A Light-curve Analysis of Gamma-Ray Nova V959 Mon: Distance and White Dwarf Mass

V959 Mon is a nova detected in gamma-rays. It was discovered optically about 50 days after the gamma-ray detection owing to its proximity to the Sun. The nova's speed class is unknown because of the lack of the earlier half of its optical light curve and a short supersoft X-ray phase due to eclipse by the disk rim. Using the universal decline law and time-stretching method, we analyzed the data on V959 Mon and obtained nova parameters. We estimated the distance modulus in the V band to be (m - M)_V = 13.15 ±0.3 for the reddening of E(B - V) = 0.38 ±0.01 by directly comparing it with novae of a similar type - LV Vul, V1668 Cyg, IV Cep, and V1065 Cen. The distance to V959 Mon is 2.5 ±0.5 kpc. If we assume that the early phase of the light curve of V959 Mon is the same as that of time-stretched light curves of LV Vul, our model fitting of the light curve suggests that the white dwarf (WD) mass is 0.9-1.15 M _o, which is consistent with a neon nova identification. At the time of gamma-ray detection the photosphere of the nova envelope extends to 5-8 R _o (about two or three times the binary separation) and the wind mass-loss rate is yr^-1. The period of hard X-ray emission is consistent with the time of appearance of the companion star from the nova envelope. The short supersoft X-ray turnoff time is consistent with the epoch when the WD photosphere shrank to behind the rising disk rim, which occurred 500 days before nuclear burning turned off.