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Overview: RXTE Mission-Long Data Products

As the RXTE mission matures, the archive of high quality data on X-ray objects of interest is becoming increasingly dense, and as a result, more valuable to a broad array of researchers. The RXTE Guest Observer Facility (GOF) has begun generating Mission-Long Data Products based on the standard data products (StdProds) from the PCA and HEXTE, for over 200 sources that have been observed many times with RXTE during its mission.

The mission-long data products are intended to give a researcher an idea of how much, and what quality of, data is available in the RXTE public archive for their sources of interest. They are not meant to replace the careful reduction and analysis by the scientist, but rather to lead her towards data that will be suitable to her needs. Users should bear in mind that only a single point is included for an observation. And thus the impressive variability seen by many RXTE targets on short timescales is entirely overlooked in the mission-long data products.


Introduction: Which Sources, Which Data?

NASA's Rossi X-ray Timing Explorer (RXTE) was launched into low-Earth orbit on December 30, 1995. On board are three science instruments: the All Sky Monitor (ASM), which scans the entire viewable sky and measures the time histories of source intensities in three energy bands from 1.5--12.0 keV; the Proportional Counter Array (PCA), composed of five co-aligned Xenon detectors (PCUs) with a total collecting area of 6500 square cm, most sensitive over the 2-25 keV energy range; and the High Energy X-ray Timing Experiment (HEXTE), two clusters of "phoswich" scintillation detectors that rock on and off source along mutually orthogonal directions for realtime background measurements, sensitive to high-energy X-rays from 15-250 keV. All instruments continue to perform well, and exciting results regularly attract the attention of the scientific community as well as the popular press.

All RXTE observing time is awarded through the Guest Observer Program. Proprietary data is available to Guest Observers online with PGP-encryption. All observations become public and enter the HEASARC archive after one year. For each observation, Standard Products add value and accessibility to the PCA and HEXTE data in this archive. These products include light curves, spectra, GIF files, and a filter file for each PCA and HEXTE observation.

The mission-long data products combine all good RXTE observations of a unique source. For most sources, this involves combining data from separate proposals across many successive observing years. Observations are considered to be from the same source if the RA and Dec coordinates are the same to within reasonable limits given RXTE's field of view. Only pointed observations are used (no scans or slews). Mission-long data product plots (GIF files) are generated for a source if at least thirty (30) good observations remain after several decisions and screenings are considered.

Briefly, these include:

A FITS file containing the data for good observations is made for all sources, regardless of the number of observations. For more information, see the "Filenames and Contents" section, below.


The Standard Products, Merged Lightcurves, and Mission-Long Data Products

Data is collected in standard modes for every PCA and HEXTE observation, in addition to data in modes selected by the original proposer. The PCA has two standard modes. Standard1 (or Std1) has a time resolution of 0.125s and no energy information, while Std2 has a time resolution of 16s and 129 energy channels (numbered 0-128) covering the full range of the PCA detectors. HEXTE's archive mode has 16s time resolution and 129 spectral channels.

Only the standard data modes are used for RXTE Standard Products generation. RXTE Standard Products are stored in the archive in a separate /stdprod subdirectory, at the obsid level. For example, the Standard Products for obsid 90130-01-01-03 are stored in the directory named xte/data/archive/AO9/P90130/90130-01-01-03/stdprod . Here, the proposal is number 90130, and the target 01 is the first accepted target in the proposal.

Some of the PCUs suffer breakdown and trip off if they are not regularly "rested". This means that any individual observation may contain data from 1 to 5 of the PCUs. All PCA standard products have therefore been normalized to 1 PCU (i.e. light curves are presented in units of c/s/PCU).

In addition to Standard Products at the obsid level, an intermediate "Merged Lightcurve" is also available at the proposal level, for each target in the proposal. For example, the merged light curve for the first target in proposal 90130 is stored in the directory named xte/data/archive/AO9//P90130/90130-01. Note that the only contents in this subdirectory are the light curves and gif files of the merged HEXTE and PCA Std2 light curves from the individual obsids of that target and proposal. In cases where the PCA gain setting changed during a proposal's observations, there will be separate lightcurves for each gain epoch. These merged products are intended to give the archival observer an "at a glance" idea of the available RXTE data on a given source in a single proposal.

By contrast, the Mission-Long Data Products combine all observations of a target from throughout the mission, gathering data from many proposals in some cases. Each observation included in the Mission-Long Data Products is represented by a single data point. For Mission-Long lightcurves, the PCA data point is the average rate of the background-subtracted PCA Std1 mode for that observation; the HEXTE data point is the average rate of the background-subtracted HEXTE cluster B archive mode. For the Mission-Long hardness ratio plots, the averages of the background-subtracted PCA standard 2 rates in several bands are used (see next section for description of bands).


Filenames and Contents

Each source may have up to three Mission-Long Data Products:
  • Mission-Long FITS file (Source_ID.fits)
  • PCA and HEXTE light curves (Source_ID_rates.gif)
  • PCA hardness ratios (Source_ID_hardness.gif)

where "Source_ID" is the unique source identifier, based on the SIMBAD- recognized source name. Sources which have less than 30 good observations will only have a Mission-Long FITS file. (See Introduction: Which Sources, Which Data? for a description of "good" observations.)

  • Mission-Long FITS file (Source_ID.fits)

    This file contains the data plotted in the two GIF products. Each row in the file is a separate RXTE observation (obsid), drawn from all proposals that included the source as a target. It is possible for a point to not appear in the corresponding plot (such as when the Std1 rate is less than zero) but the data from this obsid still appears in the FITS file. For each observation, the data table lists the obsid, the RA and Dec, the start time, stop time, and mid-time, expressed in RXTE Mission Days (following the precedent of RXTE ASM FITS files). The table also contains all Std1, Std2 and HEXTE rates and errors, with the PCA data normalized to 1 PCU (ie., rates are in units of c/s/PCU). The "exposure" column shows the total goodtime used for each observation. (Goodtime was determined by RXTE Standard Products filtering criteria.) The number of PCUs whose data went into the calculations of the PCA rate is stored in the NPCU column. There are also columns which report whether the rate for the observation appears in the PCA or HEXTE plots.

  • PCA and HEXTE light curves (Source_ID_rates.gif)

    Combined plot of the average, background-subtracted PCA Std1 Rate in ct/s/PCU (top) and the average HEXTE Cluster B rate in the 15-30 keV band in ct/s(bottom). Time, plotted on the x-axis, is shown in MJD at the bottom, and by calendar year at the top. The area under the plotted data points is shaded grey, to make the sequence of points easier to discern. Instead of drawing a diagonal line to connect two widely separated points, a pair of horizontal lines are used that meet at the half-way point between the plotted data. Please note that this is not an interpolation, and no meaning for the data value should be inferred. For many sources, the mission-long light curves span several PCA gain epoch boundaries. Black diamond symbols on the top and bottom x-axes indicate the dates at which PCA gain transitions occurred. Gain transitions can introduce sudden biases in count rates measured, or, more obviously, in hardness ratios, since the energy-to-channel relation is affected.

  • PCA hardness ratios (Source_ID_hardness.gif)

    Combined plot of two hardness ratios from PCA Std2 rates. The ratio of the rates in 4-9 keV/2-4 keV is displayed on the top panel, while the ratio of 9-20 keV/4-9 keV is shown on the bottom panel. Again, time is shown on the x-axis in MJD at the bottom, and by calendar year at the top. The area under the plotted data points is shaded grey, to make the sequence of points easier to discern. Instead of drawing a diagonal line to connect two widely separated points, a pair of horizontal lines are used that meet at the half-way point between the plotted data. Please note that this is not an interpolation, and no meaning for the data value should be inferred. As in the rates plot, black diamond symbols on the top and bottom x-axes indicate the dates at which PCA gain transitions occurred. Gain transitions can introduce sudden biases in count rates measured, or, more obviously, in hardness ratios, since the energy-to-channel relation is affected.

The GIF products for the microquasar GRS1915+105, which has been monitored nearly continously during RXTE's mission, are shown below.

mission-long light curve for GRS1915+105; rates fr
om PCA and HEXTE
Above: PCA and HEXTE rates from GRS1915+105. One point per observation. The diamonds on the time axis represent gain epoch changes. See notes on "PCA and HEXTE lightcurves," above, for further details.
mission-long hardness ratios for GRS1915+105 fr
om pca standard2 data
Above: Hardness ratios of GRS1915+105 from Std2 rates in the 2-4, 4-9 and 9-20 keV bands. The diamonds on the time axis represent gain epoch changes. See notes on "PCA hardness ratios," above, for further details.


Getting & Using Mission-Long Data Products

While the mission-long data products provide a broad overview to the user of the contents of the RXTE archive for many sources, they should not be considered as definitive products for generating publishable scientific results. There may be some cases where useful science can be performed using the mission-long data products alone, but generally the purpose of these files and figures is to enable interested archival investigators to get an accurate idea of the quality and extent of the data, and of the source behavior, for objects of interest. Since the mission-long data products contain a single point for each observation, and are based on the standard products, which themselves are generated using a general script and employ one single set of filtering criteria for all observations, the results will not typically be optimized to yield the most useful and reliable data in each individual case. As always, when doing scientific investigations, there is no substitute for performing the step-by-step analysis carefully for yourself.

The Mission-Long Data Products can be accessed in a variety of ways:

FTP/Web:

ftp://legacy.gsfc.nasa.gov/xte/data/archive/MissionLongData/

BROWSE:

Select RXTE catalog "XTEMLCAT" to search the RXTE Mission-Long Data Products source catalog in BROWSE.


The Fine Print: Details of Mission-Long Data Product Generation and Data Caveats

The same caveats that apply to the generation of RXTE Standard Products apply to the Mission-Long Data Products.

In addition, the following data caveats apply to the Mission-Long Data Products:

In some cases it may not be possible to filter out bad data perfectly. The most common cause for this would be contamination by another source in the field of view, although it is not always possible to identify a cause without investigating in detail. Users should be aware of the following potential data problems:

  • 1E 1740.7-2942 - possible field of view contamination in some observations
  • GALACTICCENTER - there are several objects in the field of view of this observation
  • PSR B1509-58 - possible field of view contamination in some observations
  • SMCPOSITION* - there are several objects in the fields of view of these monitoring observations
Also, users should be aware that these products represent the PCA and HEXTE count rates, not flux. As the calibrations of the instruments has gradually changed throughout time, and suddenly when high voltage changes have been commanded, the count rates may not be a completely faithful representation of flux. Users can check the PCA rates and hardness ratios of a canonical "constant" source such as the Crab to demonstrate these effects.


If you have a question about RXTE, please send email to one of our help desks.

This page is maintained by the RXTE GOF and was last modified on Wednesday, 02-Sep-2009 10:08:22 EDT.