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We present photometric and spectroscopic observations of SN Hunt 248, a newsupernova (SN) impostor in NGC 5806, which began a multi-stage outburst in 2014May. The initial '2014a' discovery brightening exhibited an absolute magnitudeof M~-12 and the spectral characteristics of a cool dense outflow, includingP-Cygni lines of Fe II, H I, Na I, and strong line blanketing from metals. Thesource rapidly climbed and peaked near M~-15 mag after two additional weeks.During this bright '2014b' phase the spectrum became dominated by Balmeremission and a stronger blue continuum, similar to the SN impostor SN 1997bs.Archival images from the Hubble Space Telescope between 1997 and 2005 reveal aluminous (4e5 Lsun) variable precursor star. Its location on theHertzsprung-Russell diagram is consistent with a massive (M_init~30 Msun) coolhypergiant having an extremely dense wind and an Eddington ratio just belowunity. At the onset of the recent 2014a brightening, however, the object becamesuper-Eddington. The subsequent boost in luminosity during the 2014b phaseprobably resulted from circumstellar interaction. SN Hunt 248 provides thefirst case of a cool hypergiant undergoing a giant eruption reminiscent ofoutbursts from luminous blue variable (LBV) stars. This lends support to thehypothesis that some cool hypergiants, such as Rho Cas, could be LBVsmasquerading under a pseudo-photosphere created by their extremely dense winds.Moreover, SN Hunt 248 demonstrates that eruptions stemming from such stars canrival in peak luminosity the giant outbursts of much more massive systems likeEta Car.

SN Hunt 248: a super-Eddington outburst from a massive cool hypergiant