On-ground calibration of the ART-XC/SRG mirror system and detector unit at IKI. Part III
We have performed tests of the SRG project's ART-XC telescope's flight spare units of the mirror system (MS) and the URD focal plane X-ray detector. During the tests, the MS was illuminated by a quasi-parallel X-ray beam from a microfocus tube. By changing the angle between the MS optical axis and the incoming beam we obtained data on vignetting of the effective area of the MS-URD pair at the energies of Cu K-lines. The measurements are in good agreement with our model. Using the model vignetting of the effective area, we determined the ART-XC grasp as a function of energy for singly and doubly reflected events. The grasp reaches ∼31 cm2deg2 for singly reflected events at 7.2 keV, ∼44 cm2deg2 for doubly reflected events at 8.6 keV and ∼74 cm2deg2 for the sum of singly and doubly reflected events at 7.6 keV. The grasp values at 8.1 keV are ∼29.9 cm2deg2 for singly reflected events, ∼43.8 cm2deg2 for doubly reflected events and ∼73.7 cm2deg2 for all events. The ART-XC field of view is ∼2 deg2. In its central ∼0.3 deg2, sources are imaged and the instrument works in a "telescope" mode. The outer ∼1.7 deg2 do not have an imaging capability and the instrument works as a "concentrator" there. We also discuss the possibility of using ART-XC for monitoring bright transient X-ray sources when the observatory will be operating in the survey mode.