Aspects of infalling d-branes in two-dimensional black hole

This is a brief review of Refs. 1 and 2. We study the dynamics of D0-brane falling into the Lorentzian two-dimensional black hole (2D BH), typically arising in the near-horizon limit of non-extremal NS5-brane background, by the methods of conformal field theory. We propose the exact boundary state describing the infalling D0-brane by carefully carrying out the Wick rotation from the known D1-brane solution. We evaluate the closed string radiation from the infalling brane. A thermal-like behavior at the Hawking temperature is observed in the outgoing radiation. On the other hand, it is remarkable to find the incoming radiation absorbed by the black hole effectively showing the Hagedorn-like behavior with precise alpha'-correction. We confirm this feature by exactly analyzing the imaginary part of cylinder amplitudes, as well as the saddle point approximation. The radiation rate curiously depends on the level k of SL(2)/U(1) supercoset, suggesting the "black hole/string phase transition" at k = 1 (k = 3 for the bosonic coset) discussed recently.