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Constraints on the time-scales of quasar activity are key to understandingthe formation and growth of supermassive black holes (SMBHs), quasar triggeringmechanisms, and possible feedback effects on their host galaxies. However,observational estimates of this so-called quasar lifetime are highly uncertain(t_Q~10^4-10^9 yr), because most methods are indirect and involve manymodel-dependent assumptions. Direct evidence of earlier activity is gained fromthe higher ionization state of the intergalactic medium (IGM) in the quasarenvirons, observable as enhanced Ly$\alpha$ transmission in the so-calledproximity zone. Due to the ~30 Myr equilibration time-scale of HeII in the z~3IGM, the size of the HeII proximity zone depends on the time the quasar hadbeen active before our observation t_on<t_Q, enabling up to $\pm$0.2 dexprecise measurements of individual quasar on-times that are comparable to thee-folding time-scale t_S~44 Myr of SMBH growth. Here we present the firststatistical sample of 13 quasars whose accurate and precise systemic redshiftsallow for measurements of sufficiently precise HeII quasar proximity zone sizesbetween ~2 and ~15 proper Mpc from science-grade Hubble Space Telescope (HST)spectra. Comparing these sizes to predictions from cosmological hydrodynamicalsimulations post-processed with one-dimensional radiative transfer, we infer abroad range of quasar on-times from t_on<1 Myr to t_on>30 Myr that does notdepend on quasar luminosity, black hole mass, or Eddington ratio. These resultspoint to episodic quasar activity over a long duty cycle, but do not rule outsubstantial SMBH growth during phases of radiative inefficiency or obscuration.

Dating individual quasars with the HeII proximity effect