SDSS1133: An Unusually Persistent Transient in a Nearby Dwarf Galaxy
  While performing a survey to detect recoiling supermassive black holes, we
have identified an unusual source having a projected offset of 800 pc from a
nearby dwarf galaxy. The object, SDSS J113323.97+550415.8, exhibits broad
emission lines and strong variability. While originally classified as a
supernova (SN) because of its nondetection in 2005, we detect it in recent and
past observations over 63 yr and find over a magnitude of rebrightening in the
last 2 years. Using high-resolution adaptive optics observations, we constrain
the source emission region to be <12 pc and find a disturbed host-galaxy
morphology indicative of recent merger activity. Observations taken over more
than a decade show narrow [O III] lines, constant ultraviolet emission, broad
Balmer lines, a constant putative black hole mass over a decade of observations
despite changes in the continuum, and optical emission-line diagnostics
consistent with an active galactic nucleus (AGN). However, the optical spectra
exhibit blueshifted absorption, and eventually narrow Fe II and [Ca II]
emission, each of which is rarely found in AGN spectra. While this peculiar
source displays many of the observational properties expected of a potential
black hole recoil candidate, some of the properties could also be explained by
a luminous blue variable star (LBV) erupting for decades since 1950, followed
by a Type IIn SN in 2001. Interpreted as an LBV followed by a SN analogous to
SN 2009ip, the multi-decade LBV eruptions would be the longest ever observed,
and the broad Halpha emission would be the most luminous ever observed at late
times (>10 yr), larger than that of unusually luminous supernovae such as SN
1988Z, suggesting one of the most extreme episodes of pre-SN mass loss ever
discovered.
