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 HEAO1-A2 data, expressed in terms of spherical harmonics. We model the harmonic coefficients assuming a power spectrum and an epoch-dependent bias parameter, bx(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 ( ∼ 100h −1 Mpc) and those probed by the COBE Cosmic Microwave Background (CMB) measurements ( ∼ 1000h −1 Mpc). We find that the HEAO1 harmonics are consistent with present epoch rms fluctuations of the X-ray sources bx(0)σ8 ∼ 1 − 2 in 8 h −1 Mpc spheres. Therefore the observed fluctuations in the XRB are roughly as expected from interpolating 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 HEAO1, may reveal fluctuations to significantly larger scales and therefore more strongly constrain the large scale structure of the Universe on scales of hundreds of Mpcs. Subject headings: X-rays: diffuse radiation — cosmology: observations, large-scale structure of universe 1.