Slow-blue PanSTARRS transients : high amplification microlens events?

With PanSTARRS-1 we have discovered a new class of extreme, rare, extragalactic transients which we believe represent Seyfert-like AGN at z=1 amplified by factors of 4-100 by microlensing in foreground galaxies at z=0.25. The source should be partially resolved by the lens, leading to colour and spectroscopic trends during the decay and so providing a new and unique handle on the size and structure of the accretion disc and the broad line region. HST is the only facility capable of testing this hypothesis; we should see a typical AGN offset of a few tenths of an arcsecond from the centroid of the blue light, and signs of two galaxies of very different colour that dominate B-band and J-band respectively. An HST snapshot survey will test the hypothesis, measure the sizes and luminosities of foreground galaxies, set baseline AGN level to constrain lens models, and show us how to design future more detailed studies.