Hard X-Ray Observations of the Hydrogen-poor Superluminous Supernova SN 2018hti with NuSTAR

Some hydrogen-poor superluminous supernovae are likely powered by a magnetar central engine, making their luminosity larger than common supernovae. Although a significant amount of X-ray flux is expected from the spin-down of the magnetar, direct observational evidence is still to be found, giving rise to the "missing energy" problem. Here we present NuSTAR observations of nearby SN 2018hti 2.4 yr (rest frame) after its optical peak. We expect that, by this time, the ejecta have become optically thin for photons more energetic than ∼15 keV. No flux is detected at the position of the supernova down to F10–30 keV = 9.0 × 10−14 erg cm−2 s−1, or an upper limit of 7.9 × 1041 erg s−1 at a distance of 271 Mpc. This constrains the fraction of bolometric luminosity from the putative spinning down magnetar to be fX ≲ 36% in the 10–30 keV range in a conservative case, fX ≲ 11% in an optimistic case.