Detection of O VI lambda 1033 Emission and Far-Ultraviolet Spectral Variability in the Gravitational Lens PG 1115+080

Abstract

Far-UV spectra acquired with the International Ultraviolet Explorer (IUE) of the gravitational lens PG 1115+080 and processed with the NEWSIPS software show the presence of prominent O VI λ1033 resonance line emission that is superimposed on strong far-UV continuum between 900 and 1100 A (in the z_QSO_ = 1.722 rest frame). A comparison of these spectra with an IUE Archive spectrum of PG 1115+080 indicates that O VI emission was not present in 1978, when the only other spectrum covering this wavelength range was obtained, which showed only far-UV continuum. The detection of O VI λ1033 resonance line emission in the brightest component, PG 1115+080A, suggests a high state of ionization in the broad emission line region (BELR). O VI emission is accompanied by absorption in the line core and complex absorption features that truncate the blue wing of the emission profile. The O VI absorption trough suggests outflow that is characteristic of high- ionization resonance lines in broad absorption line (BAL) quasars. Both the O VI emission and BAL absorption decreased significantly in strength relative to the continuum over a timescale of ~100 days. The equivalent widths of absorption features within the O VI BAL trough also changed on timescales of roughly weeks down to ~1 day. Rapid variations associated with absorption features may reflect ionization changes that cause column densities in the BAL outflow to fluctuate. These results emphasize the importance of monitoring emission lines of very high ionization (>100 eV) because the timescale over which O VI changes occur is comparable to the gravitational delay time of ~2-3 months estimated for images in PG 1115+080.