Fermi-LAT Observations of V549 Vel 2017: a Sub-Luminous Gamma-Ray Nova?

TL;DR

This paper reports the detection and analysis of the 2017 nova V549 Vel by Fermi-LAT, revealing it as potentially the least luminous gamma-ray nova, with implications for shock properties and progenitor characteristics.

Contribution

It provides the first detailed multi-wavelength analysis of V549 Vel, highlighting its low gamma-ray luminosity and discussing the uncertainties in its distance and progenitor nature.

Findings

Fermi-LAT detected V549 Vel with ~5.7 sigma significance.

The nova's gamma-ray luminosity could be as low as 4*10^33 erg/s.

The progenitor's spectral energy distribution suggests a G-type star if the Gaia distance is correct.

Abstract

We report on the Fermi-LAT detection (with ~5.7 sigma significance) as well as the multi-wavelength analysis of the 2017 nova eruption V549 Vel. Unlike the recent shock-powered novae ASASSN-16ma and V906 Car, the optical and gamma-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 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. Swift/XRT 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., R_bb~5*10^8 cm). However, there is also an unknown astrometric excess noise of 3.2 mas found in the Gaia data, and the inferred distance becomes controversial. If the Gaia distance is accurate, the gamma-ray luminosity of V549 Vel will be as low as L_gamma~4*10^33 erg/s, making it the least luminous gamma-ray nova known so far. This may imply that the shock properties responsible for the gamma-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.