The SWIFT BAT Software Guide Version 6.3 30 ... - HEASARC - Nasa

180 APPENDIX D. BAT SOFTWARE TOOL REFERENCE
D.19 BATTBLOCKS
D.19.1 NAME
battblocks - estimate interesting Bayesian Blocks and burst durations
• for time-variable data
D.19.2 USAGE
battblocks infile outfile
D.19.3 DESCRIPTION
battblocks estimates interesting time intervals (GTIs) based on time variable event or rate data. It
does this using the Bayesian Block algorithm, which is intended to robustly compute time intervals
based on Bayesian analysis. (see Scargle 1998)
The user provides event data or a single-channel binned light curve. The light curve should be a
standard OGIP light curve FITS file (i.e. contain the proper HDUCLASn keywords which identify
whether the light curve is background subtracted or not). Both RATE and COUNT type light
curves are supported. It must be possible to multiply RATE by the TIMEDEL per-bin exposure
to arrive at counts per bin. If the light curve has been background subtracted, then the ERROR
column must be present.
battblocks relies on the keyword information in the input file as being correct. In particular,
HDUCLAS1 should be “EVENTS” or “LIGHTCURVE”, depending on the input file type. For light
curves, HDUCLAS2 should be “NET” or “TOTAL” depending on whether the file is background
subtracted. For light curves, HDUCLAS3 should be “RATE” or “COUNT”, although the user can
override this with the hduclas3 parameter. There must also be a way to determine the per-sample
exposure, either with the standard TIMEDEL keyword, or a column named either TIMEDEL or
EXPOSURE.
The Bayesian block algorithm can be used to estimate time intervals for any time series. For
bursts, it can also estimate several measures of the burst duration, and the interval of the peak
flux. The burst duration is estimated by locating bracketing background intervals, and then locating
intervals which enclose 90% and 50% of the total counts. These are stored in the ‘durfile’ with
extension names GTI T90 and GTI T50. Users should be aware that these algorithms are sensitive
to systematic variations in the background. Some care must be taken to filter the incoming light
curve. Users can also choose their own percentage fluence with the ‘txx’ keyword; the result is
stored in the GTI TXX extension.
The interval of the peak flux is estimated by sliding a time window across the light curve or
event data and finding the epoch with the highest counts. The user can select the size of the
window. The corresponding GTI is written to ‘durfile’ with the extension name ‘GTI PEAK’.