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

Creators: Margutti, R.; Grefenstette, B. W.

Abstract

We 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 × 10^(44) 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 × 10^4 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 (M ~ 10^(-3) – 10^(-4) M⊙ Yr^(-1) for v w = 1000 km s−1). While these properties exclude ^(56)Ni-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 ~10^(50)–10^(51.5) erg over ~10^3–10^5 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.

Additional Information

© 2019 The American Astronomical Society. Received 2018 October 25; revised 2018 December 19; accepted 2018 December 19; published 2019 February 6. We are very grateful to the entire NuSTAR, INTEGRAL, Swift, XMM-Newton, VLA, and VLBA teams for making this observing campaign possible. We thank R. Lunnan, A. Suzuki for their feedback on the first version of the paper that appeared on the archive. Some of the observations reported here were obtained at the MMT Observatory, a joint facility of the University of Arizona and the Smithsonian Institution under programs (2018A-UAO-G15, 2018B-SAO-21, 2018B-UAO-G16; PIs Fong, Patnaude, Terreran). Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration under program NW254 (PI Miller). The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. The Keck and MMT observations were supported by Northwestern University and the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). This paper includes data acquired with UKIRT under program (U/18A/UA01, PI Fong). UKIRT is owned by the University of Hawaii (UH) and operated by the UH Institute for Astronomy; operations are enabled through the cooperation of the East Asian Observatory. This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile. Based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory (NOAO Prop. 2018A-0343, 2018B-0210; PI: G. Terreran), which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Based in part on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, Inovaçãos e Comunicaçãoes (MCTIC) do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). This paper uses data products produced by the OIR Telescope Data Center, supported by the Smithsonian Astrophysical Observatory. Support for this work was provided to MRD by NASA through Hubble Fellowship grant NSG-HF2-51373 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. Partly based on observations with INTEGRAL, an ESA project with instruments and science data center funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain) and with the participation of Russia and the USA. Partly based on observations obtained with XMM-Newton (IDs: 0822580401, 0822580501, AO-17, program #82258; PI Margutti), an ESA science mission with instruments and contributions directly funded by ESA member states and NASA. R.M. acknowledges partial support from NASA XMM-Newton Guest Investigator program grant 80NSSC19K0384. We acknowledge the use of public data from the Swift data archive. This work made use of data from the NuSTAR mission (IDs 90401327002, 90401327004, 90401327006, 90401327008), a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA). Observations taken with the VLA (program 18A-123; PI Coppejans) were used in this research. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. B.D.M. acknowledges support from NSF grant AST-1615084, NASA Fermi Guest Investigator Program grants NNX16AR73G and 80NSSC17K0501, and through the Hubble Space Telescope Guest Investigator Program grant HST-AR-15041.001-A. P.U. and S.M. acknowledge financial support from ASI under contracts ASI/INAF 2013-025-R0. D.G. acknowledges the financial support of the UnivEarthS Labex program at Sorbonne Paris Cité (ANR-10-LABX-0023 and ANR-11-IDEX-0005-02). K.D.A. acknowledges support provided by NASA through the NASA Hubble Fellowship grant HST-HF2-51403.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. I.C. acknowledges support by the Telescope Data Center, Smithsonian Astrophysical Observatory and the Russian Science Foundation grant 17-72-20119. D.J.P. acknowledges support through NASA Contract NAS8-03060. L.D. acknowledges support by the Bundesministerium für Wirtschaft und Technologie and the Deutsches Zentrum für Luft und Raumfahrt through the grant FKZ 50 OG 1602. N.R. acknowledges the support from the University of Maryland through the Joint Space Science Institute Prize Postdoctoral Fellowship as well support from the Center for Research and Exploration in Space Science and Technology II. M.R.D. acknowledges support from the Dunlap Institute at the University of Toronto. D.D.F. and A.V.K. gratefully acknowledge support from RFBR grant 18-02-00062. Facilities: Swift - Swift Gamma-Ray Burst Mission, XMM - , NuSTAR - , INTEGRAL - , Odyssey - , WIND (Konus) - , Fermi - Fermi Gamma-Ray Space Telescope (formerly GLAST), Magellan:Baade (LDSS2 imaging spectrograph, Boller & Chivens spectrograph), Magellan:Clay (LDSS2 imaging spectrograph) - , SOAR - , Keck:I - , Keck:II - , WIYN - , CTIO:0.9m - , UKIRT - , MMT - , VLA - , VLBA - . Software: BOXFIT (Version 2; van Eerten et al. 2010, 2012), CASA (McMullin et al. 2007), CIAO (Fruscione et al. 2006), HEAsoft (Version 6.24), HOTPANTS (Becker 2015), INTEGRAL Off-line Scientific Analysis (OSA), IRAF (Tody 1986, 1993), pwkit (Williams et al. 2017), SNOoPY (http://sngroup.oapd.inaf.it/snoopy.html), XMM-Newton SAS (Version 17.0), XSPEC (Arnaud 1996).

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