WMAP: Weight Map
A weight map (WMAP) is a 2-D image that has been binned in detector coordinates (i.e. the DET column in a Chandra event list). The WMAP for use with mkwarf should be created by the too sky2tdet, released in CIAO 4.3.
The WMAP created by sky2tdet properly weights the ARF based on how much of the source flux fell onto the bad pixels, columns, or a node boundary and which bad pixels are actually exposed. Without accounting for these effects, the ARF is significantly over-estimated.
The mkacisrmf tool can also take a WMAP as input. mkacisrmf doesn't require the level of detail in the sky2tdet WMAP, though, and using it can result in extended runtimes. In this case, create the spectrum with dmextract:
unix% dmextract "sources.evt2[bin pi]" sources.pi wmap="[bin det=8]"
or as an image by using the DM binning syntax with dmcopy:
unix% dmcopy "sources.evt2[energy=500:2000][bin det=8]" sources.wmap8