National Aeronautics and Space Administration

EDS Configurations: Generic Single Bit

• General Description
• Detailed Description
• Reduction requirements and options
• Gain and offset
• Other features

The generic Single Bit configurations are designated with a character string beginning with SB, e.g. SB_500us_50_249_2s. Files containing Single Bit data are in science array format. The science data occupy the XTE_SA extension in the form of single arrays of the time stamp and the counts accumulated within the time resolution. The column containing the arrays is called XeCnt and is preceded by the Time column which gives the time of the start of each accumulation. In addition, Single Bit configurations always:

• combine data from all five PCU and all anodes, and
• contain only one channel band.

The mode is called "single bit" because the Experiment Data System (EDS) onboard RXTE accumulates a stream of ones and zeroes from good xenon events (i.e. those which survive background rejection): a zero indicates a clock tick; a one indicates an event. The XTE Fits Formatter (XFF) converts this bit stream into a science array file which has the same format as binned data.

SB_ttt_ll_hh_rr

For example, in the SB_500us_50_249_2s configuration, data are accumulated with ~500-microsecond resolution, in one band from channels 50-249, every 2 seconds. For a complete list of available Single Bit mode configurations, see the RXTE PCA Configurations page. For more details about the different Single Bit mode configurations, please consult the Technical Appendix.

The only important property missing from the configuration name is the exact time resolution, which, for Single Bit mode, is the time between clock ticks. This is not necessarily the same as the number in the configuration name, which is an approximation. For example, in the SB_31us_0_249_500ms configuration, the clock really ticks every 1/2**15 seconds, i.e. 30.517578125 microseconds. To derive this number, work out the nearest inverse power of two from the configuration name.

Another place to find a description of the data properties is in 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 SB_125us_18_35_1s configuration is:

D[0~4] & E[X1L^X1R^X2L^X2R^X3L^X3R] & C[18~35] & T[0.0;0.0001220703125;8192]
 

• 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[18~35] - one channel range, 18-35, is included.

• T[0.0;0.0001220703125;8192] - clock ticks with zero offset, every 0.0001220703125 seconds, 8192 per array.

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