On the scattering enhancement of Nv1240 emission line of quasi-stellar
  objects
  The Nv emission line of active galactic nuclei shows peculiar behavior in the
line--continuum correlation, which may be indicative of an extra line component
in addition to that from the normal broad emission line region. In this paper,
we investigate possible contribution to the Nv emission via resonant scattering
of both continuum and Ly alpha in a broad absorption line (BAL) outflow, by
performing the Sobolev Monte Carlo simulations. The contribution is dependent
on the covering factor, optical depth and velocity profile of the outflow, as
well as the equivalent width (EW) of Ly alpha. Adopting model parameters
constrained by observations, we find that the measured Nv EW in the spectra of
non-BAL quasi-stellar objects (QSOs) could have been enhanced by a factor of
1.82~2.73 on average, while there is only moderate absorption of Ly alpha along
the BAL outflow direction. Our model can produce a relatively narrow scattering
line profile. About 80% of the total scattered flux falls within the central
+-4500km/s. We find that the resonant scattering can produce a prominent
polarized emission line around Nv. Both the broad excess emission and the
unusually large polarized flux observed around Nv in BAL QSOs are considered as
strong evidence for the scattering enhancement. Future spectropolarimetric
observations and spectroscopic monitoring of luminous QSOs may offer crucial
tests for this interpretation, and provide useful information on the physical
and geometrical properties of QSO outflows. We argue that the scattering offers
a promising and robust process for producing the peculiar behavior of Nv
emission compared to the other processes proposed previously.
