Fermi-LAT Observations of V549 Vel 2017: A Subluminous Gamma-Ray Nova?

We report on the Fermi Large Area Telescope detection (with 5.7 significance) as well as the multiwavelength analysis of the 2017 nova eruption V549 Vel. Unlike the recent shock-powered novae ASASSN-16ma and V906 Car, the optical and γ-ray light curves of V549 Vel show no correlation, likely implying relatively weak shocks in the eruption. Gaia detected a candidate progenitor of V549 Vel and found a parallax measurement of ϖ = 1.91 0.39 mas, equivalent to a mode distance of d 560 pc (90% credible interval of 380-1050 pc). The progenitor was also observed by the 2MASS and WISE surveys. When adopting the Gaia distance, the spectral energy distribution of the progenitor is close to that of a G-type star. The Swift X-Ray Telescope detected the supersoft X-ray emission of the nova (kT = 30-40 keV) since day 236, and the inferred blackbody size is comparable to that of other novae assuming d 560 pc (i.e., Rbb ∼ 5 × 108 cm). However, there is also an unknown astrometric excess noise of i = 3.2 mas found in the Gaia data, and the inferred distance becomes controversial. If the Gaia distance is accurate, the γ-ray luminosity of V549 Vel will be as low as Lγ ∼ 4 × 1033 erg s−1, making it the least luminous γ-ray nova known so far. This may imply that the shock properties responsible for the γ-ray emission in V549 Vel are different from those of the more luminous events. If the nova is located farther away, it is likely a symbiotic system with a giant companion as the observed progenitor.