2004ApJ...612..706A

Since the photons that stream from QSOs alter the ionization state of the gas they traverse, any changes to a QSO's luminosity will produce outward-propagating ionization gradients in the surrounding intergalactic gas. This paper shows that at redshift z∼3, the gradients will alter the gas's Lyα absorption opacity enough to produce a detectable signature in the spectra of faint background galaxies. By obtaining noisy (signal-to-noise ratio∼4) low-resolution (∼7 Å) spectra of several dozen background galaxies in an R∼20' field surrounding an isotropically radiating 18th magnitude QSO at z=3, it should be possible to detect any order-of-magnitude changes to the QSO's luminosity over the previous 50-100 Myr and to measure the time t_Qsince the onset of the QSO's current luminous outburst with an accuracy of ∼5 Myr for t_Q≲50 Myr. Smaller fields of view are acceptable for shorter QSO lifetimes. The major uncertainty, aside from cosmic variance, will be the shape and orientation of the QSO's ionization cone. This can be determined from the data if the number of background sources is increased by a factor of a few. The method will then provide a direct test of unification models for active galactic nuclei.