3 years ago

# Far-UV HST Spectroscopy of An Unusual Hydrogen Poor Superluminous Supernova: SN2017egm.

Kate H. R. Rubin, A. De Cia, D. A. Perley, Lin Yan, R. Quimby, P. J. Brown, R. Lunnan

SN2017egm is the closest (z=0.03) H-poor superluminous supernova (SLSN-I) detected to date, and a rare example of an SLSN-I in a massive and metal-rich galaxy. Here we present the HST UV & optical spectra covering (1000 - 5500)$\AA$ taken at +3 day relative to the peak. Our data reveal two sets of absorption systems, separated by 235 km/s, at redshifts matching the host galaxy, NGC3191 and its companion galaxy 73 arcsec apart. A weakly damped Lyman-alpha absorption line (sub-DLA) is detected with a total HI column density of $(6^{+3}_{-1})\times10^{19}$cm$^{-2}$. This is an order of magnitude smaller than HI column densities in the disks of nearby galaxies (>$10^{10}M_\odot$) and suggests that SN2017egm is on the near side of NGC3191 and has a low host extinction (E(B-V)=0.007). Using unsaturated metal absorption lines and taking into account of H ionization and dust depletion corrections, we find the metallicities of two absorption systems are >1 $Z_\odot$. These two results together can rule out the possibility that SN2017egm is located in a metal poor, dwarf companion galaxy to NGC3191. We compare the supernova continuum and absorption features of early-time UV spectra for SN2017egm, Gaia16apd, iPTF13ajg and PTF12dam. We find that the continuum at $\lambda%CONTENT%gt;2800A is well fit by a blackbody model, whereas the continuum at$\lambda%CONTENT%lt;2800A is considerably below the model. The degree of UV suppression varies significantly from source to source, with the 1400A to 2800A continuum flux ratio of 1.5 for Gaia16apd and 0.4 for iPTF13ajg. This can not be explained by the differences in magnetar power or blackbody temperature (i.e. color temperature). Finally, the UV spectra reveal a common set of seven broad absorption features and their equivalent widths are similar (within a factor of 2) among the four events. These seven features bode well for future high-z SLSN-I spectral classifications.

Publisher URL: http://arxiv.org/abs/1711.01534

DOI: arXiv:1711.01534v1

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