GRAVITATIONAL LENSING OF TYPE IA SUPERNOVAE BY GALAXY CLUSTERS

We propose a method to remove the mass-sheet degeneracy that arises wh en the mass of galaxy clusters is inferred from gravitational shear. T he method utilizes high-redshift standard candles that undergo weak le nsing. Natural candidates for such standard candles are type Ia supern ovae (SNe Ia). When corrected with the light-curve shape (LCS), the pe ak magnitude of SNe Ia provides a standard candle with an uncertainty in apparent magnitude of Delta m similar or equal to 0.1-0.2, Gravitat ional magnification of a background SN Ia by an intervening cluster wo uld cause a mismatch between the observed SN Ia peak magnitude compare d with that expected from its LCS and redshift. The average detection rate for SNe Ia with a significant mismatch of greater than or equal t o 2 Delta m behind a cluster at z similar or equal to 0.05-0.15 is abo ut 1-2 supernovae per cluster per year at J, I, R less than or similar to 25-26. Since SNe are point-like sources for a limited period, they can experience significant microlensing by massive compact halo objec ts (MACHOs) in the intracluster medium. Microlensing events caused by MACHOs of similar to 10(-4) M-. are expected to have time-scales simil ar to that of the SN light curve. Both the magnification curve by a MA CHO and the light curve of a SN Ia have characteristic shapes that all ow us to separate them. Microlensing events caused by MACHOs of smalle r mass can unambiguously be identified in the SN light curve if the la tter is continuously monitored. The average number of identifiable mic rolensing events per nearby cluster (z less than or similar to 0.05) p er year is similar to 0.02 (f/0.01), where f is the fraction of the cl uster mass in MACHOs of masses 10(-7) < M-macho/M-. < 10(-4).