The autocorrelation function and fluctuations of the soft X-ray sky on scales from 1 arcmin to 5°

L. W. Chen, A. C. Fabian, R. S. Warwick, G. Branduardi-Raymont, C. R. Barber

Research output: Contribution to journalArticlepeer-review

12 Citations (Scopus)


We investigate the spatial anisotropy and inhomogeneity of the soft X-ray sky, using one field of deep exposure and six collinear fields of medium exposure observed with the ROSAT Position Sensitive Proportional Counter. We first remove time-variable contaminations and excise bright sources from the data, followed by simultaneous spectral fitting to determine the diffuse extragalactic X-ray background count rate; we then reconstruct the strip composed of the six medium fields which span 2° (RA) x 7° (Dec.) of the sky. Using the deep field and the merged strip, we calculate the weighted angular autocorrelation function and the rms fluctuation of the surface intensity of the X-ray sky in the energy band-0.4-2.4 keV from 1 arcmin to 5°. The 2σ upper limit to excess fluctuation on scales down to a few arcmin is found to be lower than 20 per cent. As a result of using the low extragalactic background count rate determined from spectral fitting, the 2 a upper limits on the autocorrelation function are similar to previous values, some of which cover a higher energy band and some of which use a higher estimate of the extragalactic X-ray background (due to various contaminations or to the presence of brighter discrete sources). The weighted fitting of the observed autocorrelation function to a theoretical power-law model scaling as θ-0.8 reduces the uncertainty and yields a 2σ upper limit = 3.86 x 10-3θ-0.8 over the scale of θ = 2-300 arcmin. It implies that the sources contributing the residual soft X-ray background must have a correlation length r0 < 2 h-1 Mpc if they lie at redshifts < 1, otherwise in general r0 < 4 h-1 Mpc.

Original languageEnglish
Pages (from-to)846-858
Number of pages13
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Publication statusPublished - 1994
Externally publishedYes


  • Diffuse radiation
  • Large-scale structure of Universe
  • X-rays: General

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


Dive into the research topics of 'The autocorrelation function and fluctuations of the soft X-ray sky on scales from 1 arcmin to 5°'. Together they form a unique fingerprint.

Cite this