  The James Webb Space Telescope (JWST) will perform exoplanet transit
spectroscopy in the coming decade promising transformative science results. All
four instruments on board can be used for this technique which reconstructs the
atmospheric transmission or emission spectrum of an exoplanet from
wavelength-dependent light curve measurements. Astrophysical and instrumental
noise and systematics can affect the precision and accuracy of the final
spectrum, and hence, the atmospheric properties derived from the spectrum.
Correlated noise and time-dependent systematics that can bias the measured
signal must be accounted for in the final uncertainties. However quantifying
these effects can be difficult with real data or simple analytic tools.
Existing publicly-available simulators for JWST do not adequately simulate
complex time domain processes on exoplanetary transit observations. We report
JexoSim, a dedicated time domain simulator for JWST including all four
instruments for exoplanet transit spectroscopy. JexoSim models both the
astrophysics and the instrument, generating 2-D images in simulated time akin
to a real observation. JexoSim can capture correlated noise and systematic
biases on the light curve giving it great versatility. Potential applications
of JexoSim include performance testing of JWST instruments, assessing science
return, and testing data reduction pipelines. We describe JexoSim, validate it
against other simulators, and present examples of its utility.
