Wind Acceleration as a Driver of Detached Blueshifted Absorption in Quasar Disk Winds

Abstract: Active galactic nuclei (AGN) unification models often emphasize the viewing angle, $i$, but $i$ alone does not determine quasar properties. This is crucial for quasar outflows: UV absorption in Extremely High Velocity Outflow (EHVO) quasars can reach blueshifted velocities of $\sim0.2~c$. In disk-wind models, both $i$ and internal wind structure shape the emergent spectrum. We test their interplay using biconical quasar disk-wind models with different acceleration lengths, $R_v$, and generate synthetic spectra over a range of $i$. We use Monte Carlo radiative transfer to account for finite continuum sources, wind attenuation, scattering, reprocessing, and emission. Changing $R_v$ greatly alters the ionization structure, continuum shape, and absorption-line profiles. At intermediate viewing angles, sightlines pass through the fastest wind. Even there, highly detached and blueshifted \CIV\ absorption like that observed in EHVO quasars appears only in models with small $R_v$. In these models, the gas reaches high velocity before attaining the ionization and density conditions favorable for \CIV. Models with larger $R_v$ instead produce broader, less detached troughs, even when the terminal velocity is very high. Thus, highly detached and blueshifted absorption requires both a high terminal velocity and small $R_v$, making such features diagnostics of disk-wind acceleration and structure. EHVO quasars provide a clear example, but the same principle applies more broadly to highly detached and blueshifted absorption in quasar outflows. Our results support an extended disk-wind view of AGN unification: $i$ selects the observed wind region, while $R_v$ shapes the emergent spectrum and absorption morphology.