  In this paper we conjecture that the generating function of the intersection
numbers on the moduli spaces of Riemann surfaces with boundary, constructed
recently by R. Pandharipande, J. Solomon and R. Tessler and extended by A.
Buryak, is a tau-function of the KP integrable hierarchy. Moreover, it is given
by a simple modification of the Kontsevich matrix integral so that the
generating functions of open and closed intersection numbers are described by
the MKP integrable hierarchy. Virasoro constraints for the open intersection
numbers naturally follow from the matrix integral representation.
