GAMMA-RAY OBSERVATIONS CONSTRAIN THE INTERGALACTIC INFRARED RADIATION DENSITY

We have used the recent γ‐ray observations of Mrk421 to place theoretically significant constraints on the intergalactic IR radiation field (IIRF). Our 2σ upper limits are ∼10−8 w m−2 sr−1 for the 1 to 5 μm range, consistent with normal galactic IR production. They rule out exotic mechanisms proposed to produce a larger IIRF.