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Dramatic Improvement of PCA Background Models

Dear RXTE Users,

We would like to call your attention to the release of a software
patch for RXTE data analysis which provides:

   (1) A dramatic improvement to the PCA background models, which will
have a profound impact in your ability to analyze data from faint
sources, and;

   (2) Associated improvements to XTEFILT, and fixes to bugs in

This patch is for FTOOLS v4.1, to update the RXTE subpackage to

* If you have RXTE data obtained in Epoch 3 (April 15, 1996 -          *
* present) for which the source count rate is less than 40 ct/sec/PCU, *
* you should reanalyze your data using the new "L7" background model,  *
* contained in this patch, which takes into account various particle   *
* monitors and the SAA history!                                        *

The new model is described in detail on the PCA pages at


Briefly, characteristics of the L7 model include:

    * Excess variance of <0.1 ct/sec (2-10 keV; PCUs 0-3). The
       systematic error in the 2-10 keV band is now ~0.1 times 
       the 1-sigma CXB fluctuation.
    * Excess variance of <0.1 ct/sec (10-20 keV).
    * Reliable spectral subtraction below 24 keV, for energies
       where the net signal is >0.01 ct/sec/keV.

Additionally, xtefilt and its associated programs have been updated to
include information about electron contamination and time since an SAA
passage in the filter files.  Filtering based on these quantities
should result in substantially cleaner data sets with better
signal-to-noise values.

This patch also fixes bugs that were reported in the following RXTE
tools from the FTOOLS version 4.1 release: fxbary, rddescr, mkgitf.
The tool rddescr is used by pcarsp to generate response matrices.

* If you have had failures when attempting to generate response        *
* matrices or barycenter correct data under FTOOLS 4.1, you should     *
* obtain and install this patch.                                       *

Instructions on installing the patch, as well as detailed release
notes describing significant changes to the analysis tools may be
found on the RXTE GOF web page at


Efforts are currently underway to construct PCA background models that
extend to higher energies, brighter sources, and can be applied
to Epochs 1 and 2.

We hope and expect that these new background models will encourage a
flurry of interest in the analysis of RXTE faint source data.  As
always, if you have a paper or other article based on RXTE data
accepted for publication, please drop us a note so we may include it
on our web pages.


	Alan Smale (for the RXTE GOF)
	Keith Jahoda (for the PCA Team)

| Dr. Alan P. Smale                | xtehelp@athena.gsfc.nasa.gov (RXTE)|
| RXTE Guest Observer Facility     | alan@osiris.gsfc.nasa.gov (me)     |
| Code 660.2  NASA/GSFC            | (Please Email rather than phoning.)| 
| Greenbelt, MD 20771, USA         | (301) 286-1682 (FAX)               |
|              RXTE GOF homepage: http://rxte.gsfc.nasa.gov             |