Description

`dmarfadd' will add several Auxiliary Response Files (ARF) together to create a single output ARF file. In the Chandra case, there is a separate ARF for each detector element (ACIS CCD or HRC MCP). The user can use `dmarfadd' to combine these files to create an ARF that covers the region (spatial or spectral) of interest. Each CCD generally has different good-time intervals (GTI), and hence, a different EXPOSURE. `dmarfadd' weights the ARF by the exposure time and derives a mean EXPOSURE. For grating spectra, where aspect dither causes different chips to overlap the same spectral region, this produces the correct ARF.

Although 'dmarfadd' was designed to add ARFs for grating observations, it can also work on imaging ARFs. When adding imaging ARFs for point sources, it is assumed that the same RMF applies to every ARF. If the RMFs are significantly different, it is advised to NOT combine the ARFs using dmarfadd, but to analyse these data separately.

In addition to the effective area column (SPECRESP), 'dmarfadd' also averages the 'FRACEXPO' column, which is primarily needed for grating pileup modeling. FRACEXPO is a value between zero and one, which indicates the fractional time that region was exposed by a detector (e.g., it is 1.0 on chip, and drops in chip gaps).

NOTE: (1) The energy grids across the files to be combined must be the same. (2) For grating observations, the keywords, GRATING, (HETG or LETG), TG_PART (which indicates HEG or MEG grating types for HETG), and TG_SCRID must match across the files. (3) The keyword EXPOSURE must exist in all input files. (4) For gratings, TG_M must match in absolute value (that is, orders -1 and +1 may be added, but not 1 and 2). For imaging-mode (GRATING='NONE'), the grating keywords are ignored.