Measuring the response of the intergalactic medium to a blast of ionizing radiation allows one to infer the physical properties of the medium and , in principle , the lifetime and isotropy of the radiating source .
The most sensitive such measurements can be made if the source of radiation is near the line of sight to a bright background QSO .
We present results based on deep Keck/HIRES observations of the QSO triplet KP76 , KP77 and KP78 at z \simeq 2.5 , with separations of 2-3 ^ { \prime } ~ { } on the plane of the sky .
Using accurate systemic redshifts of the QSOs from near-IR spectroscopy , we quantify the state of the IGM gas in the proximity regions where the expected ionizing flux from the foreground QSOs exceeds that of the metagalactic background by factors 10 - 200 , assuming constant and isotropic emission .
Based on the unusual ionization properties of the absorption systems with detected H i , C iv and O vi , we conclude that the gas has been significantly affected by the UV radiation from the nearby QSOs .
Aided by observations of the galaxy density near the foreground QSOs , we discuss several effects that may explain why the transverse proximity effect has eluded most previous attempts to detect it .
Our observations suggest that the luminosities of KP76 and KP77 have remained comparable to current values over timescales of , respectively , \Delta t > 25 Myr and 16 Myr < \Delta t < 33 Myr - consistent with typical QSO lifetimes estimated from independent , less-direct methods .
There is no evidence that the UV radiation from either QSO was significantly anisotropic during these intervals .