Roaring to softly whispering: Persistent X-ray emission at the location of the Fast Blue Optical Transient AT2018cow $\sim$3.7 yrs after discovery and implications on accretion-powered scenarios

Abstract

We present the first deep X-ray observations of a luminous FBOT AT2018cow, at $\sim 3.7\mathrm{yr}$ since discovery, together with the re-analysis of the observation at $\hat{I}´t\sim 220$ d. X-ray emission is significantly detected at a location consistent with AT2018cow. The very soft X-ray spectrum and sustained luminosity are distinct from the spectral and temporal behavior of the LFBOT in the first $\sim 100$ d, and would possibly signal the emergence of a new emission component, although a robust association with AT2018cow can only be claimed at $\hat{I}´t\sim 220$ d, while at $\hat{I}´t\sim 1350$ d contamination of the host galaxy cannot be excluded. We interpret these findings in the context of the late-time panchromatic emission from AT2018cow, which includes the detection of persistent, slowly-fading UV emission with $\hat{I}½L_{\hat{I}}½\approx 10^{39}\mathrm{erg}{\mathrm{s}}^{-1}$. Similar to previous works, (and in analogy with arguments for Ultra-Luminous X-ray sources --ULXs), these late-time observations are consistent with thin-disks around Intermediate Mass Black Holes (IMBHs, with $M_{\bullet }\approx 10^3-10^4{\mathrm{M}}_{\odot }$) accreting at sub-Eddington rates. However, differently from previous studies, we find that smaller-mass BHs with $M_{\bullet }\approx 10-100{\mathrm{M}}_{\odot }$ accreting at $\gtrsim$ the Eddington rate cannot be ruled out, and provide a natural explanation for the inferred compact size ($R_{\mathrm{out}}\approx 40R_{\odot }$) 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 $\lesssim$ Eddington over a $\approx 100$ days time scale.

17 pages, 3 figures, accepted for publication in ApJL