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 these 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 distribution of VS 18 Per were made using reddened continuum power-law models (Fv∝v∝, and a two-component hot blackbody+steady-state (geometrically thin) accretion disk model. We found that the SEDs of GU Mus following primary outburst are equally well fit by either a power-law model with index α∼0.7, or a two-component accretion disk+35 000 K blackbody model. Following the secondary outburst of GU Mus, the SED is best fitted by a power-law model with index α∼0.2. Sixteen months following the main outburst of GU Mus, the SED is consistent with that of K5/7 V. We use this fit to derive a lower limit distance of 4 kpc to GU Mus. The SEDs of V518 Per are best fit by a range of power-law models where the spectral index a changes from ∼-0.4 to ∼-0.1 within 227 days following x-ray maximum.