2017ApJ...845L...8N

At redshift z = 0.03, the recently discovered SN 2017egm is the nearest Type I superluminous supernova (SLSN) to date and first near the center of a massive spiral galaxy (NGC 3191). Using SDSS spectra of NGC 3191, we find a metallicity ∼2 Z_☉ at the nucleus and ∼1.3 Z_☉ for a star-forming region at a radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a star formation rate of ∼15 M_☉ yr^–1 (with ∼70% dust obscured), which can account for a Swift X-ray detection and a stellar mass of ∼10^10.7M_☉. We model the early UV-optical light curves with a magnetar central-engine model, using the Bayesian light curve fitting tool MOSFiT. The fits indicate an ejecta mass of 2-4 M_☉, a spin period of 4-6 ms, a magnetic field of (0.7–1.7)×10¹⁴ G, and a kinetic energy of 1–2×10⁵¹ erg. These parameters are consistent with the overall distributions for SLSNe, modeled by Nicholl et al., although the derived mass and spin are toward the low end, possibly indicating an enhanced loss of mass and angular momentum before explosion. This has two implications: (i) SLSNe can occur at solar metallicity, although with a low fraction of ∼10%, and (ii) metallicity has at most a modest effect on their properties. Both conclusions are in line with results for long gamma-ray bursts. Assuming a monotonic rise gives an explosion date of MJD 57889 ± 1. However, a short-lived excess in the data relative to the best-fitting models may indicate an early-time "bump." If confirmed, SN 2017egm would be the first SLSN with a spectrum during the bump phase; this shows the same O II lines seen at maximum light, which may be an important clue for explaining these bumps.