Hard X-ray spectrum of NGC 4151.

The X-ray source 4U 1206+39 associated with the Seyfert galaxy NGC 4151 was observed with the high-pressure proportional chamber of the HXR76 experiment on board the Transatlantic Balloon Facility. The spectrum in the 25-190-keV range may be fitted with a power law with spectral slope alpha = 0.9 + or - 0.2. Data are consistent with previous observations in the same range, and no long-term variability is indicated. Comparison with the low-energy spectrum of Ariel 5 shows a break at about 40 keV where the spectral index becomes harder. Data are also consistent with the Miso gamma experiment in the range 0.15-20 MeV.