Hard X-ray Emission from Uhuru Sources

Abstract

Giacconi et al.¹ have published a catalogue of 125 discrete X-ray sources detected by Uhuru in the 2–6 keV band. Spectral information extending up to the hard X-ray region is available for only 8–10 of these sources, which have been studied earlier by balloon and rocket experiments. In two balloon flights carried out from Hyderabad by the TIFR group, one on April 16, 1969, and the other on May 5, 1970, a large part of the sky defined by RA = 190°−330° and δ = −20° to + 50° was scanned for hard X-ray emission in the 16–150 keV band. The sky map of hard X-ray intensity, the spectral results for Sco X-1 and Cyg X-1 and the evidence for two new hard X-ray sources obtained from these nights have already been published^2–4. In the flight of April 16, 1969, the region of the sky corresponding to 13 of the Uhuru sources was also scanned with reasonably good efficiency. We have re-analysed the data of this flight to elicit some information on the spectral characteristics of these sources (Table 1, part a). The exposures were calculated using the positional coordinates given in the Uhuru catalogue.