BLACK-HOLE GREYBODY FACTORS AND ABSORPTION OF SCALARS BY EFFECTIVE STRINGS

We compute the greybody factors for classical black holes in a domain where two kinds of charges and their anticharges are excited by the ex tra energy over extremality. We compare the result to the greybody fac tors expected from an effective string model which was earlier shown t o give the correct entropy. In the regime where the left-and right-mov ing temperatures are much smaller than the square root of the effectiv e string tension, we fmd a non-trivial greybody factor which agrees wi th the effective string model. However, if the temperatures are compar able with the square root of the effective string tension, the greybod y factors agree only at the leading order in energy. Nevertheless, the re are several interesting relations between the two results, suggesti ng that a modification of the effective string model might lead to bet ter agreement. (C) 1997 Elsevier Science B.V.