Possible Identification of an Uhuru X-ray Source in the Southern Sky

Abstract

A WEAK X-ray source 2U 0328 −52 (1.7 ± 0.4 c.p.s.) was recently detected by the Uhuru satellite1 at RA (1950) = 3 h 28 m, Dec (1950) = −52° 29′. The corresponding X-ray intensity in the range 2 to 10 keV is 2.9 × 10−14 J m−2 s−1 and the positional error box at the 90% confidence level has an area of 18 square degrees. The source lies far from the galactic plane at latitude −51° and may well be extragalactic. No radio sources were discovered in the Parkes survey2 in the error box area. Tentative identifications of 2U 0328 −52 with the bright galaxies IC 1954 at 3 h 30.2 m, −52° 5′ and NGC 1249 at 3 h 8.6 m, −53° 32′ were suggested1, but another possibility is that the source may be due to a rich cluster of galaxies. There is strong evidence3 that some rich clusters found from the Mount Palomar Sky Survey4 are X-ray sources; five of the 19 northern Uhuru sources with high galactic latitudes have been identified with rich clusters. High concentrations of galaxies have been found5–7 near the position of 2U 0328 −52. A contour map (Fig. 1) of the number of galaxies per square degree with photographic magnitudes ≤17.7 has been prepared from Shapley's catalogue of galaxies5 for the region. A notable cluster of galaxies occurs at 3 h 29.3 m, −52° 44′ near the centre of the 2U 0328 −52 error box; less prominent clusters within the error box occur at 3 h 41.4 m, −53° 47′ and 3 h 26 m, −53° 40′. Further X-ray observations are required to decide whether the Uhuru source coincides with any of these clusters. The X-ray emission from rich clusters of galaxies may be due to thermal bremsstrahlung of hot intergalactic gas8 or to inverse Compton scattering of relativistic electrons on the microwave background radiation9.