7.1 TeV Observations and Constraints on IR Background
The recent detection of Mrk 421 in TeV gamma rays by the Whipple observatory (Punch et al. 1992) suggests that other blazars may also be detectable in that energy range. Stecker et al. (1992) predicted that the observed blazar spectrum (photon index ~ 2) will soften to an index of ~ 3.5 at ~ 300 GeV due to absorption by pair production off the intergalactic infrared starlight. Stecker (1993) has computed the IR optical depths of the TeV photons for the EGRET blazars. The IR photons with wavelengths around 2 µm will contribute most to the absorption of TeV gamma rays. Ground-based Cerenkov detectors have improved their sensitivities such that this absorption break can now be studied. In fact, the measurements could be used to constrain the infrared intergalactic background which has not been well determined (Stecker et al. 1992).
7.2 GRB from Type Ia SN
The cumulative gamma-ray spectrum of Type Ia supernovae (SN Ia's) during the history of the Universe are expected to contribute significantly to the GRB. This idea was first advanced by Clayton & Silk (1969), followed by a more detailed discussion by Clayton & Ward (1975) and a recent improved calculation by The et al. (1993). SN Ia's emit strong gamma-ray lines at 847, 1238, 2599 and 3250 keV from the radioactive decay of 56Ni -> 56Co -> 56Fe produced in the explosion. This has been confirmed by the observations of SN 1987a (e.g., Leising & Share 1990). Integrating over the cosmological history of nucleosynthesis smears the lines and they become edges near the rest energies in the GRB spectrum. In particular, the SN Ia's may contribute a significant fraction of the GRB at ~ 1 MeV (Fig. 12). If high resolution spectral measurements can be made of the GRB between 0.1 and 1.0 MeV, the model of The et al. (1993) may be used to derive the history of SN Ia nucleosynthesis in the Universe. Such high resolution spectral measurements of the GRB have never been made. The current generation of germanium gamma-ray spectrometers has sufficient spectral resolution to measure such edges in the GRB spectrum. For example the GRIS balloon instrument has successfully resolved the 56Co lines of SN 1987a (Tueller et al. 1990). Observations of the GRB by GRIS are planned for October 1994. Better spatial resolution in future instruments would allow determination of the GRB fine-scale anisotropy. This is essential to distinguish between the baryon-symmetric cosmological origin and the AGN origin of the GRB (Gao et al. 1990; Cline & Gao 1990), as these models predict different intensity fluctuation patterns in the GRB.
Figure 12. The differential flux of cosmic Type Ia supernovae in the Einstein-de Sitter (q0 = 0.5) universe. The data points are measurements of the GRB (The, Leising & Clayton 1993).