Gaia17biu/SN 2017egm in NGC 3191: The closest hydrogen-poor superluminous supernova to date is in a "normal", massive, metal-rich spiral galaxy
Creators: name: Bose, Subhash
name: Dong, Subo
name: Pastorello, A.
name: Filippenko, Alexei V.
name: Kochanek, C. S.
name: Mauerhan, Jon
name: Romero-Canizales, C.
name: Brink, Thomas
name: Chen, Ping
name: Prieto, J. L.
name: Post, R.
name: Ashall, Christopher
name: Grupe, Dirk
name: Tomasella, L.
name: Benetti, Stefano
name: Shappee, B. J.
name: Stanek, K. Z.
name: Cai, Zheng
name: Falco, E.
name: Lundqvist, Peter
name: Mattila, Seppo
name: Mutel, Robert
name: Ochner, Paolo
name: Pooley, David
name: Stritzinger, M. D.
name: Villanueva, S.
name: Zheng, WeiKang
name: Beswick, R. J.
name: Brown, Peter J.
name: Cappellaro, E.
name: Davis, Scott
name: Fraser, Morgan
name: de Jaeger, Thomas
name: Elias-Rosa, N.
name: Gall, C.
name: Gaudi, B. Scott
name: Herczeg, Gregory J.
name: Hestenes, Julia
name: Holoien, T. W. -S.
name: Hosseinzadeh, Griffin
name: Hsiao, E. Y.
name: Hu, Shaoming
name: Jaejin, Shin
name: Jeffers, Ben
name: Koff, R. A.
name: Kumar, Sahana
name: Kurtenkov, Alexander
name: Lau, Marie Wingyee
name: Prentice, Simon
name: Reynolds, T.
name: Rudy, Richard J.
name: Shahbandeh, Melissa
name: Somero, Auni
name: Stassun, Keivan G.
name: Thompson, T. A.
name: Valenti, Stefano
name: Woo, Jong-Hak
name: Yunus, Sameen
Publisher: name: arXiv
Year: 2017
Subjects: subject: High Energy Astrophysical Phenomena (astro-ph.HE)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: Solar and Stellar Astrophysics (astro-ph.SR)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: Hydrogen-poor superluminous supernovae (SLSNe-I) have been predominantly found in low-metallicity, star-forming dwarf galaxies. Here we identify Gaia17biu/SN 2017egm as an SLSN-I occurring in a "normal" spiral galaxy (NGC 3191) in terms of stellar mass (several times 10^10 M_sun) and metallicity (roughly Solar). At redshift z=0.031, Gaia17biu is also the lowest redshift SLSN-I to date, and the absence of a larger population of SLSNe-I in dwarf galaxies of similar redshift suggests that metallicity is likely less important to the production of SLSNe-I than previously believed. With the smallest distance and highest apparent brightness for an SLSN-I, we are able to study Gaia17biu in unprecedented detail. Its pre-peak near-ultraviolet to optical color is similar to that of Gaia16apd and among the bluest observed for an SLSN-I while its peak luminosity (M_g = -21 mag) is substantially lower than Gaia16apd. Thanks to the high signal-to-noise ratios of our spectra, we identify several new spectroscopic features that may help to probe the properties of these enigmatic explosions. We detect polarization at the ~0.5% level that is not strongly dependent on wavelength, suggesting a modest, global departure from spherical symmetry. In addition, we put the tightest upper limit yet on the radio luminosity of an SLSN-I with &lt;5.4x10^26 erg/s/Hz (at 10 GHz), which is almost a factor of 40 better than previous upper limits and one of the few measured at an early stage in the evolution of an SLSN-I. This limit largely rules out an association of this SLSNe-I with known populations of gamma-ray burst (GRB) like central engines.
description: Accepted for publication in ApJ. Ancillary ASCII tables added: TRL.txt -- blackbody temperature, radius and luminosity; uvw2uvm2uvw1uvu.txt -- UV photometry; BgVri.txt -- optical photometry; zJHK.txt -- NIR photometry