An Embedded X-Ray Source Shines through the Aspherical AT 2018cow: Revealing the Inner Workings of the Most Luminous Fast-evolving Optical Transients

Margutti, R.R.MarguttiMetzger, B. D.B. D.MetzgerChornock, R.R.ChornockVurm, I.I.VurmRoth, N.N.RothGrefenstette, B. W.B. W.GrefenstetteSavchenko, V.V.SavchenkoCartier, R.R.CartierSteiner, J. F.J. F.SteinerTerreran, G.G.TerreranMargalit, B.B.MargalitMigliori, G.G.MiglioriMilisavljevic, D.D.MilisavljevicAlexander, K. D.K. D.AlexanderBietenholz, M.M.BietenholzBlanchard, P. K.P. K.BlanchardBozzo, E.E.BozzoBrethauer, D.D.BrethauerChilingarian, I. V.I. V.ChilingarianCoppejans, D. L.D. L.CoppejansDucci, L.L.DucciFerrigno, C.C.FerrignoFong, W.W.FongGötz, D.D.GötzGuidorzi, C.C.GuidorziHajela, A.A.HajelaHurley, K.K.HurleyKuulkers, E.E.KuulkersLaurent, P.P.LaurentMEREGHETTI, SandroSandroMEREGHETTINicholl, M.M.NichollPatnaude, D.D.PatnaudeUbertini, P.P.UbertiniBanovetz, J.J.BanovetzBartel, N.N.BartelBerger, E.E.BergerCoughlin, E. R.E. R.CoughlinEftekhari, T.T.EftekhariFrederiks, D. D.D. D.FrederiksKozlova, A. V.A. V.KozlovaLaskar, T.T.LaskarSvinkin, D. S.D. S.SvinkinDrout, M. R.M. R.DroutMacFadyen, A.A.MacFadyenPaterson, K.K.Paterson2020-12-182020-12-1820190004-637Xhttp://hdl.handle.net/20.500.12386/29004We present the first extensive radio to γ-ray observations of a fast-rising blue optical transient, AT 2018cow, over its first ∼100 days. AT 2018cow rose over a few days to a peak luminosity L pk ∼ 4 × 1044 erg s-1, exceeding that of superluminous supernovae (SNe), before declining as L ∝ t -2. Initial spectra at δt ≲ 15 days were mostly featureless and indicated large expansion velocities v ∼ 0.1c and temperatures reaching T ∼ 3 × 104 K. Later spectra revealed a persistent optically thick photosphere and the emergence of H and He emission features with v ∼ 4000 km s-1 with no evidence for ejecta cooling. Our broadband monitoring revealed a hard X-ray spectral component at E ≥ 10 keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT 2018cow showed bright radio emission consistent with the interaction of a blast wave with v sh ∼ 0.1c with a dense environment (\dot{M}∼ {10}-3-{10}-4 {M}☉ {yr}}-1 for v w = 1000 km s-1). While these properties exclude 56Ni-powered transients, our multiwavelength analysis instead indicates that AT 2018cow harbored a “central engine,” either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released ∼1050-1051.5 erg over ∼103-105 s and resides within low-mass fast-moving material with equatorial-polar density asymmetry (M ej,fast ≲ 0.3 M ☉). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black holes are disfavored by the large environmental density probed by the radio observations.STAMPAenAn Embedded X-Ray Source Shines through the Aspherical AT 2018cow: Revealing the Inner Workings of the Most Luminous Fast-evolving Optical TransientsArticle10.3847/1538-4357/aafa012-s2.0-85062033314000458043400018https://iopscience.iop.org/article/10.3847/1538-4357/aafa012019ApJ...872...18MFIS/05 - ASTRONOMIA E ASTROFISICA