title: Slowly fading super-luminous supernovae that are not pair-instability explosions.
abstract: Super-luminous supernovae that radiate more than 10(44) ergs per second at their peak luminosity have recently been discovered in faint galaxies at redshifts of 0.1-4. Some evolve slowly, resembling models of 'pair-instability' supernovae. Such models involve stars with original masses 140-260 times that of the Sun that now have carbon-oxygen cores of 65-130 solar masses. In these stars, the photons that prevent gravitational collapse are converted to electron-positron pairs, causing rapid contraction and thermonuclear explosions. Many solar masses of (56)Ni are synthesized; this isotope decays to (56)Fe via (56)Co, powering bright light curves. Such massive progenitors are expected to have formed from metal-poor gas in the early Universe. Recently, supernova 2007bi in a galaxy at redshift 0.127 (about 12 billion years after the Big Bang) with a metallicity one-third that of the Sun was observed to look like a fading pair-instability supernova. Here we report observations of two slow-to-fade super-luminous supernovae that show relatively fast rise times and blue colours, which are incompatible with pair-instability models. Their late-time light-curve and spectral similarities to supernova 2007bi call the nature of that event into question. Our early spectra closely resemble typical fast-declining super-luminous supernovae, which are not powered by radioactivity. Modelling our observations with 10-16 solar masses of magnetar-energized ejecta demonstrates the possibility of a common explosion mechanism. The lack of unambiguous nearby pair-instability events suggests that their local rate of occurrence is less than 6 × 10(-6) times that of the core-collapse rate.
authors.affiliation: Astrophysics Research Centre, School of Mathematics and Physics, Queen's University Belfast, Belfast BT7 1NN, UK. mnicholl03@qub.ac.uk
authors.forename: M
authors.initials: M
authors.lastname: Nicholl
authors.forename: S J
authors.initials: SJ
authors.lastname: Smartt
authors.forename: A
authors.initials: A
authors.lastname: Jerkstrand
authors.forename: C
authors.initials: C
authors.lastname: Inserra
authors.forename: M
authors.initials: M
authors.lastname: McCrum
authors.forename: R
authors.initials: R
authors.lastname: Kotak
authors.forename: M
authors.initials: M
authors.lastname: Fraser
authors.forename: D
authors.initials: D
authors.lastname: Wright
authors.forename: T-W
authors.initials: TW
authors.lastname: Chen
authors.forename: K
authors.initials: K
authors.lastname: Smith
authors.forename: D R
authors.initials: DR
authors.lastname: Young
authors.forename: S A
authors.initials: SA
authors.lastname: Sim
authors.forename: S
authors.initials: S
authors.lastname: Valenti
authors.forename: D A
authors.initials: DA
authors.lastname: Howell
authors.forename: F
authors.initials: F
authors.lastname: Bresolin
authors.forename: R P
authors.initials: RP
authors.lastname: Kudritzki
authors.forename: J L
authors.initials: JL
authors.lastname: Tonry
authors.forename: M E
authors.initials: ME
authors.lastname: Huber
authors.forename: A
authors.initials: A
authors.lastname: Rest
authors.forename: A
authors.initials: A
authors.lastname: Pastorello
authors.forename: L
authors.initials: L
authors.lastname: Tomasella
authors.forename: E
authors.initials: E
authors.lastname: Cappellaro
authors.forename: S
authors.initials: S
authors.lastname: Benetti
authors.forename: S
authors.initials: S
authors.lastname: Mattila
authors.forename: E
authors.initials: E
authors.lastname: Kankare
authors.forename: T
authors.initials: T
authors.lastname: Kangas
authors.forename: G
authors.initials: G
authors.lastname: Leloudas
authors.forename: J
authors.initials: J
authors.lastname: Sollerman
authors.forename: F
authors.initials: F
authors.lastname: Taddia
authors.forename: E
authors.initials: E
authors.lastname: Berger
authors.forename: R
authors.initials: R
authors.lastname: Chornock
authors.forename: G
authors.initials: G
authors.lastname: Narayan
authors.forename: C W
authors.initials: CW
authors.lastname: Stubbs
authors.forename: R J
authors.initials: RJ
authors.lastname: Foley
authors.forename: R
authors.initials: R
authors.lastname: Lunnan
authors.forename: A
authors.initials: A
authors.lastname: Soderberg
authors.forename: N
authors.initials: N
authors.lastname: Sanders
authors.forename: D
authors.initials: D
authors.lastname: Milisavljevic
authors.forename: R
authors.initials: R
authors.lastname: Margutti
authors.forename: R P
authors.initials: RP
authors.lastname: Kirshner
authors.forename: N
authors.initials: N
authors.lastname: Elias-Rosa
authors.forename: A
authors.initials: A
authors.lastname: Morales-Garoffolo
authors.forename: S
authors.initials: S
authors.lastname: Taubenberger
authors.forename: M T
authors.initials: MT
authors.lastname: Botticella
authors.forename: S
authors.initials: S
authors.lastname: Gezari
authors.forename: Y
authors.initials: Y
authors.lastname: Urata
authors.forename: S
authors.initials: S
authors.lastname: Rodney
authors.forename: A G
authors.initials: AG
authors.lastname: Riess
authors.forename: D
authors.initials: D
authors.lastname: Scolnic
authors.forename: W M
authors.initials: WM
authors.lastname: Wood-Vasey
authors.forename: W S
authors.initials: WS
authors.lastname: Burgett
authors.forename: K
authors.initials: K
authors.lastname: Chambers
authors.forename: H A
authors.initials: HA
authors.lastname: Flewelling
authors.forename: E A
authors.initials: EA
authors.lastname: Magnier
authors.forename: N
authors.initials: N
authors.lastname: Kaiser
authors.forename: N
authors.initials: N
authors.lastname: Metcalfe
authors.forename: J
authors.initials: J
authors.lastname: Morgan
authors.forename: P A
authors.initials: PA
authors.lastname: Price
authors.forename: W
authors.initials: W
authors.lastname: Sweeney
authors.forename: C
authors.initials: C
authors.lastname: Waters