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 cm 2 deg 2 for singly reflected events at 7.2 keV, ∼44 cm 2 deg 2 for doubly reflected events at 8.6 keV and ∼74 cm 2 deg 2 for the sum of singly and doubly reflected events at 7.6 keV. The grasp values at 8.1 keV are ∼29.9 cm 2 deg 2 for singly reflected events, ∼43.8 cm 2 deg 2 for doubly reflected events and ∼73.7 cm 2 deg 2 for all events. The ART-XC field of view is ∼2 deg 2 . In its central ∼0.3 deg 2 , sources are imaged and the instrument works in a “telescope” mode. The outer ∼1.7 deg 2 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.