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 (∼100 h-¹ Mpc) and those probed by the COBE cosmic microwave background (CMB) measurements (∼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(0)σ₈ ∼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.