It is now well established that the X-ray background (XRB) originates from the integrated emission of discrete sources out to cosmological distances. Hence it may be a convenient tracer of the mass distribution on scales intermediate to those explored by local galaxy redshift surveys (~100 Mpc) and by the COBE Microwave background measurements (~1000 Mpc). We present a first attempt to measure and model the large angular scale fluctuations in the XRB from the HEAO-1 A2 data, expressed in terms of spherical harmonics. From the few low-order multipoles detected, we estimate the power-spectrum normalization. We predict that an X-ray whole sky survey resolving sources a factor of 10 fainter than HEAO-1, would allow us to measure the large scale fluctuations of the XRB to order l ~20 (theta ~ pi/20), and therefore constrain the large scale structure of the Universe on scales of hundreds of Mpc.
Large Scale Structure: Tracks and Traces
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