We have used observations of Cyg X-1 from the Compton Gamma Ray Observatory and BeppoSAX to study the variation in the MeV -ray emission between the hard and soft spectral states, using spectra that cover the energy range from 20 keV up to 10 MeV. These data provide evidence for significant spectral variability at energies above 1 MeV. In particular, whereas the hard X-ray flux decreases during the soft state, the flux at energies above 1 MeV increases, resulting in a significantly harder -ray spectrum at energies above 1 MeV. This behavior is consistent with the general picture of galactic black hole candidates having two distinct spectral forms at soft -ray energies. These data extend this picture, for the first time, to energies above 1 MeV. We have used two different hybrid thermal/nonthermal Comptonization models to fit broadband spectral data obtained in both the hard and soft spectral states. These fits provide a quantitative estimate of the electron distribution and allow us to probe the physical changes that take place during transitions between the low and high X-ray states. We find that there is a significant increase (by a factor of $4) in the bolometric luminosity as the source moves from the hard state to the soft state. Furthermore, the presence of a nonthermal tail in the Comptonizing electron distribution provides significant constraints on the magnetic field in the source region.

Nature © Macmillan Publishers Ltd 1998 8 reported 7,8 ; the spectra of RX J0852:0 2 4622 are similar to these if the power-law-like spectrum of its northern limb section is applied. The X-ray spectral parameters which Willingale et al. 8 have obtained for SN1006 and our results for RX J0852:0 2 4622 are basically the same concerning the spectral index a as well as the temperatures T r,1 and T r,2 of the SNR interior. Furthermore, the X-ray morphology of the two objects is very similar for E .

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We report the detection of the 44 Sc nuclear decay lines at 67.9 keV and 78.4 keV associated with the nuclear decay of 44 Ti in Cassiopeia A. The line emission was observed by the PDS instrument on board BeppoSAX, which recently observed the supernova remnant for over 500 ks. The detection of the line emission with a flux of (2.1 ± 0.7) 10 −5 ph/cm 2 /s in each line (90% confidence) is at the 5σ significance level, if we can assume that the 12-300 keV continuum is adequately represented by a single power law. However, as the nature of the continuum is not clear we investigate various other possibilities. A more conservative estimate of the line flux is made by assuming that a power law continuum is at least a good approximation to the continuum emission for a narrower 30-100 keV energy range. With this limitation the measured line flux is (1.9 ± 0.9) 10 −5 ph/cm 2 /s, with the detection still at the 3.4σ significance level. We suggest that together with the CGRO-COMPTEL measurement of the 44 Ca line at 1157 keV of (3.3 ± 0.6) 10 −5 ph/cm 2 /s a flux for all three lines of (2.5 ± 1.0) 10 −5 ph/cm 2 /s for Cas A can be adopted. This implies an initial 44 Ti mass of (0.8 − 2.5) 10 −4 M ⊙ . 5 The observational identification numbers for the additional archival data used for our analysis are 30011001, 30011002 and 30795005. They were chosen on the availability of spectra made with the variable rise time rejection method.

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