NAME

nuhotpix -- Search for hot and flickering pixels in NuSTAR event files

USAGE

nuhotpix infile outfile [parameter = < value >]

DESCRIPTION

'nuhotpix' searches for anomalous (hot and flickering) pixels by applying statistical tests to the NuSTAR event file. For each detector of the Focal Plane Module, and for each time interval defined by the input parameter 'binsize', the events are binned into images. For each image, hot/flickering pixel are identified by comparing the counts in each pixel to the mean background counts.

First, for each pixel, the probability for its counts to be a Poisson fluctuation of the background is computed using the incomplete Gamma function Gamma(c,b), where (c) are the counts in the pixel and (b) is the background evaluated using the mean of the total counts on the detector image. The parameter 'impfac' allows the user to rescale the background level. If the pixel probability is lower than a Poisson probability threshold (set through the parameter 'logpos'), the pixel is considered a hot/flickering pixel candidate. To confirm the hot/flickering pixel candidates, their counts are compared to the mean counts of the surrounding pixels in a square cell. By setting the cell size (parameter 'cellsize') to the order of the PSF core, it is possible to discriminate a hot/flickering pixel from a pixel of the X-ray source.

As a second step, by setting the parameter 'cleanflick' to "yes" (default), hot/flickering pixels are searched using a background estimated locally in its surrounding pixels contained in a cell with size defined by the parameter 'cellsize'. If the local background is zero, the pixel is flagged as hot/flickering if the number of counts is larger than a threshold value (set by the parameter 'bthresh'). In this second step, the hot/flickering pixels search may be iterated by setting the parameter 'iterate' to "yes".

To flag events, the 'nuhotpix' module updates the 'STATUS' column of the event file (see 'nuflagbad' help file). The format of this column is a 16-bit binary number specifying the quality of the flagging.

The task also flags events if any of the 8 neighborhood pixels falls on a hot/flickering pixel. The position of these neighbor pixels is stored in a new column, named 'HOTPOS', added by the task. The format of this column is a 8-bit binary number.

The values of the 'STATUS' column used by 'nuhotpix' is the following:

Events STATUS flags
  • b0000000000100000 Event falls in a hot/flickering pixel
  • b0000000001000000 Event has a neighbor hot/flickering pixel

    • The list of hot/flickering pixels is stored in the four 'BADPIX' extensions, one for each of the four detectors, of the output event file. Optionally, if requested by the user through the parameter 'outhpfile', the hot/flickering pixels list is also written in a separate output file.

    The 'BADPIX' extensions is updated with the positional and temporal information of the hot/flikering pixels. The start and end time values of the time interval where the pixel is hot/flickering is stored in the two columns 'TIME' and 'TIME_STOP'. The 16-bit binary number column 'BADFLAG' (see 'nuflagbad' help file) is updated by 'nuhotpix' as follows:

    Hot/flickering pixels BADFLAG values
  • b0000000000100000 Event falls in a hot/flickering pixel

    • PARAMETERS

    infile [file name]
    Name of the input FITS event file.

    outfile [file name]
    Name of output FITS event file.

    (outhpfile = DEFAULT) [file name]
    Name of the output hot pixel file. If outhpfile=DEFAULT the standard naming convention is used for the output file. If outhpfile=NONE no output file is created.

    (binsize = 600) [real]
    Bin size (seconds) for count image generation.

    (cellsize = 5) [integer]
    Search cell size in units of pixels. Must be an odd integer greater than one.

    (impfac = 1.0) [real]
    Rescale factor for the background level.

    (logpos = -6) [real]
    Logarithm of the Poisson probability threshold for rejecting a pixel. Must be negative.

    (bthresh = 6.) [real]
    Counts threshold for candidate hot/flickering pixels with zero counts in the local background

    (cleanflick = yes) [boolean]
    If set to "yes", search and flag hot/flickering pixels using local background.

    (iterate = yes) [boolean]
    If set to "yes", iterate the search of hot/flickering pixels (only if 'cleanflick' is set to 'yes').

    (clobber=no) [boolean]
    If set to yes, overwrite the output file.

    (history=yes) [boolean]
    If set to yes, write parameter values and other information in HISTORY blocks.

    (chatter = 2) [integer]
    Chatter Level (min=0, max=5).

    EXAMPLES

    1. Search and flag hot/flickering pixels in the NuSTAR event file named nu40060001001A_uf.evt. The list of hot/flickering pixels are stored in the 'BADPIX' extensions of the output event file and in the output hot pixel fits file named 'nu40060001001A_outhp.fits'. 

      >  nuhotpix infile=nu40060001001A_uf.evt outfile=nu40060001001A_out.evt

      BUGS

      SEE ALSO

      LAST MODIFIED

      July 2012