We present observations of the unusually luminous Type II supernova (SN) 2016gsd. With a peak absolute magnitude of V = -19.95 +/- 0.08, this object is one of the brightest Type II SNe, and lies in the gap of magnitudes between the majority of Type II SNe and the superluminous SNe. Its light curve shows little evidence of the expected drop from the optically thick phase to the radioactively powered tail. The velocities derived from the absorption in( )H alpha are also unusually high with the blue edge tracing the fastest moving gas initially at 20 000 km s(-1), and then declining approximately linearly to 15000 km s(-1) over similar to 100 d. The dwarf host galaxy of the SN indicates a low-metallicity progenitor which may also contribute to the weakness of the metal lines in its spectra. We examine SN 2016gsd with reference to similarly luminous, linear Type II SNe such as SNe 1979C and 1998S, and discuss the interpretation of its observational characteristics. We compare the observations with a model produced by the JEKYLL code and find that a massive star with a depleted and inflated hydrogen envelope struggles to reproduce the high luminosity and extreme linearity of SN 2016gsd. Instead, we suggest that the influence of interaction between the SN ejecta and circumstellar material can explain the majority of the observed properties of the SN. The high velocities and strong H alpha absorption present throughout the evolution of the SN may imply a circumstellar medium configured in an asymmetric geometry. ; Jenny and AnttiWihuri Foundation Vilho, Yrjo and Kalle Vaisala Fund of the Finnish academy of Science and Letters UCD seed funding scheme SF1518 Science Foundation Ireland Swedish Research Council Villum Fonden 13261 Independent Research Fund Denmark (IRFD) 802100170B Instrument Center for Danish Astronomy (IDA) European Organisation for Astronomical Research in the Southern Hemisphere, Chile as part of PESSTO (the Public ESO Spectroscopic Survey for Transient Objects) ESO program 188.D-3003 191.D-0935 National Aeronautics & Space Administration (NASA) NNX08AR22G National Science Foundation (NSF) AST-1238877 Chinese Academy of Sciences KJCX2-EW-T06 Chinese Astronomical Data Center (CAsDC) National Natural Science Foundation of China 11573003 National Astronomical Observatories of China Chinese Academy of Sciences Special Fund for Astronomy from the Ministry of Finance Science & Technology Facilities Council (STFC) ST/P000312/1 National Aeronautics & Space Administration (NASA) NN12AR55G 80NSSC18K0284 80NSSC18K1575 Iniciativa Cientifica Milenio del Ministerio de Economia, Fomento y Turismo de Chile IC120009 CONICYT PAI/INDUSTRIA 79090016 Finnish Cultural Foundation National Science Foundation (NSF) AST-1313484 LSSTC Data Science Fellowship Program - LSSTC NSF Cybertraining Grant 1829740 Brinson Foundation Gordon and Betty Moore Foundation European Union (EU) 839090 European Southern Observatory under ESO programme 0103.D0338(A) EU/FP7-ERC grant 615929
In the past decade, several rapidly evolving transients have been discovered whose timescales and luminosities are not easily explained by traditional supernovae (SNe) models. The sample size of these objects has remained small due, at least in part, to the challenges of detecting short timescale transients with traditional survey cadences. Here we present the results from a search within the Pan-STARRS1 Medium Deep Survey (PS1-MDS) for rapidly evolving and luminous transients. We identify 10 new transients with a time above half-maximum (t(1/2)) of less than 12 days and -16.5 > M> -20 mag. This increases the number of known events in this region of SN phase space by roughly a factor of three. The median redshift of the PS1-MDS sample is z = 0.275 and they all exploded in star-forming galaxies. In general, the transients possess faster rise than decline timescale and blue colors at maximum light (g(P1) - r(P1) less than or similar to -0.2). Best-fit blackbodies reveal photospheric temperatures/radii that expand/cool with time and explosion spectra taken near maximum light are dominated by a blue continuum, consistent with a hot, optically thick, ejecta. We find it difficult to reconcile the short timescale, high peak luminosity (L> 10(43) erg s(-1)), and lack of UV line blanketing observed in many of these transients with an explosion powered mainly by the radioactive decay of56Ni. Rather, we find that many are consistent with either (1) cooling envelope emission from the explosion of a star with a low-mass extended envelope that ejected very little (<0.03 M-circle dot) radioactive material, or (2) a shock breakout within a dense, optically thick, wind surrounding the progenitor star. After calculating the detection efficiency for objects with rapid timescales in the PS1-MDS we find a volumetric rate of 4800-8000 events yr(-1) Gpc(-3) (4%-7% of the core-collapse SN rate at z = 0.2). ; NSF through a Graduate Research Fellowship ; David and Lucile Packard Foundation Fellowship for Science and Engineering ; Jenny and Antti Wihuri Foundation ; European Research Council under the European Union 291222 ; National Aeronautics and Space Administration NNX08AR22G ; Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation AST-1238877 ; University of Maryland ; Eotvos Lorand University (ELTE) ; Gemini Observatory GN-2011B-Q-3, GS-2012A-Q-31 ; Astronomy