  We show that the spectrum of the unusual transient SCP06F6 is consistent with
emission from a cool, optically thick and carbon-rich atmosphere if the
transient is located at a redshift of z~0.14. The implied extragalactic nature
of the transient rules out novae, shell flashes, and V838 Mon-like events as
cause of the observed brightening. The distance to SCP06F6 implies a peak
magnitude of M_I ~- 18, in the regime of supernovae. While the morphology of
the light curve of SCP06F6 around the peak in brightness resembles the slowly
evolving Type IIn supernovae SN1994Y and SN2006gy its spectroscopic appearence
differs from all previous observed supernovae. We further report the detection
of an X-ray source co-incident with SCP06F6 in a target of opportunity
XMM-Newton observation made during the declining phase of the transient. The
X-ray luminosity of L_X ~- (5+-1)e42 erg/s is two orders of magnitude higher
than observed to date from supernovae. If related to a supernova event, SCP06F6
may define a new class. An alternative, though less likely, scenario is the
tidal disruption of a carbon-rich star.

Astrophysics