SDSS1133: an unusually persistent transient in a nearby dwarf galaxy

While performing a survey to detect recoiling supermassive black holes, we\nhave identified an unusual source having a projected offset of 800 pc from a\nnearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad\nemission lines and strong variability. While originally classified as a\nsupernova (SN) because of its nondetection in 2005, we detect it in recent and\npast observations over 63 yr and find over a magnitude of rebrightening in the\nlast 2 years. Using high-resolution adaptive optics observations, we constrain\nthe source emission region to be <12 pc and find a disturbed host-galaxy\nmorphology indicative of recent merger activity. Observations taken over more\nthan a decade show narrow [O III] lines, constant ultraviolet emission, broad\nBalmer lines, a constant putative black hole mass over a decade of observations\ndespite changes in the continuum, and optical emission-line diagnostics\nconsistent with an active galactic nucleus (AGN). However, the optical spectra\nexhibit blueshifted absorption, and eventually narrow Fe II and [Ca II]\nemission, each of which is rarely found in AGN spectra. While this peculiar\nsource displays many of the observational properties expected of a potential\nblack hole recoil candidate, some of the properties could also be explained by\na luminous blue variable star (LBV) erupting for decades since 1950, followed\nby a Type IIn SN in 2001. Interpreted as an LBV followed by a SN analogous to\nSN 2009ip, the multi-decade LBV eruptions would be the longest ever observed,\nand the broad Halpha emission would be the most luminous ever observed at late\ntimes (>10 yr), larger than that of unusually luminous supernovae such as SN\n1988Z, suggesting one of the most extreme episodes of pre-SN mass loss ever\ndiscovered.\n