A First Determination of the Intergalactic IR from the MK421 Gamma- ray Spectrum
Session 29 -- Gamma Ray Astrophysics
Oral presentation, Wednesday, January 12, 2:15-3:45, Salon III Room (Crystal Gateway)

## [29.06] A First Determination of the Intergalactic IR from the MK421 Gamma- ray Spectrum

F. W. Stecker (NASA/GSFC), O. C. De Jager (Potchefstroom U.) and M. H. Salamon (Univ. of Utah)

{We have used the recent gamma-ray observations of Mrk421 obtained by the Whipple Observatory team (Mohanty, et al. 1993) and the EGRET experiment on Compton Gamma Ray Observatory (Lin, et al. 1992) to obtain a first possible measurement of the intergalactic infrared radiation field (IIRF) by following our previously proposed method (Stecker, et al. 1991). When we analyse the TeV data in the form of an integral energy spectrum, we find evidence for a sharp absorption by an IIRF with a spectrum which is roughly of the form $n(\epsilon) \sim (0.6\pm 0.3)\times 10^{-3} \epsilon^{-2.6}(H_o/75\;{\rm km/s/Mpc})$ ${\rm cm}^{-3}{\rm eV}^{-1}$ for $0.005\;{\rm eV}< \epsilon < 1.3$ eV. This IIRF is consistent with previous theoretical considerations (Stecker, et al. 1977; Stecker, et al . 1991). The sharpness of the observed rollover may indicate the potential importance of galactic evolution.}

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\noindent{Lin, Y.C., et al. , 1992, Ap.J. 401, L61.} \goodbreak \noindent{Mohanty, G., et al. , 1993,Proc. 23rd Int'l. Cosmic Ray Conf., Calgary, Canada , 1, 440.} \goodbreak \noindent{Stecker, F.W., de Jager, O.C. and Salamon, M.H., 1992, Ap.J. , 390, L49.} \goodbreak \noindent{Stecker, F.W., Puget, J.L. and Fazio, G.G., 1977, Ap.J , 214, L51.}