Bose, SubhashSubhashBoseDong, SuboSuboDongKochanek, C. S.C. S.KochanekPASTORELLO, AndreaAndreaPASTORELLOKatz, BoazBoazKatzBersier, DavidDavidBersierAndrews, Jennifer E.Jennifer E.AndrewsPrieto, J. L.J. L.PrietoStanek, K. Z.K. Z.StanekShappee, B. J.B. J.ShappeeSmith, NathanNathanSmithKollmeier, JunaJunaKollmeierBENETTI, StefanoStefanoBENETTICAPPELLARO, EnricoEnricoCAPPELLAROChen, PingPingChenELIAS DE LA ROSA, NANCY DEL CARMENNANCY DEL CARMENELIAS DE LA ROSAMilne, PeterPeterMilneMorales-Garoffolo, AntoniaAntoniaMorales-GaroffoloTARTAGLIA, LEONARDOLEONARDOTARTAGLIATOMASELLA, LinaLinaTOMASELLABilinski, ChristopherChristopherBilinskiBrimacombe, JosephJosephBrimacombeFrank, StephanStephanFrankHoloien, T. W. -S.T. W. -S.HoloienKilpatrick, Charles D.Charles D.KilpatrickKiyota, SeiichiroSeiichiroKiyotaMadore, Barry F.Barry F.MadoreRich, Jeffrey A.Jeffrey A.Rich2020-09-292020-09-2920180004-637Xhttp://hdl.handle.net/20.500.12386/27523We report a luminous Type II supernova, ASASSN-15nx, with a peak luminosity of {M}<SUB>V</SUB>=-20 mag that is between those of typical core-collapse supernovae and super-luminous supernovae. The post-peak optical light curves show a long, linear decline with a steep slope of 2.5 mag (100 day)<SUP>-1</SUP> (i.e., an exponential decline in flux) through the end of observations at phase ≈ 260 {day}. In contrast, the light curves of hydrogen-rich supernovae (SNe II-P/L) always show breaks in their light curves at phase ∼100 day, before settling onto <SUP>56</SUP>Co radioactive decay tails with a decline rate of about 1 mag (100 day)<SUP>-1</SUP>. The spectra of ASASSN-15nx do not exhibit the narrow emission-line features characteristic of Type IIn SNe, which can have a wide variety of light-curve shapes usually attributed to strong interactions with a dense circumstellar medium (CSM). ASASSN-15nx has a number of spectroscopic peculiarities, including a relatively weak and triangular-shaped Hα emission profile with no absorption component. The physical origin of these peculiarities is unclear, but the long and linear post-peak light curve without a break suggests a single dominant powering mechanism. Decay of a large amount of {}<SUP>56</SUP>{Ni} (M <SUB>Ni</SUB> = 1.6 ± 0.2 {M}<SUB>☉ </SUB>) can power the light curve of ASASSN-15nx, and the steep light-curve slope requires substantial γ-ray escape from the ejecta, which is possible given a low-mass hydrogen envelope for the progenitor. Another possibility is strong CSM interactions powering the light curve, but the CSM needs to be sculpted to produce the unique light-curve shape and avoid producing SN IIn-like narrow emission lines.STAMPAenArticle10.3847/1538-4357/aacb352-s2.0-85051843507000440045800008https://iopscience.iop.org/article/10.3847/1538-4357/aacb352018ApJ...862..107BFIS/05 - ASTRONOMIA E ASTROFISICA