Spectral Rebinning

By grouping channels in appropriately large numbers, the combined signal-to-noise of groups will jump into the Gaussian regime. There are two ways to do this: the FTOOL grppha, or the RGS pipeline. grppha can group channels using an algorithm which bins up consecutive channels until a count rate threshold is reached. This method conserves the resolution in emission lines above the threshold while improving statistics in the continuum.

To rebin with grppha in the Command Window, type:

grppha

         > Please enter PHA filename[] P0134520301R1S001SRSPEC1001.FIT
         > Please enter output filename[] P0134520301R1S001SRSPEC1001.bin.FIT
         > GRPPHA[] group min 30 
         > GRPPHA[] exit

The disadvantage of using grppha is that, although channel errors are propagated through the binning process correctly, the errors column in the original spectrum product is not strictly accurate. The problem arises because there is no good way to treat the errors within channels containing no counts. To allow statistical fitting, these channels are arbitrarily given an error value of unity, which is subsequently propagated through the binning. Consequently, the errors are overestimated in the resulting spectra.

The other approach, which involves calling the RGS pipeline, bins the data during spectral extraction. The following rebins the pipeline spectrum by a factor 3. Note that the entry stage is much later in the rgsproc chain of processes, so this takes far less time than when run with an earlier entry stage.

To rebin with rgsproc in the GUI:

1)

Call rgsproc in the ``Available Tools'' panel of the Hera GUI.

2)

Next to ``(re)start processing at'', use the pull-down menu to select ``4:spectra''. Next to ``dispersion channel rebinning factor'', enter 3; next to ``number of response energy bins'', enter 4000; verify ``background correct the spectra'' is set to ``no''.

3)

Click ``Run''.

To rebin with rgsproc in the Command Window, type:

rgsproc orders='1 2' rebin=3 rmfbins=4000 entrystage=4:spectra
$$ finalstage=5:fluxing bkgcorrect=no

where

orders - dispersion orders to extract
rebin - wavelength rebinning factor
rmfbins - number of bins in the response file; this should be greater than 3000
entrystage - entry stage to the pipeline
finalstage - exit stage for the pipeline

One disadvantage of this approach is that you can only choose integer binning of the original channel size. To change the sampling of the events, the pipeline must be run from the second stage (``angles'') or earlier.

To change the event sampling with rgsproc in the GUI:

1)

Call rgsproc in the ``Available Tools'' panel of the Hera GUI.

2)

Next to ``(re)start processing at'', use the pull-down menu to select ``2:angles''. Next to ``number of response energy bins'', enter 4000; verify ``background correct the spectra'' is set to ``no''. Next to ``number of beta bins'', enter ``1133''.

3)

Click ``Run''.

To change the event sampling with rgsproc in the Command Window, type:

rgsproc orders='1 2' nbetabins=1133 rmfbins=4000 entrystage=2:angles
$$ finalstage=5:fluxing bkgcorrect=no

where the parameters are as defined previously, and

nbetabins - number of bins in the dispersion direction; the default is 3400

The disadvantage of using rgsproc, as opposed to grppha, is that the binning is linear across the dispersion direction. Velocity resolution is lost in the lines, so the accuracy of redshift determinations will be degraded, transition edges will be smoothed, and neighboring lines will become blended.