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RXTE PCA and HEXTE Mission-Long Data Products Recipes from the RXTE Cook Book |
RXTE FAQ |
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- Overview: RXTE Mission-Long Data Products
- Introduction: Which Sources? Which Data?
- The Standard Products, Merged Lightcurves, and Mission-Long Data Products
- Filenames and Contents
- Getting & Using Mission-Long Data Products
- The Fine Print: Details of Mission-Long Data Product Generation
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.
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:
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).
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.)
The GIF products for the microquasar GRS1915+105, which has been
monitored nearly continously during RXTE's mission, are shown
below.
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 same caveats that apply to the generation of
RXTE Standard Products
apply to the mission-long data products.
Overview: RXTE Mission-Long Data Products
Introduction: Which Sources, Which Data?
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
Filenames and Contents
Each source may have up to three Mission-Long Data Products:
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.
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.
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.
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.
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
The Fine Print: Details of Mission-Long Data Product Generation
If you have a question about RXTE, please send email to one of our
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