Fermi Gamma-ray Space Telescope

The Long-Lived Gamma-Ray Nova: Nova Sagittarii 2015 No. 2

K. L. Li
A. K. H. Kong

Abstract:

We report a Fermi/LAT study of the Nova Sagittarii 2015 No. 2 (N Sgr 2015b hereafter) using the PASS 8 data (i.e., 100 MeV to 100 GeV). Gamma-ray emission was first seen in 2015 March 23 (MJD 57104; about 7 days since the discovery) until it dropped below the observing threshold around May 2 (MJD 57144). After a ~15-day period of quiescence, we detect an unexpected re-brightening at the nova position with TS=11. If it is true, N Sgr 2015b will be the longest-lived gamma-ray nova (i.e., 75 days after the LAT detection while other Fermi novae lasted for 17-27 days) since the first Fermi/LAT nova detected in 2010. In addition, a bright sub-luminous B-type companion with a color temperature of ~15000 K is discovered in the databases of PanSTARRS, 2MASS, and VVV surveys. Since B-type sub-dwarf and white dwarf binaries are progenitor candidates of Type-Ia supernovae via a double-degenerate channel, the progenitor of N Sgr 2015b could be a good laboratory to test scenarios for the formation of a Type-Ia supernova.