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EDS Configurations: Delta Binned



General Description

Files containing Delta-Binned data are in science array format. The science data occupy the XTE_SA extension in the form of histograms of the time-lags between successive events accumulated from good xenon events (i.e. those which survive background rejection). The column containing the histograms is called XeLag and is preceded by the Time column which gives the time of the start of each accumulation. Note that Delta-Binned configurations always:

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


Detailed Description

The naming convention of the configuration provides a summary of the properties of the data. For Delta-Binned mode data, the format

D_ttt_ll_hh_bbbb_rrr_X

gives

D
- mnemonic denoting a delta-binned mode
ttt
- the time bin size ("us" denotes microseconds)
ll
- the lower channel boundary
hh
- the upper channel boundary
bbbb
- the number of bins
rrr
- the readout time
X
- code denoting the number of bits per bin
For example, in the D_1us_0_249_1024_16s_F configuration, histograms of time-lags with ~1-microsecond resolution are accumulated, in one band from channels 0-249, in 1024 steps every 16 seconds. "F" means that there are 16 bits per bin, i.e. that up to 65,536 events can be put in the same 1-microsecond-wide time-lag bin per without overflowing the telemetry. "H" would mean that there are 8 bits per bin.

The only important property missing from the configuration name is the true time resolution, which, for Delta-Binned mode, is the step size of the time-lag histograms. This is not necessarily the same as the number in the configuration name, which is an approximation. For example, in the D_4us_0_249_1024_64s_F configuration, the time-lag steps are really 1/2**18 seconds wide, i.e. 3.81469726562 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 D_1us_0_249_1024_16s_F configuration is:

D[0~4] & E[X1L^X1R^X2L^X2R^X3L^X3R] & C[0~249] & L[0.0;9.5367431640&625e-07;1024]
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[0~249] - one channel range, 0-249, is included.

  • L[0.0;9.5367431640&625e-07;1024] - time-lags with zero offset, in steps 9.5367431640&625e-07 seconds wide, 1024 per histogram.

Note that the read-out or accumulation time only dictates the arrangement of the data in the file, rather than their scientific properties. The XTE ftool saextrct transparently extracts data and assigns times 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. It is written in the DELTAT keyword, while the number of steps per histogram is in TDIM2 keyword.


Warning about overflow and wrap-around

The ability of a given Delta-Binned configuration to accommodate all events depends on the count rate of the source. Of course, once you have your data in hand, it's too late to change the configuration; however, it is important to know the constraints - just in case your observation exceeds them.

For bright sources, the first few bins are susceptible to overflow: the maximum number of time-lags per bin is 256 for configurations ending with "H" and 65,536 for configurations ending with "F".

For weak sources, long time-lags can be a problem. The time-lags themselves are calculated using a 14-bit number, which means that any lags longer than than 2**14 times the step size (time resolution) will wrap around.


Reduction requirements and options

The XTE GOF does not at present supply any software that can reduce or analyze Delta-Binned data.


Gain and offset

Gain and offset corrections are applied by the EDS to generic Delta-Binned data.


Other features

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


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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 Tuesday, 14-Sep-1999 13:45:43 EDT.