Lecturer: Dr. Iair Arcavi Affiliation: Las Cumbres Observatory Global Telescope, and Kavli Institute for Theoretical Physics, University of California, Santa Barbara Abstract: We recently identified several rapidly rising (~ 10 day rise-time) transients with peak luminosities between those of supernovae (SNe) and superluminous SNe (~ -20 absolute magnitude). The rapid rise to such a luminous peak places these events in a unique part of SN phase space, challenging standard SN emission mechanisms. Spectra formally classify one of the events as a Type II SN (an explosion of a H-rich massive star), but an unusual absorption feature, which can be interpreted as either high velocity hydrogen (though deeper than in previously known cases) or Si II (as seen in Type Ia SNe, the explosions of white dwarfs), is also observed. Traditional models of nickel decay, CSM interaction, shock breakout in a wind (or steeper CSM) and magnetar spindown can not readily explain the light curve characteristics. A new form of strong CSM interaction or “non-Arnettian” nickel-power deposition are required to explain the data. The latter suggests the intriguing possibility of so- called “Type 1.5 SNe” as the interpretation for these events. More detailed models for such transients and more constraining observations of future cases should help better determine their puzzling nature. Additional details of the upcoming Astrophysics' seminars can be found on the following link. ‏האירוע הזה כולל שיחת וידאו ב-Google Hangouts. הצטרף: https://hangouts.google.com/hangouts/_/mail.huji.ac.il/astrophysics?hceid=bWFpbC5odWppLmFjLmlsX2c0czhydDlpcmhwZzRvdGNybWIzZGFqcjdvQGdyb3VwLmNhbGVuZGFyLmdvb2dsZS5jb20.5ibkccf9p7t0qeeud3q0qig7pg&hs=121