Here we sketch all the steps involved in producing the files needed by xspec, namely, source and background spectra and a response. To avoid confusing generalities, this example assumes that your observation is of a flaring AGN.
- Use XDF to find your Standard-2 datafiles:
Instructions can be found in the Getting Started Guide. We recommend naming the XDF output file something like std2.xdf.
- Use XDF to find the filter file:
select the Standard Products subsystem, and at the bottom of the AppId window, select Filter_File. We recommend naming the XDF output file something like filter.xdf. Please note:
- If the data you want to examine span more than one ObsId, then it's a good idea to merge the corresponding filter files. Use the ftool fmerge, making
sure the input files are in increasing time order and adding lastkey=TSTOP to the command line. For more information, type "fhelp fmerge".
- If you don't have a filter file, then generate one following the recipe Creating XTE Filter Files.
- Examine the filter file and create a GTI file:
The filter file tells you the status of the satellite and its instruments during your observation and can be used to identify periods of good data. To plot it, type "fplot offset=yes" followed by the name of the filter file. Enter "Time" as the x-axis. For the y-axes, it depends what in particular we want to see:
- How many PCUs were on? Plot the columns:
PCU0_ON
PCU1_ON
PCU2_ON
PCU3_ON
PCU4_ON
NUM_PCU_ON
Periods where the number of PCUs on is different should be reduced separately and combined later - see the recipe Combining 3-PCU, 4-PCU and 5-PCU Spectra for details.
Another thing you can determine from the number of PCU on is when the SAA passages occur. At these times, all PCU are turned off, so NUM_PCU_ON goes to zero.
- Were there any Earth occultations? Plot the column ELV. Here, zero is defined as pointing at the horizon, negative is looking at Earth, and positive is looking at the source. We recommend filtering out data where the elevation angle is less than 10 degrees. Some data sets have shown effects of bright Earth at ~5 degrees, so you should look at your data before including anything closer than 10.
- Was the pointing stable? Plot the column OFFSET This should vary quite regularly on scales of ~10^-3 degrees, and these minor variations will not affect your analysis. They are mainly due to the HEXTE rocking motion. Often, however, you will see steep spikes at the very beginning or end of a given observation; this is where some small amount of slew was included in the on source data file. These must be filtered out, and we recommend using OFFSET < 0.02 as the selection criterion.
- Were there any SAA passages in your observation? Plot the TIME_SINCE_SAA column, which is a parameterization of coordinates relative to the SAA which approximately represents minutes since the peak of the last SAA passage. Since it is a coordinate parameterization, it will jump to negative values before the SAA, and in orbits where there is no SAA passage, it is defined to be 100. When TIME_SINCE_SAA is between zero and, for instance, 30 minutes, the background will increase dramatically, lowering your signal to noise. For faint sources, you may want to discard this data. See below.
- Was there significant electron contamination? Plot the ELECTRONn columns. (For each PCU, these numbers should be effectively the same.) For analysis of faint sources, the PCA team recommends filtering out time when ELECTRON2 is larger than 0.1.
To list all the columns in a filter file, use the ftool flcol. For more information about plotting, please check the recipe Plotting with PLT.
Note that all but the earliest RXTE data files have two extensions called GTI, the first of which contains the times when:
- Telemetred data are present
- The satellite is pointing at the nominal source position, as derived from the spacecraft attitude
- The nominal source position is not occulted by the Earth, as predicted by mission operations
- The satellite is outside the South Atlantic Anomaly, as predicted by mission operations
The second GTI extension includes time when only the first of the above apply.
These GTI are applied by answering "APPLY" to the saextrct prompt Input GTI files to be OR'd. Filtering with only the GTIOR file, however, is not usually sufficient, which is why we recommend making a GTIAND file using the selection criteria discussed above. Mainly, this is because the SAA passages may not occur exactly as predicted, nor do the Earth occults; in addition, the GTIOR's criterion for determining occults does not take into account possible bright Earth effects.
To make your own GTI file to be AND'ed in the extractor, use the tool maketime. This tool will make a FITS GTI file from any FITS file with a time column and other columns from which to select. It will take either Fortran or C style. For example, to filter out occults and bright Earth, slews, and times where all PCU's are off during SAA and only three are on otherwise:
olegacy [84] [day] ~: maketime
Name of FITS file and [ext#][] FP_3c7a9f5-3c855c1.xfl
Name of output FITS file[] basic.gti
Selection Expression[] elv.gt.10.and.offset.lt.0.02.and.num_pcu_on.eq.3
Column containing HK parameter names[NAME]
Column containing HK parameter values[VALUE]
Column containing HK parameter times[TIME]
Flag, yes if HK format is compact[yes] no
Note that the tool is case insensitive with regard to column names and that the last input must be changed from the default 'yes' to 'no' for XTE data. This file can then be put into either saextrct or seextrct as the GTI file to be AND'ed.
For analysis of faint sources, we recommend the additional expressions (TIME_SINCE_SAA > 30 || TIME_SINCE_SAA < 0.0) && ELECTRON2 < 0.1, which will filter out data within thirty minutes of the peak of SAA and that with high electron contamination.
- Create column selection files:
After verifying which PCUs were on during your observation, create ASCII files containing the columns from which you want to extract spectra. We recommend two files: one containing all the layers of all the PCUs that were on; the other containing just the top layer of all the PCUs that were on.
For example, if PCU0, PCU1 and PCU2 were the only PCUs on during your observation, you could create the file pcu012.col containing:
X1LSpecPcu0
X1RSpecPcu0
X2LSpecPcu0
X2RSpecPcu0
X3LSpecPcu0
X3RSpecPcu0
X1LSpecPcu1
X1RSpecPcu1
X2LSpecPcu1
X2RSpecPcu1
X3LSpecPcu1
X3RSpecPcu1
X1LSpecPcu2
X1RSpecPcu2
X2LSpecPcu2
X2RSpecPcu2
X3LSpecPcu2
X3RSpecPcu2
and the file pcu012_top.col containing:
X1LSpecPcu0
X1RSpecPcu0
X1LSpecPcu1
X1RSpecPcu1
X1LSpecPcu2
X1RSpecPcu2
For more information about PCU and anode selection, please check the recipe Selecting Data by Detector and Anode.
- Extract a light curve:
To verify that you've removed all the bad data and to identify parts of the light curve that you might want to focus on - the flare, say - extract a light curve with saextrct, as described in the
ABC Guide. Input std2.xdf, your GTI file (at the prompt Input GTI file to be AND'd with INFILE) and column selection file (at the prompt Name of COLUMNS to be accumulated).
Plot your light curve with "fplot offset=yes", entering TIME as the x-axis and RATE[ERROR] as the y-axis. Looking at the plot:
- Check whether there are any suspicious looking spikes or drop-outs indicative of bad data. These are usually found at the extrema of sections of good data.
- Note down the start and stop times of the parts of the light curve you want a spectrum from, avoiding any bad data. In our example, we'll be isolating the AGN flare and a stretch of non-flare emission.
Then run timetrans to generate time filters. Here, we'll call then flare.tint and quiescent.tint.
- Extract spectra:
Rerun saextrct to extract the PHA files, this time applying your time filters at the prompt Input time intervals t1-t2, t3-t4 in seconds. You should extract two spectra for each of your time filters: one for all the layers and one for just the top layer. In our example, we'll apply pcu012.col and pcu012_top.col to produce the PHA files:
flare_all.pha quiescent_all.pha
flare_top.pha quiescent_top.pha
- Correct for deadtime:
Deadtime exceeds 1 percent when the count rate per PCU exceeds 1000 counts per second. To correct for deadtime, please consult the recipe PCA Deadtime.
- Create background datafiles:
This is a fast-changing topic. The
program "pcabackest" is used to create pca background files for your
data, based on models formulated by the PCA team. Please check the
PCA Digest page for the latest
information on which background models to use, and follow the recipe for
Running Pcabackest to
apply those models and create appropriate PCA backgrounds for your data.
- Extract background spectra:
Use exactly the same extraction parameters as you did for your
source data, but with the list of pcabackest-generated datafiles as input instead of std2.xdf. In our example, we'd end up with, say:
flare_back_all.pha quiescent_back_all.pha
flare_back_top.pha quiescent_back_top.pha
- Create response matrices:
PCA response matrices should match the PHA in epoch (to ensure that the high-voltage setting is the same), and in PCU and anode. Follow the recipe Creating PCA Responses to make responses corresponding to the column-selection files used. For our example, we'd use pcarsp to create:
pca_LR1_961029.rsp
pca_xe_961029.rsp
