* Astronomy

Title: The superluminous supernova SN 2017egm in the nearby galaxy NGC 3191: a metal-rich environment can support a typical SLSN evolution
Author: Matt Nicholl, Edo Berger, Raffaella Margutti, Peter K. Blanchard, James Guillochon, Joel Leja, Ryan Chornock

At a redshift of z=0.03, the recently discovered SN2017egm is the nearest Type I superluminous supernova (SLSN) to date. It is the first to be found in a massive spiral galaxy (NGC 3191). Using SDSS spectra of NGC 3191, we find a metallicity ~2 Zsun at the nucleus and ~1.3 Zsun for a star forming region at a radial offset similar to SN2017egm. Archival photometry from radio to UV reveals a star formation rate ~15 Msun/yr (with ~70% obscured by dust), which can account for a Swift X-ray detection, and a stellar mass ~10^10.7 Msun. We model the UV and optical light curves over the first month after explosion with a magnetar central engine model, using the Bayesian light curve fitting tool MOSFiT. The fits indicate an ejecta mass of 2-6 Msun, a spin period of 4-6 ms, a magnetic field of (0.7-1.7) x 10^14G, and a kinetic energy of 2 x 10^51 erg. These parameters are consistent with the overall distributions for SLSNe, modeled by Nicholl et al (2017), although we note that the derived mass and rotation rate are at the low end of the distribution, possibly indicating enhanced loss of mass and angular momentum prior to explosion. This leads to two critical implications: (i) Type I SLSNe can occur at solar metallicity, although with a low fraction of ~10%; and (ii) metallicity has at most a modest effect on the SLSN properties. Both of these conclusions are in line with results for long gamma-ray bursts. Our modelling suggests an explosion date of MJD 57890±4. A short-lived excess in the data relative to the best-fitting models may indicate an early-time 'bump' similar to those seen in other SLSNe. If confirmed, SN2017egm would be the first SLSN with an observed spectrum during the bump phase; this early spectrum shows the same characteristic oxygen lines seen at maximum light, which may be an important clue in understanding the underlying mechanism for the bumps.

Read more (2403kb, PDF)

ATel 10531: X-ray Emission likely not from Supernova 2017egm

ATel 10498: Re-classification of Gaia17biu/SN 2017egm: the closest hydrogen-poor superluminous supernova yet found, located in a massive host galaxy