On the scattering enhancement of Nv1240 emission line of quasi-stellar objects

TL;DR

This paper investigates how resonant scattering in quasar outflows enhances the Nv emission line, explaining peculiar behaviors observed in active galactic nuclei through simulations and suggesting observational tests.

Contribution

It introduces a model showing resonant scattering significantly enhances Nv emission in quasars, supported by simulations and polarization data, offering a new explanation for observed anomalies.

Findings

Resonant scattering can enhance Nv EW by a factor of 1.82 to 2.73.

Approximately 80% of scattered flux is within +-4500 km/s.

Polarized emission around Nv supports scattering enhancement hypothesis.

Abstract

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.