Response files are now available. Please note that cross talk between orders (order overlap) affects all orders, and should not be ignored, as doing so will lead to significant errors. What this means is, if you want to simulate a spectrum in, say, the -6 order, you need to load the responses for the -6 order plus the contributing responses for the -5 and -7 orders. Arcus relies on the CCD intrinsic energy resolution to sort geometrically overlapping spectral orders. There are different ways to assign photons to the different spectral orders which have an effect on the amount of cross talk there is: With very conservative PHA filtering, the cross talk is held to a minimum -- but this reduces the effective area, so you will need to compensate with a longer exposure time. The flip side of this is to increase range of PHAs sorted into any one order; this will give you a larger effective area and reduce the exposure time, but it will increase the amount of cross talk. (A more in-depth discussion of order sorting can be found here.)
These two approaches are included in the response file tarball. The "osiptouch" directory corresponds to the maximum extraction area; Arcus's full effective area is used, but cross talk can be very substantial (up to ~20%). The "osip60" directory corresponds to a very narrow extraction region; cross talk is minimal, but this comes at the cost of a ~40% reduction in Arcus's effective area.
All in all, simulating spectra with Arcus can get complicated rather quickly if you are not yet familiar with it. To address this, the Arcus team has made a guide for how to simulate spectra using PyXSPEC, ISIS, XSPEC, and SPEX. A guide on how to use the response files in Sherpa can be found here.
Anyone with questions or concerns is strongly encouraged to contact Dr. H. Moritz Günther (hgunther [at] mit [dot] edu) or Dr. David Huenemoerder (dph [at] space [dot] mit [dot] edu) for more information.