A template-free approach for detecting a gravitational wave stochastic background with LISA.

The Laser Interferometer Space Antenna will be the first Gravitational Wave observatory in space developed by the European Space Agency. It is scheduled to fly in the early 2030's. LISA design predicts sensitivity levels that enable the detection a Stochastic Gravitational Wave Background signal. This stochastic type of signal is a superposition of signatures from sources that cannot be resolved individually and which are of various types, each one contributing with a different spectral shape. In this work we present a fast methodology to obtain the posterior probability to measure this signal in a set of frequency bins, combining information from all the available data channels. We use this methodology in order to obtain an efficient and computationally cheap first assessment of the stochastic gravitational wave backgrounds, but also generate sensible prior densities for more computationally expensive template-based searches. We also derive useful detectability bounds as a function of frequency and prior knowledge on the instrumental noise spectrum.