Have the Diffuse Cosmic X-rays an Anisotropic Component?

Abstract

WE have recently shown¹ that if one assumes that the far-infrared radiation² is galactic, then inverse Compton scattering of these photons with cosmic ray electrons would explain the flux and the energy spectrum of diffuse X-rays. We also predicted a large anisotropy which seemed to be at variance with experimental data which indicated that diffuse X-rays are isotropic within about 10 per cent. Recent measurements by Cooke, Griffiths and Pounds³, however, now present evidence in support of the existence of anisotropy. They observed excess X-rays from the region of the galactic equator in the 1.4 to 18 keV region with an intensity of 0.3 photons (cm² s rad)⁻¹ near l = 260°, increasing to 0.5 near l = 300° in the Centaurus region. Perhaps these results indicate that the diffuse X-rays result from two separate mechanisms, one producing an isotropic component and the other an anisotropic component ! Thus, in the light of these new results, we feel it is worth while to look more closely at the predictions of our model regarding anisotropy. The X-ray flux I is directly proportional¹ to the integration path-length L. Restricting our present discussion to the galactic equatorial plane corresponding to the measurements of Cooke et al.³, it follows by simple geometry that the X-ray flux at a longitude l, I(l) say, is proportional to where R is the radius of the galaxy and a is the distance from the Earth to the centre of the galaxy. Taking a/R = 2/3 we obtain in good agreement with the value of 1.7 obtained by Cooke et al.³. We also obtain to be compared with the experimental upper limit³ of 3.3. This gives us added confidence in our model, but it is clearly desirable to have much more experimental confirmation of the anisotropy measured by Cooke et al.³.