Intense CIII] 1907,1909 emission from a strong Lyman continuum emitting galaxy

We have obtained the first complete ultraviolet (UV) spectrum of a strong Lyman continuum(LyC) emitter at low redshift -- the compact, low-metallicity, star-forming galaxy J1154+2443 -- with a Lyman continuum escape fraction of 46% discovered recently. The Space Telescope Imaging Spectrograph spectrum shows strong Lya and CIII] 1909 emission, as well as OIII] 1666. Our observations show that strong LyC emitters can have UV emission lines with a high equivalent width (e.g. EW(CIII])$=11.7 \pm 2.9 \AA$ rest-frame), although their equivalent widths should be reduced due to the loss of ionizing photons. The intrinsic ionizing photon production efficiency of J1154+2443 is high, $\log(\xi_{\rm ion}^0)=25.56$ erg$^{-1}$ Hz, comparable to that of other recently discovered $z \sim 0.3-0.4$ LyC emitters. Combining our measurements and earlier determinations from the literature, we find a trend of increasing $\xi_{\rm ion}^0$ with increasing CIII] 1909 equivalent width, which can be understood by a combination of decreasing stellar population age and metallicity. Simple ionization and density-bounded photoionization models can explain the main observational features including the UV spectrum of J1154+2443.