Discovering novae in early-type galaxies with MUSE: A chance find in NGC 1404, and 12 more candidates from an archival search

Abstract

I report the discovery of a transient broad-Hα point source in the outskirts of the giant elliptical galaxy NGC 1404, discovered in archival observations taken with the Multi-Unit Spectroscopic Explorer (MUSE) integral field spectrograph. The Hα line width of 1950 km s−1 full width at half-maximum, and luminosity of (4.1 ± 0.1) × 1036 erg s−1, are consistent with a nova outburst, and the source is not visible in MUSE data obtained 9 months later. A transient soft X-ray source was detected at the same position (within <1 arcsec), 14 yr before the Hα transient. If the X-ray and Hα emission are from the same object, the source may be a short-time-scale recurrent nova with a massive white dwarf accretor, and hence a possible Type-Ia supernova progenitor. Selecting broad-Hα point sources in MUSE archival observations for a set of nearby early-type galaxies, I discovered 12 more nova candidates with similar properties to the NGC 1404 source, including five in NGC 1380 and four in NGC 4365. Multi-epoch data are available for four of these twelve sources; all four are confirmed to be transient on ∼1 yr time-scales, supporting their identification as novae.