# The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution

## TL;DR

SN 2017egm, the nearest Type I superluminous supernova, occurred in a metal-rich environment, challenging previous assumptions that such events only occur in low-metallicity galaxies, and its detailed modeling offers new insights into SLSN properties.

## Contribution

This study provides the first detailed analysis of a nearby, metal-rich SLSN, demonstrating that such supernovae can occur at near-solar metallicity and modeling their properties with a magnetar engine.

## Key findings

- SLSNe can occur at solar metallicity, though infrequently (~10%).
- The magnetar model fits suggest low ejecta mass and spin, indicating possible pre-explosion mass and angular momentum loss.
- SN 2017egm may have exhibited an early-time bump, providing clues to SLSN explosion mechanisms.

## Abstract

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$_\odot$ at the nucleus and ~1.3 Z$_\odot$ for a star forming region at a radial offset similar to SN 2017egm. Archival radio-to-UV photometry reveals a star-formation rate ~15 M$_\odot$ yr$^{-1}$ (with ~70% dust-obscured), which can account for a Swift X-ray detection, and stellar mass ~$10^{10.7}$ M$_\odot$. 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 ejecta mass 2-4 M$_\odot$, spin period 4-6 ms, magnetic field (0.7-1.7)$\times 10^{14}$G, and kinetic energy 1-2 $\times10^{51}$ erg. These parameters are consistent with the overall distributions for SLSNe, modeled by Nicholl et al (2017), although the derived mass and spin are towards the low end, possibly indicating 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 ~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 MJD $57889\pm1$. 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.