Hard X-rays from Sco X-1

BASED on the optical and soft X-ray emission (up to ≈30 keV), the spectrum of Sco X-1 has been described in terms of bremsstrahlung radiation from a transparent isothermal plasma at kT ≈5 eV (refs. 1–3). During a balloon-borne observation in June 1965, Peterson and Jacobson 4 obtained evidence in hard X-rays (> 30 keV) for another spectral component. We observed a similar feature in hard X-rays 5 during a balloon flight launched from Mildura, Australia, on December 15, 1966. The results of these and more recent observations show that the hard X-ray component is definitely not compatible with a simple extension of the thermal spectrum at lower energies 6 .