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Document integer OBJECT fields in bayestar-localize-coincs

Leo P. Singer requested to merge leo-singer/ligo.skymap:docuent-integer-row-id into master

The script bayestar-localize-coincs saves FITS files with integer-valued OBJECT cards in their FITS headers. This is somewhat unusual, because the OBJECT card is usually a string used to give the designation of an astronomical object (such as M83 or Feige 55). The OBJECT cards saved by bayestar-localize-coincs, on the other hand, are integers that refer to a coinc_event table row in the input LIGO-LW file. Add an explanation to the bayestar-localize-coincs command-line help message and also to the COMMENT in the FITS header.

Here is an example FITS headere generated by bayestar-localize-coincs:

SIMPLE  =                    T / conforms to FITS standard
BITPIX  =                    8 / array data type
NAXIS   =                    0 / number of array dimensions
EXTEND  =                    T

XTENSION= 'BINTABLE'           / binary table extension
BITPIX  =                    8 / array data type
NAXIS   =                    2 / number of array dimensions
NAXIS1  =                   40 / length of dimension 1
NAXIS2  =                19200 / length of dimension 2
PCOUNT  =                    0 / number of group parameters
GCOUNT  =                    1 / number of groups
TFIELDS =                    5 / number of table fields
TTYPE1  = 'UNIQ    '
TFORM1  = 'K       '
TTYPE2  = 'PROBDENSITY'
TFORM2  = 'D       '
TUNIT2  = 'sr-1    '
TTYPE3  = 'DISTMU  '
TFORM3  = 'D       '
TUNIT3  = 'Mpc     '
TTYPE4  = 'DISTSIGMA'
TFORM4  = 'D       '
TUNIT4  = 'Mpc     '
TTYPE5  = 'DISTNORM'
TFORM5  = 'D       '
TUNIT5  = 'Mpc-2   '
PIXTYPE = 'HEALPIX '           / HEALPIX pixelisation
ORDERING= 'NUNIQ   '           / Pixel ordering scheme: RING, NESTED, or NUNIQ
COORDSYS= 'C       '           / Ecliptic, Galactic or Celestial (equatorial)
MOCORDER=                    9 / MOC resolution (best order)
INDXSCHM= 'EXPLICIT'           / Indexing: IMPLICIT or EXPLICIT
OBJECT  =                    0 / Unique identifier for this event
INSTRUME= 'H1,L1,V1'           / Instruments that triggered this event
DATE-OBS= '2012-07-17T22:13:12.220679' / UTC date of the observation
MJD-OBS =    56125.92583588749 / modified Julian date of the observation
DATE    = '2020-09-25T17:26:49.000000' / UTC date of file creation
CREATOR = 'BAYESTAR'           / Program that created this file
ORIGIN  = 'LIGO/Virgo'         / Organization responsible for this FITS file
RUNTIME =         53.403897459 / Runtime in seconds of the CREATOR program
DISTMEAN=     299.256659137929 / Posterior mean distance (Mpc)
DISTSTD =    92.05707284960283 / Posterior standard deviation of distance (Mpc)
LOGBCI  =    4.326877727439793 / Log Bayes factor: coherent vs. incoherent
LOGBSN  =    7.485118470191388 / Log Bayes factor: signal vs. noise
VCSVERS = 'ligo.skymap 0.5.1.dev5+g5fa7938.d20200925' / Software version
HISTORY
HISTORY Generated by calling the following Python function:
HISTORY ligo.skymap.bayestar.localize(event=..., waveform='o2-uberbank', f_low=3
HISTORY 0, min_inclination=0.0, max_inclination=1.5707963267948966, min_distance
HISTORY =None, max_distance=None, prior_distance_power=2, cosmology=False, mcmc=
HISTORY False, chain_dump=None, enable_snr_series=True, f_high_truncate=0.95)
HISTORY
HISTORY This was the command line that started the program:
HISTORY bayestar-localize-coincs runs/O3/bns_astro/events.xml.gz --coinc-event-i
HISTORY d 0 -o /tmp
COMMENT
COMMENT The integer value in the OBJECT card in this FITS header is a row ID
COMMENT that refers to a coinc_event table row in the input LIGO-LW document.
COMMENT

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