# Status of the GIS gain calibration

Last updated on November 19, 2002

- Accuracy of the GIS gain determination
- Variation of the calibration source peak energies
- Method of the gain correction
- Limit of the radial gain correction
- Recalculate PI values using new calibration

## Accuracy of the GIS gain determination

Accuracy of the absolute GIS energy scale is being monitored using the instrumental copper fluorescent line at 8.05 keV. The background spectra are accumulated using the dark-earth data for each about 12 months, and the detector region is divided into 10 concentric rings. The line energy is determined for each energy spectrum extracted for each period from each ring.The line energies for different periods and for each concentric ring are shown in the figure below (postscript file is available).

Range of the ordinate for each diagram is 7.9-8.2 keV, and the horizontal dotted lines indicate +/-1 % of the correct energy (8.048 keV). Abscissa is the radius (in channel) of the concentric rings. In the GIS2 diagrams for the periods 8 to 12, circles are determined with the parabola radial correction, while asterisks are due to the 4-th order polynomial correction (see below for the radial correction). The diagrams from 1 to 12 correspond to the following overlapping periods:

Data # | Data Period |
---|---|

1 | 1993/04-1993/12 |

2 | 1993/07-1994/06 |

3 | 1994/01-1994/12 |

4 | 1994/07-1995/06 |

5 | 1995/01-1995/12 |

6 | 1995/07-1996/06 |

7 | 1996/01-1996/12 |

8 | 1996/07-1997/06 |

9 | 1997/01-1997/12 |

10 | 1997/07-1998/06 |

11 | 1998/01-1998/12 |

12 | 1998/07-1999/06 |

It is seen that the line energy is within +/- 1 % of the correct value
for all the periods and rings. Therefore, ** we conclude that accuracy of the current
GIS energy determination is within +/- 1 % at 8.05 keV** over the detector,
at least until l 1999 June.

## Variation of the calibration source peak energies

GIS has the iron 55 calibration sources attached at the detector rims. They produce fluorescent lines at 5.89 keV (K-alpha) and at 6.49 keV (K-beta). The calibration source peak is being monitored by the GIS team at Univ. of Tokyo/ISAS and by the GSFC ASCA Processing team, Systematic energy shift from the correct energy and the secular energy variation indicate the current limit of the GIS gain determination. Note that the calibration sources are used to correct time variations of the gains, but not used to determine the absolute gain, since gain drops rapidly near the detector rim where absolute energy determination is inevitably difficult.## Method of the gain correction

There are four components to take into account in order to determine the GIS gain and correct energies for the GIS events:### Short-term variation

The short-term gain variation is known to be caused by temperature variation of the sensor due to day-night transitions and/or satellite aspect changes. This can be corrected using the temperature housekeeping parameters. For each observation sequence, a GIS gain history file is created by the`temp2gain`ftool by looking at variation of the temperature housekeeping parameters. The gain history file, which is named like

`ft970210_0223_1750.ghf`, has the gain values for every 600 sec, for which the following long-term variation and positional variation are taken into account. The

`ascalin`ftool uses the gain history file to carry out the PHA to PI conversion.

### Long-term variation

The following figure indicates the long term variation of the raw pulse-heights of the calibration sources at fixed temperatures (postscript file is available).

The long term gain change is regularly monitored by the GIS team, and
the temperature-gain conversion coefficients are determined for every
one month period. The time-dependent temperature-gain coefficients
thus determined are in the 1-st extension of the
` gis_temp2gain.fits` calibration file which is regularly released by the
GIS team. This files is used by the `temp2gain` ftool
to determine the instantaneous gain values for a given observation.

### Position dependence

The position dependence of the gain is corrected using the*gain-maps*which are in the calibration files

`gis2_ano_on_flf_180295.fits`and

`gis3_ano_on_flf_180295.fits`, which have been determined through the pre-flight ground calibration. The

`ascalin`ftool reads these calibration files, and carry out positional gain correction for each event.

### Long-term variation of the position dependence

Secular change of the GIS gain map had been discovered. That is expressed with a radial function in the first order approximation as follows:corrected PI values = original PI values / ( GMAPC0 + GMAPC1*r + ... + GMAPCn*r**n )

where r**2 = (DETX - DET_XCEN)**2 + (DETY - DET_YCEN)**2 , and GMAPCn are time-dependent coefficients determined through calibration.

Coefficients of the polynomial function are determined by the GIS team every six months or so,
and released in the ` gis_temp2gain.fits` file (written in the second extension).
Both for GIS2 and GIS3, a parabola function (only GMAP0 and GMAPC2) had been
adopted until the end of 1998 September. However, it had been found that
a fourth order polynomial describes the GIS2 radial gain variation better,
and new coefficients (GMAPC0 and GMAP4) have been
determined for the period
after October 1997 (see
the original announcement from the GIS team).
The fourth-order correction for GIS2 has been introduced since
version 4.3.17 of the `gis_temp2gain.fits` file, released on November 4, 1999.

Data taken between October 1997 to September 1998 have been originally processed with the parabola method when delivered to the Guest Observers, but these data in the archives have been reprocessed with the fourth order method (although the difference is insignificant; see the figures above and below).

The `temp2gain ` ftool determines the instantaneous radial
gain correction coefficients for each observation, through either interpolation or extrapolation
of the values in the ` gis_temp2gain.fits` file, and these values are
written in the gain history files which are used by `ascalin` to calculate PI values.

The following figure shows the secular change of the polynomial function coefficients for the gain-map correction (postscript file is available).

### Extrapolation of the gain parameters

As explained above, the temperature-gain coefficients and the gain-map correction coefficients are determined by the GIS team every one month and six months respectively. When observation periods are covered by the periods for which these correction coefficients have been determined, interpolation is used to determine the instantaneous gain values. If these coefficients have not yet been determined, appropriate coefficients have to be calculated through extrapolation.It is possible that the extrapolation is made as far as one-month for the temperature coefficients or six-month for the gain-map correction coefficients. It has been confirmed that the extrapolation method gives consistent answer with the interpolation method. In panel 12 of the figure shown at the top, data were accumulated between 1998 July and 1999 June, while the gain-map correction coefficients were determined partly through extrapolation using the data taken between 1997 October and 1999 March; the Cu line energies are in the right place.

Unless GIS performance starts changing rapidly, we expect the extrapolation method will also be
reliable in future. In any case, in order to see if your gain history file
is made with extrapolation or interpolation, you may look at the processing information
on `aux/ad*_hdr_page.html` in the data package.
Here is an example of `/cdrom/46009000/aux/ad46009000_001_hdr_page.html `(`/cdrom`
is assumed to be the directory for mounting CD-ROM):

Observation Information Object: 1E1024.0-5732 Nominal Pointing (degrees) R.A.=156.645 Dec.=-57.795 Lii=284.579 Bii=-0.18 Observation Date: December 27, 1998 Duration: 102.3 kiloseconds (with 4 kiloseconds of telemetry gaps) Approximate SIS Exposure: 36.8 kiloseconds (approximately 38% with telemetry saturated) Approximate GIS Exposure: 28 kiloseconds Processing Information Sequence Version: 001 Processing Version: 7.3.0 (Revision 2) FTOOLS Version: 4.2 Processing Date: March 27, 1999 GIS Secular Gain Calibration Date: November 11, 1998 GIS Gain Map Calibration Date: April 1, 1998 SIS CTI Calibration Date: October 3, 1997In this case, it is seen that both the Secular Gain (temperature correction) and Gain Map Calibration Dates are after the observation, so these coefficients are determined by extrapolation.

These calibration dates correspond to the following CALDB keywords in
` gis_temp2gain.fits`.

In the first extension (temperature coefficients):

CVED0001= '11/11/98' / Dataset validity end date (UTC) CVET0001= '0:0:0 ' / Dataset validity end time (UTC, of day CVSD)In the second extention (gain-map correction coefficients):

CVED0002= '1/4/98 ' / Dataset validity end date (UTC) CVET0002= '0:0:0 ' / Dataset validity end time (UTC, of day CVSD)Also, they correspond to dates of the last entries in the 1st extension and the 2nd extension respectively.

## Limit of the radial gain correction

Because the gain drops rapidly near the detector edge, as well as the particle background is prominent there, as a default,`ascascreen`. In the course of the secular change of the detector performance, azimuthal dependence of the GIS3 gain has become noticeable after 1998 July at the outermost region which is usually screened out (r > 23 arcmin or 90 channel). Because of the azimuthal gain fluctuation, which is not corrected in the current gain calibration, the copper line peak azimuthally integrated at the outermost region no longer fits with a single gaussian model (figure below; postscript file is available), although discrepancy of the line center energy is at most 0.5 %. GIS team once again suggests Guest Observers not to use the GIS3 data at the outermost region for precise spectral analysis.

## Recalculate PI values using new calibration

As explained above, a new version of the`gis_temp2gain.fits`calibration file is released once every few months, and your GIS data may not have been processed with the latest one. If you want to recalculate the PI values in your GIS event files using the latest

`gis_temp2gain.fits`file, here is the procedure:

- Obtain the latest
`gis_temp2gain.fits`file from ftp://heasarc.gsfc.nasa.gov/asca/data/gis_reprocess_2002-June/gis_temp2gain.fits . This is the most recent update. - Create a new gain history file with the
`temp2gain`ftool using the new`gis_temp2gain.fits`file. - Run
`ascalin`to calculate new PI values using the new gain history file and fill PI column of your files.

The `update_gis_gain.pl`
script will carry out the step 2 and 3 above automatically (please do not forget to edit the
first line of the script to point your local perl installation).

### References

*The present study was mostly conducted by H. Kubo, A. Kubota and M. Tashiro of the GIS team. If you have questions about issues presented in this page, please contact the ASCA GOF: visit the Feedback form.*

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