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  • Roaring to Softly Whispering: X-Ray Emission after ∼3.7 yr at the Location of the Transient AT2018cow and Implications for Accretion-powered Scenarios*
Title: Roaring to Softly Whispering: X-Ray Emission after ∼3.7 yr at the Location of the Transient AT2018cow and Implications for Accretion-powered Scenarios*
Abstract We present the first deep X-ray observations of luminous fast blue optical transient (LFBOT) AT 2018cow at ∼3.7 yr since discovery, together with the reanalysis of the observation atδt∼ 220 days. X-ray emission is significantly detected at a location consistent with AT 2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first ∼100 days and would possibly signal the emergence of a new emission component, although a robust association with AT 2018cow can only be claimed atδt∼ 220 days, while atδt∼ 1350 days contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT 2018cow, which includes the detection of persistent, slowly fading UV emission withνLν≈ 1039erg s−1. Similar to previous works (and in analogy with arguments for ultraluminous X-ray sources), these late-time observations are consistent with thin disks around intermediate-mass black holes (withM•≈ 103–104M☉) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass black holes withM•≈ 10–100M☉accreting at ≳the Eddington rate cannot be ruled out and provide a natural explanation for the inferred compact size (Rout≈ 40R☉) of the accretion disk years after the optical flare. Most importantly, irrespective of the accretor mass, our study lends support to the hypothesis that LFBOTs are accretion-powered phenomena and that, specifically, LFBOTs constitute electromagnetic manifestations of super-Eddington accreting systems that evolve to ≲Eddington over a ≈100-day timescale.  more » « less
Award ID(s):
2224255 2221789
PAR ID:
10535657
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ;
Publisher / Repository:
ApJ
Date Published:
Journal Name:
The Astrophysical Journal Letters
Volume:
963
Issue:
1
ISSN:
2041-8205
Page Range / eLocation ID:
L24
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
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