Large-scale fluctuations in the X-ray background

We present an attempt to measure the large angular scale fluctuations in the X-ray background (XRB) from the HEAO 1 A2 data, expressed in terms of spherical harmonics. We model the harmonic coefficients assuming a power spectrum and an epoch-dependent bias parameter, b(X)(z), and using a phenomenological scenario describing the evolution of the X-ray sources. From the few low-order multipoles detected above shot noise, we estimate the power-spectrum normalization on scales intermediate between those explored by local galaxy redshift surveys (similar to 100 h(-1) Mpc) and those probed by the COBE cosmic microwave background (CMB) measurements (similar to 1000 h(-1) Mpc). We find that the HEAO 1 harmonics are consistent with present-epoch rms fluctuations of the X-ray sources b(X)(O)sigma(8) similar to 1-2 in 8 h(-1) Mpc spheres. Therefore, the observed fluctuations in the XRB are roughly as expected from interpolation between the local galaxy surveys and the COBE CMB experiment. We predict that an X-ray all-sky surface brightness survey resolving sources a factor of 10 fainter than HEAO 1 could reveal fluctuations to significantly larger scales, and therefore more strongly constrain the large-scale structure of the universe on scales of hundreds of Mpc.