Late-time Hubble Space Telescope Observations of AT 2018cow. I. Further Constraints on the Fading Prompt Emission and Thermal Properties 50--60 Days Post-discovery

Abstract

The exact nature of the luminous fast blue optical transient AT 2018cow is still debated. In this first of a two-paper series, we present a detailed analysis of three Hubble Space Telescope (HST) observations of AT 2018cow covering $\sim$50--60 days post-discovery in combination with other observations throughout the first two months, and derive significantly improved constraints of the late thermal properties. By modeling the spectral energy distributions (SEDs), we confirm that the UV-optical emission over 50--60 days was still a smooth blackbody (i.e., optically thick) with a high temperature ($T_{\mathrm{BB}}\sim 15000\mathrm{K}$) and small radius ($R_{\mathrm{BB}}\lesssim 1000R_{\odot }$). Additionally, we report for the first time a break in the bolometric light curve: the thermal luminosity initially declined at a rate of $L_{\mathrm{BB}}\propto t^{-2.40}$, but faded much faster at $t^{-3.06}$ after day 13. Reexamining possible late-time power sources, we disfavor significant contributions from radioactive decay based on the required ${}^{56}$Ni mass and lack of UV line blanketing in the HST SEDs. We argue that the commonly proposed interaction with circumstellar material may face significant challenges in explaining the late thermal properties, particularly the effects of the optical depth. Alternatively, we find that continuous outflow/wind driven by a central engine can still reasonably explain the combination of a receding photosphere, optically thick and rapidly fading emission, and intermediate-width lines. However, the rapid fading may have further implications on the power output and structure of the system. Our findings may support the hypothesis that AT 2018cow and other "Cow-like transients" are powered mainly by accretion onto a central engine.

22 pages, 10 figures. This is paper I of a two-paper series. v2: updated to the accepted version

References in corpus (54)

• Intermediate-Mass Black Holes
• GROND - a 7-channel imager
• Rapidly-Evolving and Luminous Transients from Pan-STARRS1
• SkyMapper Southern Survey: Second Data Release (DR2)
• The Carnegie Supernova Project I: Third Photometry Data Release of Low-Redshift Type Ia Supernovae and Other White Dwarf Explosions
• The Koala: A Fast Blue Optical Transient with Luminous Radio Emission from a Starburst Dwarf Galaxy at $z=0.27$
• A mildly relativistic outflow from the energetic, fast-rising blue optical transient CSS161010 in a dwarf galaxy
• A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients
• Connecting the progenitors, pre-explosion variability, and giant outbursts of luminous blue variables with Gaia16cfr
• iPTF16asu: A Luminous, Rapidly-Evolving, and High-Velocity Supernova
• Radio and X-ray observations of the luminous Fast Blue Optical Transient AT2020xnd
• Properties of Type Ibn Supernovae: Implications for the Progenitor Evolution and the Origin of a Population of Rapid Transients
• A Model for Fast Blue Optical Transient AT2018Cow: Circumstellar Interaction of a Pulsational Pair-instability Supernova
• Wind-reprocessed Transients
• Luminous Millimeter, Radio, and X-ray Emission from ZTF20acigmel (AT2020xnd)
• Rapidly Evolving and Luminous Transients Driven by Newly Born Neutron Stars
• Circumstellar Interaction Powers the Light Curves of Luminous Rapidly Evolving Optical Transients
• Evidence for a Compact Object in the Aftermath of the Extra-Galactic Transient AT2018cow
• Shocked jets in CCSNe can power the zoo of fast blue optical transients
• Investigating the Diversity of Type Ia Supernova Spectra with the Open-Source Relational Database Kaepora
• Fast Optical Transients from Stellar-Mass Black Hole Tidal Disruption Events in Young Star Clusters
• Supernova ejecta interacting with a circumstellar disk. I. two-dimensional radiation-hydrodynamic simulations
• A Blast from the Pasture: Studying the environment of AT 2018cow with MUSE
• The Host Galaxies of Rapidly Evolving Transients in the Dark Energy Survey
• The Peculiar Transient AT2018cow: A Possible Origin of A Type Ibn/IIn Supernova
• Optical to X-ray Signatures of Dense Circumstellar Interaction in Core-Collapse Supernovae
• uGMRT observations of a Fast and Blue Optical Transient -- AT 2018cow
• A common envelope jets supernova (CEJSN) impostor scenario for fast blue optical transients
• Magnetar Engines in Fast Blue Optical Transients and Their Connections with SLSNe, SNe Ic-BL, and lGRBs
• The nearby luminous transient AT2018cow: a magnetar formed in a sub-relativistically expanding non-jetted explosion
• A Swift Fix for Nuclear Outbursts
• AT 2018cow VLBI: No Long-Lived Relativistic Outflow
• A Wind-Driven Model: Application to Peculiar Transients AT2018cow and iPTF14hls
• X-ray transients from the accretion-induced collapse of white dwarfs
• Rapidly Evolving Transients from the Hyper Suprime-Cam SSP Transient Survey
• Deep Hubble Space Telescope Observations of GW170817: Complete Light Curves and the Properties of the Galaxy Merger of NGC 4993
• Analytic Light Curves of Dense CSM Shock Breakout and Cooling
• A hot and luminous source at the site of the fast transient AT2018cow at 2-3 years after its explosion
• Supernova explosions interacting with aspherical circumstellar material: implications for light curves, spectral line profiles, and polarization
• HI 21cm mapping of the host galaxy of AT2018cow: a fast-evolving luminous transient within a ring of high column density gas
• An environmental analysis of the fast transient AT2018cow and implications for its progenitor and late-time brightness
• A systematic study on the rise time-peak luminosity relation for bright optical transients powered by wind shock breakout
• ALMA Polarimetry of AT2018cow
• Dust Echoes from Luminous Fast Blue Optical Transients
• Fast and Luminous Transients from the Explosions of Long Lived Massive White Dwarf Merger Remnants
• On the nature of the unusual transient AT 2018cow from HI observations of its host galaxy
• A flash of polarized optical light points to an aspherical "cow"
• Moving-mesh radiation-hydrodynamic simulations of wind-reprocessed transients
• ALMA observations of molecular gas in the host galaxy of AT2018cow
• The Study of Dust Formation of Four Type Ibn Supernovae
• AT2018lqh: Black Hole Born from a Rotating Star?
• iPTF 16asu Revisited: A Rapidly Evolving Superluminous Broad-Lined Ic Supernova?
• A Population Study on the Radio Emission of Fast Blue Optical Transients
• Terminating a common envelope jets supernova impostor event with a super-Eddington blue supergiant

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• Minutes-duration Optical Flares with Supernova Luminosities