We present a first catalog of sources detected by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory in the 4-12 keV energy band during its on-going all-sky survey. The catalog comprises 867 sources detected on the combined map of the first two 6-month scans of the sky (Dec. 2019 - Dec. 2020) - ART-XC sky surveys 1 and 2, or ARTSS12. The achieved sensitivity to point sources varies between ~5x10-12 erg/s/cm2 near the Ecliptic plane and better than 10-12 erg/s/cm2 (4-12 keV) near the Ecliptic poles, and the typical localization accuracy is ~15 arcsec. Among the 750 sources of known or suspected origin in the catalog, 56% are extragalactic (mostly active galactic nuclei (AGN) and clusters of galaxies) and the rest are Galactic (mostly cataclysmic variables (CVs) and low- and high-mass X-ray binaries). For 116 sources ART-XC has detected X-rays for the first time. Although the majority of these (~80) are expected to be spurious (for the adopted detection threshold), there can be a significant number of newly discovered astrophysical objects. We have started a program of optical follow-up observations of the new and previously unidentified X-ray sources, which has already led to the identification of several AGN and CVs. With the SRG all-sky survey planned to continue for a total of 4 years, we can expect the ART-XC survey in the 4-12 keV band to significantly surpass the previous surveys carried out in similar (medium X-ray) energy bands in terms of the combination of angular resolution, sensitivity, and sky coverage.
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