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M. Lampton, R. Lieu, J.H.M.M. Schmitt, S. Bowyer, W. Voges, J. Lewis, and X. Wu
We present a list of 534 objects detected jointly in the Extreme Ultraviolet Explorer (EUVE) 100 A all-sky survey and in the ROSAT X-Ray Telescope 0.25 keV band. The joint selection criterion permits use of a low count rate threshold in each survey. This low threshold is roughly 60% of the threshold used in the previous EUVE all-sky surveys, and 166 of the objects listed here are new EUV sources, appearing in neither the Second EUVE Source Catalog nor the ROSAT Wide Field Camera Second Catalog. The spatial distribution of this all-sky catalog shows three features: an enhanced concentration of objects in Ursa Major, where the Galactic integrated HI column reaches its global minimum; an enhanced concentration in the third quadrant of the Galaxy (l_II from 180 deg to 270 deg) including the Canis Major tunnel where particularly low HI columns are found to distances beyond 200 pc; and a particularly low number of faint objects in the direction of the fourth quadrant of the Galaxy, where nearby intervening HI columns are appreciable. Of particular interest is the composition of the 166 detections not previously reported in any EUV catalog. We offer preliminary identifications for 105 of these sources. By far the most numerous (81) of the identifications are late-type stars (F,G,K,M), while 18 are other stellar types, only 5 are white dwarfs (WDs), and none is extragalactic. The paucity of WDs and extragalactic objects may be explained by a strong horizon effect wherein interstellar absorption strongly limits the effective new-source search volume and, thereby, selectively favors low-luminosity nearby sources over more luminous but distant objects.
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