1 Introduction

Sky surveys are of particular importance in high energy astronomy, where many sources exhibit irregular long-term variability which cannot be conveniently monitored by pointed observations. Observations conducted while maneuvering between targets can provide a substantial bonus to the scientific return of pointed missions (e.g., Elvis et al. 1992). Such observations are complementary to the dedicated all-sky surveys conducted by scanning instruments such as the Uhuru, Ariel-V, HEAO A-1, and ROSAT bright source catalogs (Forman et al. 1978; Warwick et al. 1981; McHardy et al. 1981; Wood et al. 1984; Voges et al. 1996). Here we report the second major X-ray slew survey, derived from observations made by the European Space Agency's EXOSAT X-ray astronomy satellite (White & Peacock 1988).

EXOSAT performed 1780 pointed observations of a wide variety of objects between 1983 June and 1986 April. The 90 hr orbit had an apogee of 190000 km and perigee of 350 km, with the science payload operated when the satellite was above the Earth's radiation belts at 50000 km. This allowed uninterrupted observations of up to 76 hr duration. The satellite was three axis stabilized, and at any given time about half the sky could be viewed. X-ray sources were simultaneously observed with up to 4 coaligned instruments. Two Channel Multiplier Array detectors (CMA; de Korte et al. 1981) each at the focus of an X-ray mirror provided images in the low-energy (0.04-2.0 keV) energy range, while the Medium Energy Detector Array (ME; Turner et al. 1981) and the Gas Scintillation Proportional Counter (GSPC; Peacock et al. 1981) covered the 1-50 keV and 2-35 keV energy ranges, respectively. In addition to the the pointed observations, a series of slews along parts of the galactic plane were performed (Warwick et al. 1985, 1988).

When EXOSAT manoeuvred between targets the ME and GSPC instruments were usually operated in order to search for new sources, monitor known ones, and to measure background counting rates. The relatively slow manoeuvre rates of EXOSAT (either 42, 85, or 170$^\circ$ hr^-1), together with a good knowledge of the pointing direction during slews, allows the construction of a catalog with high sky-coverage and sensitivity. Slew manoeuvres were usually performed in three stages or legs, rather than along the great circles directly between sources. First there would be a slew to place the instruments' pointing axis 90$^\circ$ from the Sun (at a $\beta$ angle of $90^{\circ}$), followed by a slew along the $\beta = 90^{\circ}$ line and then a final slew off the $\beta = 90^{\circ}$ line to the new pointing position. Apart from maximizing the efficiency of solar power collection, this procedure resulted in greater sky coverage than if the slews were along connecting great circles. During EXOSAT operations the ME slew data were routinely checked for the presence of X-ray sources. This led to the discovery of four previously unknown X-ray sources, all of which were found to be X-ray binaries (see White & Peacock 1988).