  The origin of the heavily fractionated reservoir of nitrogen in comets
remains an issue in the theory of their formation and hence of the solar
system. Whether the fractionated reservoir traced by comets is inherited from
the interstellar cloud or is the product of processes taking place in the
protostar, or in the protoplanetary disk, remains unclear. So far, observations
of nitrogen isotopic ratios in protostars or prestellar cores have not securely
identified such a fractionated reservoir owing to the intrinsic difficulty of
direct isotopic ratios measurements. In this article, we report the detection
of 5 rotational lines of \ce{HC3N}, {including the weaker components of the
hyperfine multiplets}, and two rotational lines of its $^{15}$N isotopologue,
towards the L1544 prestellar core. Based on a MCMC/non-LTE multi-line analysis
at the hyperfine level, we derive the column densities of \ce{HC3N}
($8.0\pm0.4\tdix{13}$\cc) and \ce{HC3^{15}N} ($2.0\pm0.4\tdix{11}$\cc) and
derive an isotopic ratio of 400$\pm$20(1$\sigma$). This value suggests that
\ce{HC3N} is slightly depleted in $^{15}$N in L1544 with respect to the
elemental $^{14}$N/$^{15}$N ratio {of $\approx$330} in the present-day local
interstellar medium. Our study also stresses the need for radiative
calculations at the hyperfine level. Finally, the comparison of the derived
ratio with those obtained in CN and HCN in the same core seems to favor
CN+C$_2$H$_2$ as the dominant formation route to HC$_3$N. However,
uncertainties in the isotopic ratios preclude definitive conclusions.
