RMF converts energy (keV) to output channel (PI: Pulse Invariant, i.e., gain-corrected PHA).

• RTI-based background rejection is assumed.

• The efficiency of the RMFs is normalized to unity: they should be used in conjunction with an ARF file which contains the XRT/GIS efficiency.

• The GIS2 and GIS3 RMFs are the same except for identifying keywords in the headers.

Charles Day, Ken Ebisawa (ASCA GOF) & Makoto Tashiro (GIS Team, University of Tokyo)

Due to the release of the new GIS blank sky event lists, the original warning announcement on the use of the old blank sky event database has become obsolete. ASCA users are encouraged to use the new GIS blank sky event lists found at ftp://heasarc.gsfc.nasa.gov/caldb/data/asca/gis/bcf/bgd/no_sources. The original announcement is kept for archival purposes.

This file was last modified on Tuesday, 19-Oct-2021 16:25:07 EDT