RXTE Helpdesk/FAQ RXTE What's New HEASARC Site Map


EDS Configurations: Binned Burst Catcher



General Description

The generic Binned Burst Catcher configurations are designated with a character string beginning with CB, e.g. CB_500us_4M_0_249_H. Before describing the structure of Binned Burst Catcher data, we review how the burst catching works.

Two Event Analyzers are required to catch bursts: one for the trigger, the other for the catcher. The Data Buffer in the catcher EA is divided into four equal parts, one per "Interval". If, say, the trigger is activated during Internal N, the trigger EA will discover this fact during Interval N+1 and the burst flag will be set. During Interval N+2, the flag causes the catcher EA to read into its memory the data for Interval N-1 (the oldest in the Data Buffer) while at the same time accumulating data from Interval N+2 in the appropriate quarter buffer. During the accumulation of N+3 data, the N data are read out, as are the post-onset N+1 and N+2 data. The readouts then cease, yielding a train of data comprising four quarters the second of which contains the burst onset. For more details about how Binned Burst Catcher mode works, please consult the Technical Appendix.

Files containing Binned Burst Catcher data are in science array format. The science data occupy the XTE_SA extension in the form of histograms accumulated from good xenon events (i.e. those which survive background rejection) according to time, in one or more channel band. The column containing the histograms is called XeCnt and is preceded by the Time column which gives the time of the start of each accumulation. For Binned Burst Catcher mode, the accumulation time is the same as the Interval. This means that for each trigger activation, there will be a group of four contiguous Intervals - one per row. Binned Burst Catcher data always combine PCU.


Detailed Description

The name of the configuration provides a summary of the properties of the corresponding data. For Binned Burst Catcher data, the format

CB_ttt_ccX_ll_hh_b

gives

CB
- mnemonic denoting a binned burst cather mode
ttt
- the time bin size ("us" denotes microseconds)
cc
- the number of energy channels
X
- the code denoting the binning of the energy channels
ll
- the lower channel boundary
hh
- the upper channel boundary
b
- the code denoting the number of bits per bin

For example, in the CB_500us_4M_0_249_H configuration, four histograms with ~500-microsecond resolution are accumulated from channels 0 to 249, with the "M" channel distribution/binning scheme. "H" means that there are 8 bits per bin, i.e. that up to 256 counts can be accumulated per ~500 microseconds without overflowing the telemetry. For a complete list of Binned Burst Catcher configurations, see the RXTE PCA Configurations page. For more details about these configurations, please consult the Technical Appendix.

The configuration name does not, however, give a complete description of the properties of the data. This key information is written in the file itself and is accessible via the ftools fstruct, flcol, fdump and fkeyprint. We recommend that you run these tools on sample files (one, say, per configuration), starting with fstruct, the least verbose. Here are the key pieces of information, and where to find them:

  1. What is the true time resolution? The time resolution for Binned Burst Catcher configurations is the step size of the histograms. This is not necessarily the same as the number in the name of the configuration, which is an approximation. For example, in the CB_250us_8A_0_249_Q configuration, the step size is really 1/2**12 seconds, i.e. 244.140625 microseconds. To derive this number, work out the nearest inverse power of two from the configuration name.

  2. What are the channel boundaries and energy resolution? The configuration name gives the number of channel bands, but not the channel boundaries themselves, which are denoted by a code letter. To derive the channel boundaries, either look them up in the Technical Appendix, or run fdump on the data file and look at the header of the second column, i.e. the one containing the science data. The value of the 2CPIX2 keyword gives the channel boundaries. For example, in the CB_500us_4M_0_249_H configuration, the value of 2CPIX2 is '0~35,36~79,80~159,160~249' which tells you that the four bands cover channels 0-35, 36-79, 80-159 and 160-249. Fkeyprint can also give the value of 2CPIX2. The energy resolution also depends on the configuration: the bins in any spectrum you extract will be the same as the channel bands in the configuration

  3. What is the Interval time? The Interval time for Binned Burst Catcher configurations is the same as the histogram accumulation time. It is not encoded in the configuration name. To determine it, use fkeyprint to look at the DELTAT keyword. For example, in the CB_250us_2A_0_249_H configuration, the value of DELTAT is 1 second, so each trigger activation will result in 4 seconds of data.

    Note, however, that while the Interval time determines the duration of the data train, it is otherwise unimportant. For the accumulation time of the histograms only dictates the arrangement of the data in the file, rather than their scientific properties. The RXTE ftool saextrct transparently extracts data without your having to know how many steps each histogram contains or how often the histograms are accumulated. However, if you intend to write your own data reduction software, the accumulation time is crucial.

The above information - time resolution, channel boundaries etc - is also available in one place in the data file, namely the DDL string which occupies the TTDES2 keyword. DDL - Data Description Language - is a compact way of describing the properties of the data for downstream software. Although somewhat terse, it is not difficult to read. For example, the value of TTDES2 for the CB_250us_2A_0_249_H configuration is:

D[0~4] & E[X1L^X1R^X2L^X2R^X3L^X3R] & C[0~35,36~79,80~159,160~249] &  T[0.0;0.00048828125;128]
 
which, broken down into its parts, means:

  • D[0~4] - events come from detectors 0-4, i.e. PCUs 0-4 (DDL's ~ symbol denotes a range).

  • E[X1L^X1R^X2L^X2R^X3L^X3R] - each event comes from only one of the six elements per PCU, i.e. the six Xe anodes (DDL's ^ symbol denotes an exclusive or).

  • C[0~35,36~79,80~159,160~249] - four channel ranges, 0-35, 36-79, 80-159 and 160-249, are included.

  • T[0.0;0.00048828125;2048] - time steps with zero offset, 0.00048828125 seconds wide, 2048 per histogram.


Burst Trigger Modes

The Binned Burst Catcher modes are triggered by a second EA running a Burst Trigger Mode. The trigger EA is synchronized with the catcher EA, so the data have the same Intervals. The ModeSpecific column in the XTE_SA extension of the trigger data file contains the times of trigger activation. A zero denotes no activation. These configurations will also trigger HEXTE.

There are two flavors of Burst Triggers: High Priority and Low Priority. The corresponding configurations have names beginning with T, e.g. or TLA_1s_10_249_1s_500_F and TLM_31us_0_249_500ms_QN, respectively. The format for the High Priority Trigger is

TLA_ttt_ll_hh_rrr_ccc_b

gives

TLA
- mnemonic denoting a high priority burst trigger mode
ttt
- the time bin size
ll
- the lower channel boundary (usually 10)
hh
- the upper channel boundary (usually 249)
rrr
- the read out time
ccc
- the count threshold (in counts/bin)
b
- the code denoting number of bits per bin

Low priority triggers are denoted TLM, THM, or TEM, for triggers based on count rate, hardness ratio, or edge, respectively.

For a complete list of Burst Trigger configurations, see the RXTE PCA Configurations page. For more details about these configurations, please consult the Technical Appendix.


Reduction requirements and options

Apart from adjusting screening criteria, your primary reduction options will always include:

  • Increasing the size of time bins when extracting a light curve
  • Selecting a channel range when extracting a light curve

Depending on the configuration, you may also be able to extract a spectrum (crude, though, for most configurations).


Gain and offset

Gain and offset corrections are applied by the EDS to Binned Burst Catcher data.


Other features

In common with other configurations in science array format, Binned Burst Catcher files have two additional columns called Spillage and ModeSpecific, as well as a coda of keywords summarizing EDS status.


Return, if you like, to the PCA 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 13:45:43 EDT.