  The galaxy SDSSJ0952+2143 showed remarkable emission-line properties first
reported in 2008 (paper I), which are the consequence of a powerful high-energy
flare. Here we report follow-up observations of SDSSJ0952+2143, and discuss
outburst scenarios in terms of stellar tidal disruption by a SMBH, peculiar
variability of an AGN, and a supernova explosion. The optical spectrum of
SDSSJ0952+2143 exhibits several peculiarities: an exceptional ratio of [FeVII]
transitions over [OIII], a dramatic decrease by a factor of 10 of the
highest-ionization lines, a very unusual and variable Balmer line profile
including a triple-peaked narrow component with two unresolved horns, and a
large Balmer decrement. The MIR emission measured with the Spitzer IRS in the
narrow 10-20mu band is extraordinarily luminous (3.5 x 10^{43} erg\s). The IRS
spectrum shows a bump around ~11mu and an increase towards longer wavelengths,
reminiscent of silicate emission. The strong MIR excess over the NIR implies
the dominance of relatively cold dust. The X-ray luminosity of 10^{41} erg\s
measured with Chandra is below that typically observed in AGN. Similarities of
SDSSJ0952+2143 with some extreme supernovae suggest the explosion of a
supernova of Type IIn. However, an extreme accretion event in a low-luminosity
AGN or inactive galaxy, especially stellar tidal disruption, remain
possibilities, which could potentially produce a very similar emission-line
response. If indeed a supernova, SDSSJ0952+2143 is one of the most distant
X-ray and MIR detected SNe known so far, the most MIR luminous, and one of the
most X-ray luminous. It is also by far the most luminous (>10^{40} erg\s) in
high-ionization coronal lines, exceeding previous SNe by at least a factor of
100 [abridged].
