Monitoring Ly alpha Emission from the Blazar 3C 279

Abstract

The blazar 3C 279 is well studied and shows frequent large continuum flares from radio to gamma-ray wavelengths. There have been a number of multi-wavelength observations of 3C 279, and hence there are many ultraviolet data for this object available in the UV archives. In this paper we present Ly alpha emission line measurements for 3C 279 using all the archival IUE SWP spectra from 1988 to 1996 and all archival HST/FOS G190H spectra from 1992 to 1996. Individual archival IUE spectra of 3C 279 show weak Ly alpha emission at ~1868 A (z = 0.536), which is easily seen in the co-added data. The Ly alpha emission is observed in all the HST/FOS spectra. The strength of Ly alpha is nearly constant (~5E-14 erg/cm2/s), while the 1750 A continuum varies by a factor of ~50, from ~0.6 to 31.6E-15 erg/cm2/s/A. The behavior of the Ly alpha emission line flux and continuum flux is similar to that of the only other well observed blazar, 3C 273, which shows constant line flux while the continuum varies by a factor of ~3. This near-constancy of emission line flux in the two best-studied blazars, suggests that the highly variable beamed continuum is not a significant source of photoionization for the gas. Some other source, such as thermal emission from an accretion disk, must be providing a significant fraction of the photoionizing flux in these objects. The large amplitude variability seen at gamma-ray energies must be due to changes in the energetic electrons in the jet rather than changes in the external photon field.