GTEXPOSURE:

Given a counts light curve prepared using gtbin for a specific
location on the sky, this tool computes the exposure (cm2-s)
associated with each time bin,allowing for a light curve in photons/s
to be computed.

If the light curve file contains DSS keywords from gtselect that
describe an acceptance cone on the sky from which the events are
extracted, then this tool will perform an aperture correction, i.e.,
it will adjust the exposure as a function of energy to account for the
tails of the psf that lie outside the acceptance cone.

In order to provide reasonably accurate flux estimates for broad
energy bands, the exposure must be weighted by a function that
approximates the spectral shape of the target source. This function
may either be specified using an XML model definition such as those
used with gtlike, or it may be given as a simple spectral index.

Usage: gtexposure infile scfile irfs srcmdl target specin ra dec rad
apcorr emin emax enumbins

General Parameters

infile [file]
Input count-space light curve file produced using gtbin.
              
scfile [file]
spacecraft pointing history file, obtained from the FSSC LAT data server.
              
irfs [string]
Reference to the LAT Instrument response functions e.g. P6_V3_DIFFUSE.
              
srcmdl = none [string]
XML file containing the Likelihood model for the target source.
              
target [string]
The name of the target source in the XML model file.
              
specin = -2.1 [double]
Photon spectral index to use for weighting the exposure as a function
of energy. If srcmdl=none and target="", then this is used in lieu of
the full Likelihood model.
              
(ra = 0) [double]
Right Ascension (degrees J2000) of the source (or extraction
region). This is used only if infile lacks DSS keywords describing the
acceptance cone.
              
(dec = 0) [double]
Declination (degrees J2000) of the source (or extraction region). This
is used only if infile lacks DSS keywords describing the acceptance
cone.
              
(rad = 180) [double]
Radius (degrees) of the extraction region. This will be used for the
aperture correction.
              
(apcorr = yes)
Flag to apply aperture-correction. Users may wish to disable this to
determine the full exposure as a function of time at a given location.
              
(emin = 100)
Minimum energy (MeV) over which to integrate the exposure. This is
used only if infile lacks DSS keywords describing the energy range of
the events.
              
(emax = 2e5)
Maximum energy (MeV) over which to integrate the exposure. This is
used only if infile lacks DSS keywords describing the energy range of
the events.
              
(enumbins = 30)
Number of energies, logrithmically-spaced, at which to evaluate the
exposure for the weighted integral. 30 over 4.5 decades in energy
should be sufficient for most spectral shapes.
