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 multi-wavelength outburst in late 2017, when the γ-ray flux increased to 2.5×10−6photcm−2s−1
Nearby in the stack
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.