The Persistent Eruption of UGC 2773-OT: finally, a decade-long extragalactic Eta Carinae analogue

While SN impostors resemble the Great Eruption of eta Car in the sense that\ntheir spectra show narrow H lines and they have typical peak absolute\nmagnitudes of -13 to -14 mag, most extragalactic events observed so far are\nquite different from eta Car in duration. Their bright phases typically last\nfor 100~d or less, rather than persisting for several years. The transient\nobject UGC2773-OT had a similar peak absolute magnitude to other SN impostors,\nbut with a gradual 5-yr prediscovery rise. In the 6 yr since discovery, it has\nfaded very slowly (0.26 mag/yr). Overall, we suggest that its decade-long\neruption is so far the best known analog of eta Car's 19th century eruption. We\ndiscuss extensive spectroscopy of the ongoing eruption. The spectra show\ninteresting changes in velocity and line shape that we discuss in detail,\nincluding an asymmetric Halpha emission line that we show is consistent with\nthe ejection of a bipolar nebula that could be very much like the Homunculus of\neta Car. Moreover, changes in the line width, line profile, blue excess\nemission resembling that of Type IIn supernovae, and the intensity of Halpha\nsuggest the presence of strong circumstellar interaction in the eruption at\nlate times. This supports the hypothesis that the extended plateau of eta Car's\neruption may have been powered by shock interaction as well. One interesting\ndifference compared to eta Car, however, is that UGC2773-OT so far does not\nexhibit the repeated brief spikes in luminosity that have been associated with\nbinary periastron events.\n