Evidence of a cyclotron line at 70 keV from 4U0531+21

The HXR79 hard X-Ray experiment was flown the August 26th 1979 from the Milo Base (Sicily, Italy) and a 27-hour flight was achieved. The scientific payload consisted of two Multiwire Spectroscopic Proportional Counters (MWSPC) with 900 cm² sensitive geometric area each, in the hard X-Ray range (16–180 keV). A single drift scan was performed on the Crab Nebula region after the balloon reached the flotation point (2.9 mbar). The observation gave a maximum counting rate of about 25 counts/s superimposed on a background of about 75 counts/s. The Crab power-law spectrum was observed, and these data provide evidence for a line emission enhancement around 70 keV.