Is the Universe More Transparent to Gamma Rays Than Previously Thought?

Abstract

The MAGIC collaboration has recently reported the detection of the strong gamma-ray blazar 3C279 during a 1-2 day flare. They have used their spectral observations to draw conclusions regarding upper limits on the opacity of the Universe to high energy gamma-rays and, by implication, upper limits on the extragalactic mid-infrared background radiation. In this paper we examine the effect of gamma-ray absorption by the extragalactic infrared radiation on intrinsic spectra for this blazar and compare our results with the observational data on 3C279. We find agreement with our previous results, contrary to the recent assertion of the MAGIC group that the Universe is more transparent to \gray s than our calculations indicate. Our analysis indicates that in the energy range between ~80 and ~500 GeV, 3C279 has a best-fit intrinsic spectrum with a spectral index ~1.78 using our fast evolution model and ~2.19 using our baseline model. However, we also find that spectral indices in the range of 0.0 to 3.0 are almost as equally acceptable as the best fit spectral indices. Assuming the same intrinsic spectral index for this flare as for the 1991 flare from 3C279 observed by EGRET, viz., 2.02, which lies between our best fit indices, we estimate that the MAGIC flare was ~3 times brighter than the EGRET flare observed 15 years earlier.Comment: version accepted for publication in ApJ Letter