Very fast stochastic gravitational wave background map making using folded data

A stochastic gravitational-wave background (SGWB) is expected from the superposition of a wide variety of independent and unresolved astrophysical and cosmological sources from different stages in the evolution of the Universe. Radiometric techniques are used to make sky maps of anisotropies in the SGWB by cross-correlating data from pairs of detectors. The conventional searches can be made hundreds of times faster through the folding mechanism introduced recently. Here we present a newly developed algorithm to perform the SGWB searches in a highly efficient way. Taking advantage of the compactness of the folded data we replaced the loops in the pipeline with matrix multiplications. We also incorporated well-known HEALPix pixelization tools for further standardization and optimization. Our Python-based implementation of the algorithm is available as an open source package PyStoch. Folding and PyStoch together has made the radiometer analysis a few thousand times faster; it is now possible to make all-sky maps of a stochastic background in just a few minutes on an ordinary laptop. Moreover, PyStoch generates a skymap at every frequency bin as an intermediate data product. These techniques have made SGWB searches very convenient and will make computationally challenging analyses like blind all-sky narrowband search feasible.