Measurement of the broad-line region size in a luminous macho quasar

We measure the broad emission line region (BLR) size of a luminous, L 10⁴⁷ erg s^-1, high-z quasar using broadband photometric reverberation mapping. To this end, we analyze 7.5 years of photometric data for MACHO13.6805.324 (z ≃ 1.72) in the B and R MACHO bands and find a time delay of 180 ± 40days in the rest frame of the object. Given the spectral-variability properties of high-z quasars, we associate this lag with the rest-UV iron emission blends. Our findings are consistent with a simple extrapolation of the BLR size-luminosity relation in local active galactic nuclei to the more luminous, high-z quasar population. Long-term spectroscopic monitoring of MACHO13.6805.324 may be able to directly measure the line-to-continuum time delay and test our findings.