On probing the properties of QSOs through their proximity effects on the
  intergalactic medium
  The proximity effect (PE) of QSOs is believed to be useful in constraining
the QSO lifetime. Observations on the PE so far, however, give apparently
contradictory results -- some are consistent with a long QSO lifetime (>~ a few
10^7 yr), but others appear to be only consistent with a short QSO lifetime <~
10^6 yr. In this paper, we show that this apparent contradiction may be solved
by simultaneously taking into account both the effect due to the density
enhancement in the QSO near zones and that due to the obscuration of the tori
associated with the QSOs, using a large number of Monte-Carlo generated
synthetic Lyman alpha forest spectra. We demonstrate that the QSO properties
and environment can be constrained simultaneously by the transverse PE and the
line of sight PE of bright type 1 QSOs together. The current available
measurements on the PEs of type 1 QSOs suggest that (1) the density is
significantly enhanced in the vicinity of the QSOs; (2) the QSO lifetime is
consistent with being as large as a few 10^7 yr and a substantially shorter
lifetime (e.g., <~10^6 yr) is not required; and (3) the half opening angle of
the tori associated with QSOs is ~60 deg, consistent with some other
independent estimates. Our simulations also show that the TPE of type 2 QSOs
can be significantly different from that of type 1 QSOs, which may be useful to
put further constraints on the QSO properties and the QSO environment.
