We have calculated gamma-ray radiative transport in regions of high energy density, such as gamma-ray burst source regions, using a discrete ordinate, discrete energy group method. The calculations include two-photon pair production and annihilation, as well as three-photon pair annihilation. The radiation field itself acts as an absorbing medium, and the optical depth depends on its intensity, so the problem is intrinsically nonlinear.Spherical divergence produces effective collimation of the flux. At high optical depth the high energy (E > 1 MeV) portion of the emergent spectrum assumes a nearly universal form. An approximate limit is derived for the high energy flux from a gamma-ray burst source region of given size, and the implications of this limit for the distance to the March 5, 1979 event are briefly discussed. We discuss more generally the problem of very luminous bursts, and implications of Galactic halo distances for flare models.

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CCD images of the fourteenth-magnitude variable BM Ursae Majoris were taken in the spring of 1991 at Lowell Observatory. An RCA CCD camera system and a standard PMT were used. Four times of minimum light were determined and improved linear and quadratic ephemerides were calculated. A period study, spanning over a thirty-year interval, shows that the system is undergoing a small but continuous period decrease of -5X10 8 d/yr. This may be due to angular momentum loss (AML) caused by stellar winds. The VRI light curves formed from the present precision observations, show that BM UMa is a W-type W UMa system. The first synthetic light curve solution of BM UMa is also presented. This solution reveals that BM UMa is a contact binary consisting of two early-K spectral-type components with a fill out of -20% and a mass ratio of -0.5. A substantial temperature difference of -400 K was determined. Both unspotted and spotted models were calculated. The spotted model indicates the presence of a region of enhanced brightness in the neck of the secondary component. This may be attributed to fluid dynamics of mass in transit rather than to magnetic activity.

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