  We report the discovery of an off-nuclear ultrasoft hyper-luminous X-ray
source candidate 3XMM J141711.1+522541 in the inactive S0 galaxy SDSS
J141711.07+522540.8 (z=0.41827, d_L=2.3 Gpc) in the Extended Groth Strip. It is
located at a projected offset of ~1.0 (5.2 kpc) from the nucleus of the galaxy
and was serendipitously detected in five XMM-Newton observations in 2000 July.
Two observations have enough counts and can be fitted with a standard thermal
disk with an apparent inner disk temperature kT_MCD ~ 0.13 keV and a 0.28-14.2
keV unabsorbed luminosity L_X ~ 4X10^{43} erg/s in the source rest frame. The
source was still detected in three Chandra observations in 2002 August, with
similarily ultrasoft but fainter spectra (kT_MCD ~ 0.17 keV, L_X ~ 0.5X10^{43}
erg/s). It was not detected in later observations, including two by Chandra in
2005 October, one by XMM-Newton in 2014 January, and two by Chandra in 2014
September-October, implying a long-term flux variation factor of >14. Therefore
the source could be a transient with an outburst in 2000-2002. It has a faint
optical counterpart candidate, with apparent magnitudes of m_F606W=26.3 AB mag
and m_F814W=25.5 AB mag in 2004 December (implying an absolute V-band magnitude
of ~-15.9 AB mag). We discuss various explanations for the source and find that
it is best explained as a massive black hole (BH) embedded in the nucleus of a
possibly stripped satellite galaxy, with the X-ray outburst due to tidal
disruption of a surrounding star by the BH. The BH mass is ~10^5 Msun, assuming
the peak X-ray luminosity at around the Eddington limit.
