3 GIS ISSUES

 

3.1 Introduction

This chapter describes some aspects of the GIS performance which users should be aware of when reducing and analyzing GIS data.

For the latest information about the GIS, please consult the Web pages, `GIS News' at the URL

http://heasarc.gsfc.nasa.gov/docs/asca/gisnews.html

and `Calibration Uncertainties' (§1.7)

3.2 Description of GIS Modes

 

Although the GIS has four modes, in practice the pulse height (PH) mode has been used for virtually all scientific observations. The multi-channel pulse count (MPC) mode was used occasionally at the beginning of the mission. The FITS keywords which pertain directly to the GIS modes are described in §3.4.

3.2.1 PH mode

 

In this mode, the on-board CPU calculates the event position and discards background events using a specially designed event selection algorithm. Only accepted X-ray events are sent to the ground. With the standard bit assignments, Pulse-height spectra consist of 1024 channels for each detector.

PH mode allows some flexibility in how the telemetry is allocated. There are 31 bits per event to describe the PHA value, X and Y position, rise time, spread and timing. Note that changing the bit assignments in PH mode only changes the digital resolution, i.e., the binning, of the data. For example, the standard 1024-channel PH mode spectral responses can be used for 256-channel PH mode spectra, provided the response has been rebinned (using the FTOOL rbnrmf) .

In the following table, the various alternative choices are given in parentheses, while the standard values are not:

PH: 10 (8)

X-pos: 8 (6) (4) (2)

Y-pos: 8 (6) (4) (2)

RiseTime: (8) (6) 5 (0)

Spread: 0 (8)

Timing: 31-(PH_bit)-(X-pos_bit)-(Y-bit)-(RiseTime_bit)-(Spread_bit) =0 for nominal setting.

The standard temporal resolution (timing bit=0) is 62.5 msec (in HIGH bit rate) and 500 msec in MEDIUM bit rate: using N bits for timing makes the temporal resolution times better. It is often useful to increase the number of timing bits to achieve higher time resolution. For example, for several observations of fast X-ray pulsars, the following bit assignments were used and found effective:

8(PH)-6(X)-6(Y)-5(RT)- 6(Timing) in high bit rate (1.0 ms resolution)

8(PH)-6(X)-6(Y)-0(RT)-10(Timing) in medium bit rate (0.5 ms resolution)

The bit assignment in a PH mode events file is described by the header keyword, `TIMEDEL'.

Telemetry capacity limits in the PH mode are 128, 16, and 4 cts/s/sensor for the HIGH, MEDIUM, and LOW telemetry rates respectively.

3.2.2 MPC mode

 

This mode is optimized for bright sources, trading temporal resolution and telemetry capacity for positional information: only pulse-height information is recorded - in the form of histograms, similar to proportional counter data. This is also a back-up mode against CPU failure, since the data are processed without the intervention of the CPU. Pulse-height spectra consisting of 256 channels for each time bin are produced. Alternatively, each time bin can contain a set of N spectra of 256/N channels for N equal portions of the time bin.

You can extract light curves and spectra, but not images, for MPC mode data using XSELECT. The lack of positional information (for the spatial gain calibration) makes analyzing MPC data problematic. Spectral response matrices do not exist for MPC mode, so accurate spectral analysis is not possible for the general user.

3.3 Imaging with the GIS

A single GIS event invokes electric pulse-heights on the 16 16 anode wires of the Imaging Photo Multiplier Tube. The GIS calculates the event position as the centroid of these 16 16 pulse-heights. As a consequence of the position determination algorithm, the GIS has a limited spatial resolution. The point spread function (PSF) of the GIS alone is a Gaussian with a FWHM of arcmin, where E is the energy in keV. Note that this is smaller than the 3-arcminute half power diameter of the XRT PSF, but at soft X-ray energies is larger than its sharp core (the FWHM of the XRT PSF is 50 arcsec). As a result, the convolved PSF of the XRT+GIS becomes much broader than that of XRT alone, and the original detailed structures in the XRT image are smeared in the GIS image. Studying small structures on a scale of an arcminute, which may be carried out with the SIS, is very difficult with the GIS.

Although the distributed GIS data contain all the events from all over the detector (50 arcmin diameter), the GIS usable Field of View (FOV) is limited by several factors. Firstly, most non-X-ray background events in the GIS occur close to the walls of the detector, i.e., at the edge of the FOV. Secondly, there is an internal calibration source which emits monochromatic X-rays and which appears at the edge of the GIS FOV. Third, not only is background high, but the gain is also less accurately known at the edge of the FOV. Indeed, because of the high background and uncertain gain, only events within the central 44 arcmin diameter of the GIS FOV are aspected (assigned RA and DEC). It is a standard practice to exclude data taken outside of a central diameter of 40 arcmin. For more details, see §5.5.2.

3.4 GIS Mode-Dependent Keywords

 

In xselect, once the datamode has been set, the various mode-dependent quantities, such as the number of energy channels, are set automatically. It is, however, useful to know what the corresponding keywords are in the GIS data file.

• The keyword DATAMODE can take the value PH for PH mode or MPC for MPC mode. It appears in all the headers.
• The keyword PHA_BINS gives the number of PHA bins, both for PH and MPC modes. It appears in the primary header and in the headers of the EVENTS (PH mode) and SPECTRUM (MPC mode) extensions.
• The following keywords appear only in PH mode files - in the primary header and in the header of the EVENTS extension. Together with PHA_BINS, they describe the bit assignment:

       PHA_BINS=                 1024 / number of bins for PHA
       RISEBINS=                   32 / number of bins for RISE_TIME
       TIMEBINS=                    1 / number of bins for TIME
       SP_BINS =                    1 / number of bins for SPREAD
       RAWXBINS=                  256 / number of bins for RAWX
       RAWYBINS=                  256 / number of bins for RAWY

  Note that the values in this example correspond to the standard bit assignment. Furthermore, when RISEBINS=1, the file contains no rise time information; consequently, background rejection based on the RTI, or invariant rise-time, using gisclean, cannot be performed.

• The keyword TIMEDEL gives the time resolution of both PH and MPC data. It appears in the primary header and in the headers of the EVENTS (PH mode) and SPECTRUM (MPC mode) extensions.
• The following three keywords appear in the primary headers of PH and MPC mode files. They give the status of the the high voltage:

       HV_RED  = 'OFF     '           / HV-reduction on/off
       HVH_LVL =                    3 / HV-High level (0 - 7)
       HVL_LVL =                    4 / HV-Low level (0 - 7)

  The values given here correspond to valid data: files which have different values are garbage, and are recognized as such by XSELECT which does not, by default, include them in `obscats' (observation catalogs).

• Finally, the keyword S_DSCR indicates whether the on-board Spread Discriminator, which rejects events based on their UV light distribution, is on or off. The Spread Discriminator was first switched on in May 93: observations before then, and a small number of observations after this, were carried out with S_DSCR='OFF'. The intrinsic background is much smaller when S_DSCR='ON', which is its normal setting.

3.5 Coping with the GIS3 Bit Problem

 

Note that all GIS3 PH data acquired in PH mode between 1994 February 10 and 1994 April 8 suffered from an on-board software-related problem, resulting in a loss of digital resolution. The data taken with the standard bit-assignment have 128 spectral channels, as opposed to the ordinary 1024. Please consult the original announcement from the GIS team for details on this problem. This is available at

http://heasarc.gsfc.nasa.gov/docs/asca/op_status/maxima940410.html.

For spectral analysis of the data taken during this period, RMFs with 128 channels have to be used corresponding to the 128 channels of the PHA data. The 128 channel RMF is available in the ASCA calibration database:

/FTP/caldb/data/asca/gis/cpf/95mar06.

This file, gis3v4_0_128ch.rmf, was made with the FTOOL rbnrmf with the following command from the 1024 channel RMF gis3v4_0.rmf:

rbnrmf infile=gis3v4_0.rmf nchan=128 cmpmode=linear 
outfile=gis3v4_0_128ch.rmf

when making an ARF with ascaarf, the 128 channel RMF should be used so that the ARF will have 128 channels.

3.6 Coping with the GIS Gain Problem

 

It has been found that the absolute gain of GIS3 is apparently overestimated by 1 - 2 % since around August of 1995. Details can be found in a report from the GIS team at the URL

http://heasarc.gsfc.nasa.gov/docs/asca/gisnews.html#gis3_gain_problem.

This problem is corrected in ascalin v0.9t (and later) when used with the program temp2gain v4.1 (and later). Data processed at GSFC after 1997 March (including REV2 products) are processed with this new software. Alternatively, Guest Observers can either correct the gain by themselves, as described in the above report, or else use the quick and rough method described below. Other occasions when Guest Observers may encounter the GIS gain problems are when (1) analyzing sources located close to the rim of the sensor where gain uncertainty is large, and (2) analyzing bright sources (> 0.1 Crab) which might cause an artificial gain shift. See also the `Calibration Uncertainties' Web page (§1.7).

It is possible for Guest Observers to manually systematically shift the PI values of GIS events in a single events file using a new gain value. Here is an example to re-determine PI values that are 0.978 (=1/1.022) times the original values, for the events file ad43001000g300370h.evt using any version of ascalin (e.g. that released in FTOOLS v3.6 or earlier).

ascalin datafile=ad43001000g300370h.evt calfile=caldb tempofile=
ft950908_0323_0800.ghf attitude=none gainnorm=1.022

Here, ft950908_0323_0800.ghf is the gain history file which is in the aux directory of the data distribution package or the ASCA archives. The correct gain normalization factor has to be determined independently, by trial and error, by for example, using the XSPEC gain command. Note that the new PI values are the old PI values divided by the input gainnorm. This is made so that the output of the XSPEC gain command can be directly used here. Instrumental spectral features at the gold M-edge (2.2 keV) and Xenon L-edge (4.8 keV) may be used as fiducial marks for the gain correction. Please consult the `GIS Gain Correction' Web page at the URL

http://heasarc.gsfc.nasa.gov/docs/asca/gain_correction.html

for more details and another example.

3.7 Other Issues Related to GIS Data

 

Here are some other important points to bear in mind with the GIS.

• Events files from GIS2 and GIS3 cannot be mixed to extract images and spectra. The two GIS detectors have different spatial characteristics and background. It should be possible, however, to make a combined light curve from GIS2 and GIS3 data which, when analyzed, would not yield misleading timing results. It is possible to combine two energy spectra from GIS2 and GIS3 and to superpose GIS2 and GIS3 images taking account of the correct detector responses, after they are separately extracted from separate events files.
• Nearly all GIS data have been taken in PH mode with standard bit assignments. The rare exceptions are non-standard PH mode, used to to increase temporal resolution, or MPC mode used for very bright sources.
• Temporal resolution depends on bit rate and bit assignment. Combining data with different bit rates is acceptable, provided the highest temporal resolution is not required throughout. However, if the very highest time resolution is required (e.g., for pulsar timing analysis), then only HIGH bit-rate data should be analyzed.
• Data taken in LOW bit-rate can be used. Although hot and flickering pixels completely saturate the portion of telemetry devoted to SIS data, LOW bit-rate GIS data are not affected in this way there is no reason to exclude them for spectral work. Note, however that the telemetry limit is low, at 4 count/s/GIS. This will be important only if accurate fluxes are required and for temporal analysis. Telemetry saturation will not affect spectral shapes and images, but the normalization (flux) will be reduced.
• To obtain the correct flux, a correction will be needed for the electronic deadtime. GIS deadtime becomes important for sources with > 10 cts/s/GIS. To carry out the deadtime correction, the FTOOLS deadtime (for spectra; see §8.10) and ldeadtime (for light curves; see 9.8) can be used.