THERMAL DIVERGENCES ON THE EVENT HORIZONS OF 2-DIMENSIONAL BLACK-HOLES

The expectation value of the stress-energy tenser [T-mu upsilon] of a free conformally invariant scalar field is computed in a general stati c two-dimensional black hole spacetime when the held is in either a ze ro temperature vacuum state or a thermal state at a nonzero temperatur e. It is found that for every static two-dimensional black hole the st ress-energy diverges strongly on the event horizon unless the held is in a state at the natural black hole temperature which is defined by t he surface gravity of the event horizon. This implies that both extrem e and nonextreme two-dimensional black holes can only be in equilibriu m with radiation at the natural black hole temperature.