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RELEASE NOTES

These Release Notes accompany the release of a patch to FTOOLS 4.1 which updates the RXTE subpackage to v4.1.1. This patch contains upgrades to the PCA background subtraction tools and models which provide a significant improvement in the PCA background subtraction for faint sources. The improvement applies to data from sources fainter than 40 ct/sec/PCU obtained during Epoch 3 (i.e since April 15, 1996). In addition, this patch also provides fixes for several known bugs.

These notes provide details on the changes to PCABACKEST and XTEFILT, and how to run these tools in combination with the two new model files and the SAA History File to obtain the best possible background subtraction for your RXTE data. All other code changes are bug fixes, and are described in the README file.

OVERVIEW

We envision users applying the background related tools in this Patch in the following way:

  1. Install the RXTE FTOOLS v4.1.1 Patch.
  2. Run XTEFILT v1.5 - generate new filter file.
  3. Run maketime on new filter file with new filtering criteria (below), producing a Good Time Interval (GTI) file for clean data.
  4. Run PCABACKEST v2.0c (or RUNPCABACKEST) with appropriate models (see the PCABACKEST section below), to produce background files for your data.
  5. Run SAEXTRCT on background data from step 4 with the GTI file from Step 3 to produce background lightcurves and/or spectra for your data. (Note: the GTI file from step 3 should go in as the GTIANDFILE parameter in SAEXTRCT.)
  6. Run SAEXTRCT on RXTE observation data with same GTI file and inputs as in step 5, to produce raw "source" lightcurves and spectra. This will insure that both your background data and your observational data have had the same filtering criteria applied to them. (i.e., same GTI's applied, same number of PCU's and layers extracted, etc.)
  7. Use LCMATH to produce background subtracted lightcurves and/or XSPEC to produce background subtracted spectra.
  8. Enjoy!

Here are the details of the most significant changes to the tools in this patch:

PCABACKEST v2.0c

PCABACKEST v2.0c is capable of evaluating two new model components, and is delivered with two corresponding model files. Most of the background is correlated with the "L7" rate, the sum of 7 rates, each of which measures events which trigger exactly two of the signal layers. Unlike the previous particle based models (Q6 or VLE) a plot of X-ray rate vs L7 is single valued; we conclude that a large fraction of the activation background is tracked by this rate.

However, examination of blank sky pointings after the L7 model is removed, show residuals which are correlated with the recent SAA history as measured by the HEXTE particle monitor. A second model component provides an activation related component with halflife of 240 minutes. The driving function is acceptably modelled by the particle monitor integrated dosage per SAA pass, accounting for the 240 minute decay time. Evaluating this component requires access to the complete SAA history (i.e. more data than is contained within any GO data set). This information can be found in the file pca_saa_history_v04 delivered with this patch and valid through May 1998. This file will be updated periodically for later datasets and distributed via the GOF, as information becomes available.

These two new model files produce very good backgrounds for sources fainter than 40 cts/sec/PCU. THEY ARE NOT APPROPRIATE FOR SOURCES BRIGHTER THAN THIS LIMIT. Users with brighter sources should continue to use models from the CALDB or the RXTE PCA team page: http://asd.gsfc.nasa.gov/docs/xray/xte/pca/index.html

The new model files are included in this patch. Once the patch is installed, you will find the new models in the $LHEA_DATA directory:

    pca_bkgd_faintl7_e03v01.mdl pca_bkgd_faint240_e03v02.mdl
Details of the performance of these models is documented, and will continue to be updated, on the PCA page (http://asd.gsfc.nasa.gov/docs/xray/xte/pca/index.html) A description and example of how to use the new PCABACKEST can be found in the RXTE Cook Book at: http://heasarc.gsfc.nasa.gov/docs/xte/xhp_proc_analysis.html
  • These models are suitable only for faint sources (< 40 ct/sec/PCU).
  • These models are currently only implemented for epoch 3 (April 15, 1996 - present)
  • The SAA history file only contains data trhough May 1998; consult the XTE GOF pages for updates.

The PCA team is working on models suitable for brighter sources. These models will not require a new version of PCABACKEST, but will require new model files.

XTEFILT v1.5

XTEFILT has been modified to calculate new quantities useful in filtering data. New filtering criteria based on these quantities result in cleaner data and substantially better backgrounds, when used in conjunction with the new pcabackest.

Running XTEFILT:

There have been no changes to the inputs for XTEFILT, however, a new appidlist is *NECESSARY*, and available from the fhelp. Simply type: 

fhelp xtefilt
and copy the appidlist that appears there into a separate ascii file. Then run XTEFILT as normal, giving the new appidlist in place of any old appid file you may have had. For further details on running xtefilt, see the fhelp or our recipe at:

http://heasarc.gsfc.nasa.gov/docs/xte/recipes/xtefilt.html

New Quantities in Filter File:

The following new quantities appear in filter files produced with XTEFILT 1.5:

TIME_SINCE_SAA

    Time (in minutes) since peak in last SAA passage; may be negative! This quantity is derived from a parameterized coordinate system related to the SAA and as such has a discontinuous jump to negative values in the time *before* peak SAA passage. See filtering criteria below for how to select data using this quantity.

ELECTRON0

    For each PCU (numbered 0-4), a ratio showing the relative contribution to in-orbit noise of electrons trapped in the Earth's magnetic field. (unitless, since a ratio)

Recommended Filtering Criteria:

These are the filtering criteria currently recommended by the PCA team to produce clean, low-noise data sets and well-behaved backgrounds. These criteria should be used in MAKETIME with a filter file from the new PCABACKEST to produce Good Time Interval files (GTI files) for input to the RXTE extractors (SAEXTRCT or SEEXTRCT).

The criteria are written in C programming language syntax, which is acceptable input for MAKETIME. They may also be written in FORTRAN syntax; see the fhelp for MAKETIME for details.

Note that these criteria may be ANDed together (&& or .AND.) to produce one long, single input to maketime, or may be used to create separate GTI files for each set of conditions. The separate GTI files can then be given to the extractors at the GTIANDFILE prompt, or the names of the GTI files may be placed in an ascii text file. This text file would then be given at the GTIANDFILE prompt, *preceded* by an "@" sign. See the fhelp for SA- or SEEXTRCT for further information.

To filter out contamination from the SAA: 

TIME_SINCE_SAA < 0 || TIME_SINCE_SAA > 30.0
This selects time before the SAA passage and times at least 30 minutes after (or whatever number you judge appropriate for your data.)

To filter out electron contamination*: 

( ELECTRON0 < 0.1) && ( ELECTRON1 < 0.1 ) && ( ELECTRON2 < 0.1 )
* NB: if you will be extracting either of the last two PCU's (3 & 4), add similar electron criteria for these. The above covers only the first three PCU's (0-2).

To filter out Earth occults and bits of slews, plus make sure the PCU's you wish to extract are on*: 

( ELV > 10.0 ) && ( OFFSET < 0.02 ) && ( PCU0_ON == 1 ) &&
( PCU1_ON == 1 ) && ( PCU2_ON == 1 )
* NB: The PCU quantities have a value of "0" when the PCU is off, and "1" when it is on. Include all PCUs you wish to extract in this statement (only PCU's 0-2 are shown in this example). The "OFFSET" parameter above should only be used for pointed observations. Observations which scan across a source or area of sky should not have the "OFFSET < 0.02" criterion applied to them.