Emission-line Variability during a Nonthermal Outburst in the Gamma-Ray Bright Quasar 1156+295

We present multi-epoch optical spectra of the γ-ray bright blazar 1156+295 (4C +29.45, Ton 599) obtained with the 4.3 m Lowell Discovery Telescope. During a multiwavelength outburst in late 2017, when the γ-ray flux increased to 2.5 × 10^-6 phot cm^-2 s^-1 and the quasar was first detected at energies ≥100 GeV, the flux of the Mg II λ2798 emission line changed, as did that of the Fe emission complex at shorter wavelengths. These emission-line fluxes increased along with the highly polarized optical continuum flux, which is presumably synchrotron radiation from the relativistic jet, with a relative time delay of ≲2 weeks. This implies that the line-emitting clouds lie near the jet, which points almost directly toward the line of sight. The emission-line radiation from such clouds, which are located outside the canonical accretion-disk related broad-line region, may be a primary source of seed photons that are up-scattered to γ-ray energies by relativistic electrons in the jet.