The HEAO 1 A-2 experiment's operations began on day 224 of 1977 (12 August 1977) and ended on day 739 of 1977 (9 January 1979). The A-2 experiment performed two independent, low-background, high-sensitivity surveys of the entire sky 6 months apart, the first scan during days 248 to 437 of 1977 (5 September 1977 - 13 March 1978) and the second scan during days 73 to 254 of 1978 (14 March 1978 - 11 September 1978). The authors analyzed the A-2 data in order to obtain a complete flux-limited sample of extragalactic X-ray sources. The region between galactic latitudes of -20 and +20 degrees was excluded to minimize contamination from galactic sources. A circle of 6 degrees radius around the Large Magellanic Cloud sources was also excluded to prevent confusion problems. Therefore, there remained 65.5% of the sky (8.23 steradians) covered by this survey. The lowest statistical significance for the existence of the sources included in this catalog is 5 sigma, as required by the maximum likelihood methods used by the authors to determine the log N - log S parameters. Taking into account this statistical significance requirement, the authors estimated the completeness level of the first and second scans to be 1.25 and 1.8 R15 ct/s, respectively. 1 R15 ct/s is approximately 2.17 x 10<sup>-11</sup> erg/cm<sup>2</sup>/s in the 2-10 keV energy band for a power-law spectrum with a photon index of 1.65. This catalog contains data for 68 non-galactic sources (61 extragalactic and 7 unidentified sources) which were listed in Table 1 of the published catalog. The identified sources fall into several categories, including narrow emission line galaxies, broad emission line galaxies, BL Lacertae objects, and clusters of galaxies. This table was last revised by the HEASARC in November 2004 when 2 sources (H1257-042 and H1325-020) which had for some reason been omitted from the previous HEASARC version (the provenance of which is somewhat uncertain) were added back in to the table. This is a service provided by NASA HEASARC .
