3HSP J095507.9+355101: a flaring extreme blazar coincident in space and time with IceCube-200107A.

The day after the arrival of the IceCube neutrino 200107A 3HSP J095507.9+355101, an extreme blazar with synchrotron peak frequency > 5E17 Hz, z=0.557, and well within the neutrino event error region, was found to exhibit a very hard X-ray flare. We report here on its multi-messenger data and classify the source as an unlikely "masquerading" BL Lac, contrary to TXS 0506+056, the first source so far associated with IceCube neutrinos. Similarly to TXS\,0506+056, however, 3HSP J095507.9+355101 is also way off the so-called "blazar sequence". Finally, we discuss some theoretical implications in terms of neutrino production. We show that, based on all of the above, 3HSP J095507.9+355101 is a very plausible counterpart to an IceCube neutrino.