The Grism Lens-Amplified Survey from Space (GLASS). XI. Detection of CIV
  in Multiple Images of $z=6.11$ Ly$\alpha$ Emitter Behind RXCJ2248.7-4431
  The CIII] and CIV rest-frame UV emission lines are powerful probes of the
ionizations states of galaxies. They have furthermore been suggested as
alternatives for spectroscopic redshift confirmation of objects at the epoch of
reionization ($z>6$), where the most frequently used redshift indicator,
Ly$\alpha$, is attenuated by the high fraction of neutral hydrogen in the
inter-galactic medium. However, currently only very few confirmations of carbon
UV lines at these high redshifts exist, making it challenging to quantify these
claims. Here, we present the detection of CIV$\lambda\lambda$1548,1551\AA\ in
\HST\ slitless grism spectroscopy obtained by GLASS of a Ly$\alpha$ emitter at
$z=6.11$ multiply imaged by the massive foreground galaxy cluster RXJ2248. The
CIV emission is detected at the 3--5$\sigma$ level in two images of the source,
with marginal detection in two other images. We do not detect significant
CIII]$\lambda\lambda$1907,1909\AA\ emission implying an equivalent width
EW$_\textrm{CIII]}<20$\AA\ (1$\sigma$) and $\textrm{CIV/CIII}>0.7$ (2$\sigma$).
Combined with limits on the rest-frame UV flux from the HeII$\lambda$1640\AA\
emission line and the OIII]$\lambda\lambda$1661,1666\AA\ doublet, we put
constraints on the metallicity and the ionization state of the galaxy. The
estimated line ratios and equivalent widths do not support a scenario where an
AGN is responsible for ionizing the carbon atoms. SED fits including nebular
emission lines imply a source with a mass of log(M/M$_\odot)\sim9$, SFR of
around 10M$_\odot$/yr, and a young stellar population $<50$Myr old. The source
shows a stronger ionizing radiation field than objects with detected CIV
emission at $z<2$ and adds to the growing sample of low-mass
(log(M/M$_\odot)\lesssim9$) galaxies at the epoch of reionization with strong
radiation fields from star formation.
