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