National Aeronautics and Space Administration

This page contains older "news items" from the PCA Digest page, in chronological order.

• 2001-06-13: Description of PCU 0 Background Issues After Loss of Propane
• 2002-02-26: New Background Models for Epoch 3
• 2002-07-09: Revised versions of PCABACKEST and PCARSP
• 2003-03-01: New Mission-Long Background Models
• 2004-04-14: New Background Models Correct for Long Term Linear Drift
• 2006-08-06: New Background Models Remove Long Term Trends
• 2007-01-28: Propane Loss in PCU 1
• 2007-09-18: Problems and Corrections to RXTE PCA Background
• 2008-12-01: New PCA Breakdown history files
• 2008-12-12: Redesign of PCA Digest Page
• 2009-08-17: Improvements to the PCA Response Matrix
• 2009-12-12: Correction to PCABACKEST spike artifacts
• 2011-06-09: Improvements to the PCA Effective Area (HEASoft 6.11)

Background models for the PCA detectors are created from observations of the "blank" sky (i.e. the recorded signals are the sum of the Cosmic X-ray Background and instrumental background). We observe that the background is time variable, so we parameterize the background as a function of spacecraft and instrument conditions. In particular, all useful models to date parameterize the background as a function of one or more of the coincidence rates reported in the Standard 2 data with an additional time dependent term to account for an activation component due to passage through the radiation belts in the South Atlantic anomaly.

The faint models use a rate called "L7", which is the sum of all 2-fold coincidences between adjacent xenon signal chains (L1+R1, L2+R2, L3+R3, L1+L2, L2+L3, R1+R2, and R2+R3). Even without the propane layer, this quantity is unambiguously defined, and our first attempt was to examine whether the L7 rate still tracks apparent X-ray rate for PCU 0. Summarized in a phrase, the answer is "not well enough". There are at least two contributing factors. Without the propane layer, there is additional noise in the apparent X-ray rate due to electrons which deposit energy in the first layer (and which previously would have stopped in the propane layer or the propane+first xenon layer). Additionally, without the proane layer, our electron screening criteria is undefined. The scatter is sufficiently large that we have abandoned the idea that we can model the PCU 0 background as a function of PCU 0 parameters.

However, we know that the behaviour adjacent and nominally identical detectors is highly correlated, and PCU 2 is always on. Over the course of summer 2001, we expect to explore a model for PCU 0 background that is based on the rates in PCU 2. The electron screening criteria can be applied based on PCU 2 data; we will explore whether additional screeing criteria are required based on combinations of data from PCU 0 and PCU 2. If successful, we will release a new background model file, and (if necessary) a slightly updated pcabackest. Users will be only slightly affected by the (potential) need to apply different screening criteria to PCU 0 than to PCU 2, and may therefore find that PCU 0 does not have an acceptable model for all the times that it operates. The GOF will work to update the REX script to accomodate this.

Keith Jahoda

New and improved models are now available for Epoch 3 to the present (both faint and bright). Briefly, the new models feature:

• sensitivity to X-ray variations improved by factor of ~2 over previous models;
• applicability to Epoch 5 PCU0 (needs new FTOOLS to fully exploit);
• new model packaging means fewer files to deal with;
• gain Epoch 3 is now divided into Epochs 3a and 3b for faint sources;

For the best possible background subtractions throughout the very long Epoch 3, a new boundary has been established, breaking it into Epoch 3a and Epoch 3b. The new Epoch boundaries are listed below.

Epoch "3a" 1996Apr15 at 00:24:05TT - 1998Feb09 at 00:01:00TT
Epoch "3b" 1998Feb09 at 00:01:00TT - 1999Mar22 at 00:18:38TT

The faint model components are:

1. "cml7" - the part of the PCA rate accounted for by the L7 rate
2. "cml7act240" - the part of the PCA rate accounted for by 240 minute radioactive decay
3. "cml7tlin" - long term secular drift of background rate

The new bright background models require the SAA_HISTORY file. The table at the top of this document organizes the models and extra files needed by Epoch and source count rate. For all epochs, the various model components have been combined into one file for ease of use. No changes have been made to the Epoch 1 and 2 models other than the merging of the separate components.

There are again three components, but not the same components as the faint model:

1. cmvle - depends solely on VLE rate
2. cmvleact24 - activation component - 24 minute decay time
3. cmvleact240 - activation component - 240 minute decay time

All new model files can be used as-is with the existing PCABACKEST for Epochs 1-4 and for PCUs 1,2,3,4 in all Epochs. For spectral analysis of data from Epoch 5 PCU0, FTOOLS v5.2 (released June 25, 2002) or later is required.

Information about the latest pcabackest and pcarsp in FTOOLs 5.2 (released June 25, 2002).

FTOOLS 5.2 includes software for improved response matrix generation as well as background subtraction.

The latest FTOOLS release, v5.2 (June 25, 2002) includes new versions of PCARMF, XPCAARF, and the wrapper script PCARSP as well as a new energy-to-channel FITS file pca_e2c_e05v02.fits. PCARMF v8.0 has the following features:

1. Calculation of sigma in the response is greatly improved.
2. Energy channels in the matrix are now spaced logarithmically.
3. Each detector can now have a separate value for the thickness of the "dead layer" between xenon volumes (via new parameters in pcarmf.par).
4. There is now an opportunity to specify a second a+bx law (and date when it starts) for the amount of xenon in the propane layer. At the moment this is only used for PCU0 (to set the amount to zero after the loss of that layer).

Derived fluxes from PCU0 still show a slight discontinuity around the time period of the loss of the propane layer, but much less so than before.

The only change to XPCAARF is to the default values (in xpcaarf.par) for the geometric areas of four of the five PCUs (PCU 2 was left unchanged). These were chosen to make the flux of the Crab similar when fit to the five detectors (epoch 3/4 chosen for the comparison). Note that this will likely result in a step function in monitoring fluxes unless the whole data set is reanalyzed with a constant set of XPCAARF coefficients.

PCARSP (v8.0) has been updated to ensure that the default value for the partial charge fraction matches the value in PCARMF (previously 0.0, now 0.02). A bug was also fixed in the previous version which caused the "-n" flag to fail to properly rename the final output response file in single-detector cases.

PCABACKEST has been updated to support the new Combined Models (CM) and to include more information in the output file headers documenting which models were used. In addition, XTEFILT (ie, FCOLLECT and XTEDERIVE) has been modified to add a new derived quantity, L6CNTPCU0, to the filter file. This can be used to filter out times when large flares appear in the background model due to the loss of the propane layer in PCU0. Six new quantities must be added to the input AppID list to allow L6CNTPCU0 to be computed (the full AppID list is given in the fhelp for XTEFILT). The new quantities are:

  74 X2LX2RCntPcu0
  74 X3LX3RCntPcu0
  74 X1LX2LCntPcu0
  74 X1RX2RCntPcu0
  74 X2LX3LCntPcu0
  74 X2RX3RCntPcu0

Download one complete mission-long background file for bright or faint sources, and let the FTOOLs work hard so you don't have to! The background files available here were created in March 2003, and contain substantially more Epoch 5 data than previous versions of the model files, which only included data through mid-2001.
Mission-long Faint Model File as of 2003-11-23 (now obsolete)
Mission-long Bright Model File as of 2003-03-30 (now obsolete)
...and don't leave without the latest pca_saa_history file!

Last updated April 13, 2004 with links to new background model files that correct for a slow linear trend in the background rate. Available for Epochs 3a, 3b, 4 and 5 as individual models files, as well as in an updated Mission-long Faint Model File as of 2003-11-23. (now obsolete) New faint model files are available through the link above and through the model table below. As the orbit of XTE degrades, the PCA background is gradually decreasing. Earlier, a new "linear drift" temporal model was incorporated in the "CM" faint background models which attempted to subtract this drift out. However due to an errant keyword in the background models, the computed term was CONSTANT and only correct at the START of the gain epoch. Eventually, this has led to an oversubtraction of background rates late in any Epoch. The performance degrades with time, and is worse at the present day, for epoch 5).

New models are being released that correct this problem, and eliminate these trends. One model is mission-long, the others contain the same data, but separated out into individual gain Epochs. The details of the faint model are the same in both representations; the mission long model is made available to simplify the analysis tasks for users.

The new models have been tested, and work well, on a variety of PCA data.

A detailed discussion of the differences between the old models and the new is available from the following PCA Team 2004 report.

The page includes several informative graphs of the cumulative error versus time, as well as pulse height dependences.

New PCA background models have been developed that remove long term trends that have become apparent over the years. Only models for "Epoch 5" -- May 2000 to the present -- were changed. The original "Epoch 5" has been divided up into three new intervals, "5A", "5B" and "5C" to account for the trends and improve the background model. New faint and bright source models are available from the links in the table above. The new models have been tested extensively against the background data themselves, as well as by select observers on AGN data. There is marked improvement for faint sources in the energy range 25-30 keV, where a background feature caused artifacts in the previous models.

A more complete description of the new models can be found in this report by Craig Markwardt.

January 28, 2007

On 2006 Dec. 25 at 12:56:08 UT, PCU 1 (in the 0-4 numbering scheme) developed a pinhole leak in its propane layer, consistent with a micrometeorite hit. The PCA team recommends that observers should not use PCU1 for light curves and spectra until new response matrices and background models have been developed. Work is beginning on those issues, and expected to take several months. High frequency timing with PCU1 should still be possible after the propane loss.

Problems were discovered in the SAA history file that result in errors in background rate estimation. A new file that corrects these problems is available.

In addition, a bug was discovered in the FTOOL PCABACKEST for faint models only, in Epoch 5c. The parameter maxmodels=2000 (instead of the default value maxmodels=600) must be set in order to return a complete faint background model.

Details can be found on the PCA Team SAA History and Backgrounds Problem Report Page

This page is maintained by the RXTE GOF and was last modified on Wednesday, 24-Aug-2022 11:10:32 EDT.