This review summarizes the observational constraints on the mass spectrum of compact objects in X-ray binaries. We currently have 20 X-ray binaries with confirmed black holes, based on dynamical information (i.e. mass in excess of 3 Msun). In two cases, V404 Cyg and GRS 1915+105, the black hole mass exceeds the maximum predicted by current Type Ib

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A considerable fraction of the massive stars of our galaxy is obscured by dust. Dust extinction is preventing us from using powerful criteria, developed at UV and optical wavelength ranges, to infer physical properties of hot stars. At wavelengths longwards of about 5 microns, emission from warm dust is typically dominating the spectra. It is

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The energy emission involved in a Gamma-Ray Burst evidently can be estimated only provided that we know the distance to the source. The same is true for the peak energy of the event in the source rest frame. Redshifts have been actually measured only for about 40 events. In order to check if it is possible to extend the Epeakrest E relation

The quiet-Sun photospheric plasma has a variety of magnetic field strengths going from zero to 1800 G. The empirical characterization of these field strengths requires a probability density function (PDF), i.e., a function P(B) describing the fraction of quiet Sun occupied by each field strength B. We show how to combine magnetic field strength