The importance of transient X-ray sources for gravitational wave physics

The recent discovery of gravitational waves has opened a new window in observational astronomy. The X-ray sources are interesting targets of gravitational astronomy, among them X-ray binaries. Accreting neutron stars are candidates for continuous emission of gravitational waves in the sensitivity band of the advanced LIGO/Virgo interferometers. The X-ray binaries are also potential emitters of low frequency gravitational radiation in the frequency band of the forthcoming space based interferometer eLISA. The accurate knowledge of the orbital period of the X-ray binaries is necessary to estimate the gravitational emission at low frequency and to reduce the computational cost of searching the signal from accreting neutron stars. The systematic search for periodicities in the archives of X-ray missions can detect the required orbital periods and, in addition, superorbital periods. The preliminary results of a search for periodicities in the Swift/BAT and RXTE/ASM archives is presented, showing also how the estimated periodicities can provide information about the characteristics of the astrophysical sources.