OPTICAL PHOTOMETRY OF THE X-RAY NOVAE-GU-MUS(=NOVA-MUSCAE-1991) AND V518-PER-(=GRO-J0422+32)

We present UBVRI photometry of the x-ray novae GU Mus and V518 Per. We compare the temporal evolution of the optical and x-ray emission of t hese two objects to that of the prototype black hole x-ray nova system V616 Mon, and find that the outbursts of all three systems exhibited similar behavior, but on slightly different time scales. We calculate decline rates for the light curves and delay times between primary and secondary x-ray maxima. Fits to the optical and ultraviolet spectral energy distributions (SEDs) of GU Mus and the optical energy distribut ion of V518 Per were made using reddened continuum power-law models (F (nu)(proportional to)nu(alpha)), and a two-component hot blackbody+ste ady-state (geometrically thin) accretion disk model. We found that the SEDs of GU Mus following primary outburst are equally well fit by eit her a power-law model with index alpha similar to 0.7, or a two-compon ent accretion disk+35 000 K blackbody model. Following the secondary o utburst of GU Mus, the SED is best fitted by a power-law model with in dex alpha similar to 0.2. Sixteen months following the main outburst o f GU Mus, the SED is consistent with that of K5/7 V. We use this fit t o derive a lower limit distance of 4 kpc to GU Mus. The SEDs of V518 P er are best fit by a range of power-law models where the spectral inde x ct changes from similar to-0.4 to similar to-0.1 within 227 days fol lowing x-ray maximum. (C) 1996 American Astronomical Society.