The ABC of XTE

HEXTE Issues

Postscript version of this chapter

• Introduction
• HEXTE Structure, Operation and Properties
• HEXTE Modes

1. The HEXTE comprises two independent clusters of detectors. The clusters are known as Cluster-A and Cluster-B or as Cluster-0 and Cluster-1. Each cluster produces its own telemetry streams, i.e. no HEXTE files contain data from more than one cluster.

2. During normal operation, the two clusters rock on and off target to collect background data. The rocking has the following properties:
   • When Cluster-0 is on target, Cluster-1 is off target and vice versa.

   • The period of the rocking is set by the observer. The default setting is 32 seconds.

   • The amplitude is also set by the observer and can be +/- 1.5 degrees or +/- 3 degrees. The default is +/- 1.5 degrees.

   • The rocking axes of the two clusters are orthogonal resulting in a total of four different background fields (two per cluster) for each on-source pointing.

   • Regardless of the period and amplitude of the rocking, it takes 2 seconds for the clusters to change position, during which no data are taken. As a result, each background dwell is 4 seconds shorter than each on-source dwell.

   • Data acquisition is synchronized to the cluster rocking; that is, the boundaries of data frames occur when the Cluster changes position.

   • Reflecting the contents of the telemetry, HEXTE data files are not initially segregated by cluster position.

3. Each cluster comprises four NaI(Tl)/CsI(Na) phoswich detectors. The detectors are numbered 0-3. Depending on the configuration, the data files may contain either the summed signal from all the detectors or separate signals from the individual detectors.

   Important note: As described in a HEXTE Team memo, Detector-2 in Cluster-1 has been incapable of providing spectra (PHA) information since March 6, 1996. It should be omitted from spectral analysis, but may be included in non-spectral temporal analysis.

4. Each detector is split into two volumes, the primary NaI detector crystal and the CsI shield/light guide crystal. X-rays produce scintillations in the NaI crystal, while particles can produce scintillations in the both the NaI and CsI crystals. Since the scintillations in the two crystals, as recorded by the photomultiplier tube, have different rise times, the two kinds of event can be distinguished.

5. Events that survive rise-time rejection pass through - or not, as the case may be - the lower and upper pulse-height discriminators and through the pulse-shape discriminators. These clean NaI-only events are then subjected to pulse-height and pulse-shape analyses.

6. Each detector is equipped with a collimator comprising a honeycomb of hexagonal tubes that restrict the FOV to about 1 degree FWHM and 2.2 degree FWZI (full width at zero intensity).

7. Each detector is covered by a thin window of beryllium. The effect of the window is to curtail the lower boundary of the pass band at around 5 keV. The default setting for the lower level discriminator, however, is set at 10 keV.

8. Each detector contains a calibration source which allows the gain to be continually monitored and maintained.

9. Each detector measures deadtime and lost events continuously. In practice, however, deadtime is corrected with software as part of the reduction procedure.

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   HEXTE Properties

   The instrumental properties of the HEXTE are:

   
      Energy range:       10 - 250 keV 
      Energy resolution:  15% at 60 keV 
      Time resolution:    8 microsec 
      Spatial resolution: collimator with 1 degree FWHM 
      Collecting area:    2 x 800 square cm 
      Sensitivity:        1 Crab yields 360 counts/s per Cluster 
      Background:         50 counts/s per Cluster  
    
   

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   What HEXTE Data Are Like Before Being Telemetered

   Before looking at HEXTE data modes, it's a good idea to review briefly the nature of HEXTE data before they are packaged for telemetry. In particular, we'll examine the information associated with each event. Telemetry provides a subset of this information.

   HEXTE events are processed separately by each Cluster at a rate of 4.915 MHz (i.e. every 1/2**22 seconds). For the clean NaI-only events, i.e. the good ones, information from the various detectors and subsystems are combined to form a 56-bit event code. Of these 56 bits:

   • 8 bits are devoted to the 256 PHA channels

   • 17 bits are devoted to encoding the time of the event

   • 6 bits are devoted to the pulse shape

   • 2 bits are devoted to the detector ID

   The remaining 23 bits are used to count the science frames and for various flags - more details in the HEXTE chapter of the Technical Appendix.

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   HEXTE Modes

   A HEXTE mode is the broad scheme for packaging the data, while the configuration is the specific implementation of the mode. When you work with HEXTE data, the configuration is more conspicuous than the mode itself - in XDF, for example, you'll specify "E_8us_256_DX0F" data to reduce rather than "Event List Mode" data. In fact, as a general rule, data in different configurations - even of the same mode - should not be reduced together. And of course, contemporaneous data in different configurations must never be combined since they're the same events!

   Note that although there are five HEXTE data modes, there are only two basic formats used for the FITS data files, namely, Science Array (for Archive, Histogram Bin and Multiscalar Bin modes) and Science Event (for Event List mode). Nearly all targets can be observed using Event List mode.

   Here, we've summarized the modes and provided links to detailed descriptions of their configurations listed below. These descriptions focus on the data reduction aspects of the configurations, and, where file formats and keywords are discussed, complement the more general information in the Data Files chapter. Given that your data set will include, per cluster, at most two configurations from the much larger number of possibilities, the descriptions are written to be complete and self-contained: you only need to read about your particular configurations:

   1. Archive Mode is roughly equivalent to PCA Standard-1 and Standard-2. Always run, it is meant to provide a uniform and continuous archive of data collected over the lifetime of the mission. Under XDF, it appears as:

         CL0ArchData Archive
         CL1ArchData Archive
      

      and always provides 64-channel spectra per detector, as well as 1.0-second lightcurves in four energy bands.

   2. Event List Mode provides time-tagged information about every event that survives background rejection and passes through the various discriminator windows. A typical Event List configuration is E_8us_256_DX1F.

   3. Histogram Bin Mode provides PHA histograms, separated by Detector (or not, as the case may be) every 1-16 seconds. A typical Histogram Bin configuration is B_16s_256_0_255_DF.

   4. Multiscalar Bin Mode provides time histograms, with a resolution of 0.5 ms to 1 s, separated by Detector or not, in 2, 4, 6 or 8 PHA bands. A typical Histogram Bin configuration is B_1ms_1T_15_250_Q

   5. Burst List Mode is a triggered version of Event List Mode providing event-by-event information for a short time period, thereby staying within average telemetry limitations. The configuration CE_8us_256_DX0F is an example.

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   • Introduction
   • HEXTE Structure, Operation and Properties
   • HEXTE Modes

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   Please proceed to the Screening chapter. Or return to the Table of Contents.

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   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 Thursday, 16-Sep-1999 08:53:41 EDT.