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

• Published in: The Astrophysical journal Vol. 858; no. 2; pp. 108 - 123
• Main Authors: Hachisu, Izumi, Kato, Mariko
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10.05.2018; IOP Publishing
• Subjects: Astrophysics; Companion stars; Curve fitting; Dwarf novae; Emission; Gamma rays; Light curve; Neon; Novae; novae, cataclysmic variables; Parameter estimation; Photosphere; stars: individual (IV Cep, LV Vul, V959 Mon, V1065 Cen); Stellar winds; White dwarf stars; X-ray emissions; X-rays: stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aabee0

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 , 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 (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.