Absorption of High Energy Gamma-rays by Low Energy Intergalactic Photons

Following our previously proposed technique, we have used the recent-ray observations of Mkr421 to place theoretically signiicant constraints on and possible estimates of the intergalactic infrared radiation eld (IIRF). Our 2 upper limits and estimates are consistent with normal galactic IR production by stars and dust. They rule out exotic mechanisms proposed to produce a larger IIRF. We nd possible evidence for intergalactic absorption of-rays above 3 TeV energy in the observed spectrum of Mkr421. The implied IIRF is of the magnitude expected to be produced by stellar emission and reprocessing in galaxies. The sharpness of the spectral turnover is not consistent with the interpretation of this feature as a cutoo in the emission from the source. However, should the cutoo primarily be a result of absorption within the source itself, our possible extragalactic ux estimate becomes the strongest upper limit extant on the IIRF. Using models for the low energy intergalactic photon spectrum from microwave to ultraviolet energies, we calculate the opacity of intergalactic space to-rays as a function of energy and redshift. These calculations indicate that the GeV-ray burst recently observed by the CGRO EGRET detector originates at a redshift less than 1:5.