celldetect: Data products description

Source candidate list

The main output of celldetect is a source candidate list. If signal-to-noise ratio in a detect cell ex- ceeds the threshold set by the user, the cell location is considered a source candidate and the following properties are sent to output:2

Data item Unit Description

RA deg Source candidate right ascension

DEC deg Source candidate declination

RA ERR deg RA error (see notes)

DEC ERR deg DEC error (see notes)

X pixel Source candidate X position

Y pixel Source candidate Y position

X ERR pixel X error (see notes)

Y ERR pixel Y error (see notes)

DETSIZE pixel Size of the detect cell

BKGSIZE pixel Size of the background cell

NPIXSOU pixel Number of pixels in source cell

NPIXBKG pixel Number of pixels in background cell

NET COUNTS count Net source counts

NET COUNTS ERR count Error in net source counts

BKG COUNTS count Background counts (scaled to source cell)

BKG COUNTS ERR count Error in bkg counts (scaled to source cell)

NET RATE† count/s Source count rate

NET RATE ERR† count/s Source count rate error

BKG RATE† count/s/pixel Background count rate

BKG RATE ERR† count/s/pixel Background count rate error

EXPTIME† s Effective exposure time

SIG MINOR pixel Minor axis of the source ellipsoid (see notes)

SIG MAJOR pixel Major axis of the source ellipsoid (see notes)

INCL ANGLE deg Position angle of the major axis (see notes)

SNR Source candidate signal-to-noise

PSFRATIO Ratio of source size to PSF size at source location

BLOCK Block factor at which the source was detected

Notes on individual items:

X, Y: Location of the detect cell. If input parameter centroid is set to ‘yes’, then X and Y are centroids of photon distributions in the detect cell. If centroid is set to ‘no’, then X and Y are merely locations of the cells.

X ERR, Y ERR: Uncertainties on centroid location. Calculated only if input parameter centroid set to ‘yes’, otherwise filled with ‘INDEF’.

RA, DEC: Right ascension and declination corresponding to X and Y. Calculated only if WCS key- words are present in the data, otherwise filled with zeros.

RA ERR, DEC ERR: RA and Dec errors, corresponding to X ERR and Y ERR. Calculated only if input parameter centroid set to ‘yes’, otherwise filled with ‘INDEF’.

DETSIZE, NPIXSOU: The current options for determining the size of the detect cell are either a fixed cell size or to let the code calculate a cell size based on the encircled energy specified by the user

and a knowledge of the PSF from libraries. The final cell size must be unity or an integer which is divisible by 3. NPIXSOU is the actual number of pixels in the detect cell, and it is equal to the square of DETSIZE.

BKGSIZE, NPIXBKG: If either background map or background value are provided then these values are equal to DETSIZE and NPIXSOU, respectively. If background map and background value are not provided then background is estimated from the input data, via calculating events in the frame surrounding the detect cell. The size of the background frame is calculated from DETSIZE, by multiplying it by√2 and then rounding it up or down to the closest number of the same parity as the detect cell size (in that way two goals are achieved: first, the area of the background frame is roughly the same as the area of the detect cell, and second, the background frame is centered on the same location as the detect frame). NPIXBKG is the actual number of pixels in the background frame. If the background frame is entirely within the data set, then NPIXBKG is equal to BKGSIZE2− DETSIZE2; it is reduced appropriately if the background frame falls outside the data set.

NET COUNTS, NET COUNTS ERR: Net source counts and the corresponding error. Counts in the detect cell reduced by the number of background counts scaled to the detect cell.

BKG COUNTS, BKG COUNTS ERR: Background counts scaled to the detect cell area. SNR: Signal-to-noise ratio in the detect cell, as calculated by the tool.

EXPTIME: Effective exposure time in the detect cell, taken from the exposure map. Not implemented at present; filled with zeros.

* RATE, * RATE ERR: Rates in the detect cell; they are simply the appropriate * COUNTS and * COUNTS ERR values divided by EXPTIME. Not implemented at present; filled with zeros. SIG MAJOR, SIG MINOR: Major and minor principal axes of the distribution of events in the detect

cell (i.e. axes of the 1-sigma ellipsoid). Calculated only if input parameter centroid set to ‘yes’, otherwise filled with ‘INDEF’.

INCL ANGLE: The position angle of the major axis of the ellipsoid. Calculated only if input parameter centroid set to ‘yes’, otherwise filled with ‘INDEF’. Note: INCL ANGLE is equal to zero if the major axis is pointing north; this follows the “astronomical” convention for position angle, and is used by all detect tools.

PSFRATIO: The ratio of the source size, defined as sqrt(SIG MINOR*SIG MAJOR), to PSF size, defined as the effective radius at which PSF contains 0.393 encircled energy (this value corresponds to 1 sigma for a 2-D Gaussian). Calculated only if centroid set to ’yes’.

BLOCK: For data sets larger than 2048 x 2048, celldetect utilizes the ”recursive blocking scheme”: first the inner 2048x2048 pixel region of the data set is searched for sources, then the inner 4096x4096 pixel region (excluding the part already analyzed) is blocked by 2 and searched for sources, then the inner 8192x8192 is blocked by 4, etc. For each consecutive search the part that has already been searched for sources in higher resolution is removed from analysis, so that for each region on the data set just one blocking factor is used. The value of BLOCK provides the information on the blocking factor used at the position of the source.

Signal-to-noise ratio map (snrmap)

celldetect can be run with convolve parameter set to ‘yes’. This option causes the tool to use a Fourier transform, instead of the direct calculation of counts in the detect cell.

If convolve is set to ‘yes’, one may request, by setting the parameter snrfile, to save the signal-to-noise map (snrmap). This is a FITS primary image containing one HDU, with no extensions. The pixels in snrmap correspond to pixels in the data, and the values in them give the signal to noise ratio as calculated by the tool.