Per-Observation Detections ARF Files (arf3.fits)
The auxiliary response file consists of a single FITS format file for each source region of each observation that records the weighted ARF for the areas of the detector over which the detection region for an observation dithers.
Source Region ARF
![[example of source region arf]](imgs/arf.png)
Source Region ARF
Example of a source region ARF.
The per-observation ARF FITS file is named: 〈i〉〈s〉〈obs〉_〈obi〉N〈v〉[_〈c〉]_〈r〉_arf3.fits
where 〈i〉 is the instrument designation, 〈s〉 is the data source; 〈obs〉 is the observation identification; 〈obi〉 is the observation interval identification; 〈v〉 is the data product version number; 〈c〉 is the cycle; and 〈r〉 is the region ID, formatted as "rnnnn", where nnnn is the 4-digit region number, formatted with leading zeros. The optional discriminator identified in square brackets is included only for ACIS alternating exposure (interleaved) mode observations.