The ambiguous transient ASASSN-17hx. A possible nova impostor

Aims: Some transients, although classified as novae based on their maximum and early decline optical spectra, cast doubts on their true nature, and raise the question of whether nova impostors might exist. Methods: We monitored a candidate nova that displayed a distinctly unusual light curve at maximum and early decline through optical spectroscopy (3000-10 000 Å, 500 < R < 100 000) complemented with Swift UV and AAVSO optical photometry. We use the spectral line series to characterize the ejecta dynamics, structure, and mass. Results: We find that the ejecta are in free ballistic expansion and have a typical classical nova structure. However, their derived mass is at least an order of magnitude higher than the typical ejecta masses obtained for classical novae. Specifically, we find Mej ≃9 × 10-3 M⊙ independent of the distance for a filling factor ɛ = 1. By constraining the distance we derived ɛ in the range 0.08-0.10, giving a mass 7 × 10-4 ≲ Mej ≲ 9 × 10-4 M⊙. The nebular spectrum, characterized by unusually strong coronal emission lines, confines the ionizing source energy to the range 20-250 eV, possibly peaking in the range 75-100 or 75-150 eV. Conclusions: We link this source to other slow novae that show similar behavior, and we suggest that they might form a distinct physical subgroup. The sources may result from a classical nova explosion occurring on a very low-mass white dwarf or they may be impostors for an entirely different type of transient.