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HEXTE Configurations: Event List



General Description

Event List mode data configurations are designated with a character string beginning with E, e.g. E_8us_256_DX1F. Files containing Event data are in science event format. The science data occupy the XTE_SE extension in the form of individual time-stamped binary event words, one per line, which fill the Event column. The words themselves are strings of ones and zeros, the combinations of which define the properties of each event with respect to a template of all possible properties within the configuration. This template is broken up into sections which, depending on the particular configuration, refer to things like Detector ID, PHA channel, etc. Thus, an individual event word, with its particular combination of ones and zeros, picks out, say, one Detector, one PHA channel, etc. The time stamps occupy the Time column, but are also encoded in the event word. The cluster position (see below) occupies the ClstrPosition column.


Detailed Description

The naming convention of the configuration provides a summary of the properties of the data. > For Event List data, the format

E_ttt_ccc_Dxxx

gives

E
- mnemonic denoting a generic binned mode
ttt
- the time bin size ("us" denotes microseconds)
ccc
- the number of energy channels
xxx
- hexadecimal byte mask indicating which bytes are telemetered

For example, in the E_8us_256_DX1F configuration, events are time-stamped with 8-microsec resolution, in 256 PHA channel bands. "DX1F" designates that 5 event bytes are to be included in the telemetry, giving full spectral and timing resolution with detector ID. > For more details about the different HEXTE Event List mode configurations, please consult the Technical Appendix.

The configuration name does not, however, give a complete description of the data. Rather, the key to understanding your Event data lies in "decoding" the event word template. The template itself occupies the TEVTB4 keyword in the header of the XTE_SE extension. It is written in DDL - the Data Descriptor Language - and can be displayed on the screen using the ftool fkeyprint. But since the TEVTB4 keyword can be somewhat long, it is often better to look first at the neighbouring - and complimentary - TDDES4 keyword. Also written in DDL, the TDDES4 keyword describes which events have been selected by the HEXTE processors. As an example, here is the value of TDDES4 in the E_8us_256_DX1F configuration:

TDDES4  = 'D[0~3] & E[0~131071] & C[0~255] & T[0~16]'
which, broken into its parts, means:

  • D[0~3] - events come from Detectors 0-3 (DDL's ~ symbol denotes a range).

  • E[0~131071] - each event is tagged with various kinds of detector information in addition to time, detector and channel.

  • C[0~255] - events come from channels 0-255.

Now let's look at the TEVTB4 keyword for the same E_8us_256_DX1F configuration:

TEVTB4  = 'E[0:63] {6}, E[CAL] {1}, T[0:15;1] {4}, D[0:3] {2}, 
           E[LE1,LE0] {2}, T[0:1;7.62939453125e-6] {17}, C[0:255] {8}'  
This looks a bit confusing at first, but the key to understanding what it means is to focus on the non-E tokens:

  • D[0:3] {2} - Detectors 0-3 (DDL's : symbol indicates a range). The {2} means that two bits are used to identify the Detector: 00 identifies Detector-0, 01 Detector-1, and so on.

  • T[0:1;7.62939453125e-6] {17} - Timing information. The {17} means that seventeen bits are used to specify the time when the event occurred - one bit for each inverse power of 2 from 1 to 17 giving a time resolution of 1/2**17 seconds. The zero at the beginning of the token is the value of the offset to be added to the time. When the original telemetry is converted on the ground to FITS, this timing information is decoded and written into the Time column.

  • C[0:255] {8} - Channels 0-255, i.e. the full 256 channel range of the HEXTE (256 = 2**8).

The E-tokens are for supplemental information about the events, most of which is actually redundant:

  • E[0:63] {6} - There are 64 channels devoted to pulse shape. In principle, pulse shape forms a third dimension of the response matrix, but present response matrices apply to all accepted pulse shape values together. There is no need to select events based on this token.

  • E[CAL] {1} - Each detector has a radioactive calibration source in its field of view. A value of 1 for this token indicates a calibration event, not a cosmic photon, but since calibration events are vetoed, they do not appear in data files. There is no need to select events based on this token.

  • E[LE1,LE0] {2} - "Lost events" are those events that are detected but cannot be assigned a PHA because the processor is occupied with another event. They are not included in data file and so there is no need to select events based on this token.

  • T[0:15;1] {4} - Not an E-token, but four bits devoted to the Science Major Frame Counter, equivalent to the column IDFNo in Archive Mode. There is no need to select events based on this token.

The cluster position, encoded as a single integer, occupies the ClstrPosition column. The values it can assume are:
   1.5-degree rocking     3.0-degree rocking
                                          
   1 or 65   on-target    8 or 72   on-target  
   2 or 66   +1.5 deg     4 or 68   +3.0 deg   
  32 or 96   -1.5 deg    16 or 80   -3.0 deg   

In common with other RXTE science event files, Event mode files have two additional extensions containing good time intervals (GTI). Both are called GTI.


Time and energy resolution

The time resolution for Event configurations is the resolution of the time stamps. This is not necessarily the same as the number in the name of the configuration, which is an approximation. For example, in the E_2ms_256_X05 configuration, the step size is really 1/2**9 seconds, i.e. 1.953125 milliseconds.

The configuration name gives the number of channel bands. For current HEXTE Event configurations there are just two possibilities: the full, 256 channels (e.g. E_2ms_256_X05), or no PHA information at all (e.g. E_8us_0_X06)


Reduction requirements and options

Light curves and spectra may be extracted from HEXTE Event list data using seextrct Apart from adjusting screening criteria, your primary reduction options include:

  • Selecting by applying a bitmask, in particular:
    • Detector IDs (depends on configuration - some do not have Detector ID)
  • Binning the events into a light curve
  • Binning the events into a spectrum (depends on configuration - some have only one channel)

Note that both light curves and spectra must be corrected for deadtime using hxtdead. See the RXTE Cookbook recipe Reduction and Analysis of HEXTE Data for further details on working with HEXTE Archive data.


Return, if you like, to the HEXTE Issues chapter or to the Table of Contents.

The ABC of XTE is written and maintained by the RXTE GOF. Please email xtehelp@athena.gsfc.nasa.gov if you have any questions or comments. This particular page was last modified on Tuesday, 14-Sep-1999 13:45:42 EDT.