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 ∼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 BB ∼ 15,000 K) and small radius (R BB ≲ 1000 R ⊙). Additionally, we report for the first time a break in the bolometric light curve: the thermal luminosity initially declined at a rate of L BB ∝ 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 56Ni 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.

Roaring to Softly Whispering: X-Ray Emission after ∼3.7 yr at the Location of the Transient AT2018cow and Implications for Accretion-powered Scenarios*

“…In addition, at δt ∼ 220 days, we added a power law (Γ ∼ 2.0) with an exponential cutoff at ≈3 keV, mimicking a fading Comptonized component. 23 In the following analysis we adopt the HST photometry values presented in Chen et al (2023aChen et al ( , 2023b. However, we note that consistent findings would be obtained by using the photometry presented in the other studies, without any significant impact on our major conclusions.…”

“…The long-term evolution of the X-ray emission from the location of AT 2018cow is characterized by three regimes: (i) an initial, highly luminous, slowly decreasing phase at δt  24 days with fast temporal variability superimposed; (ii) a rapid decay (a drop of about three orders of magnitude in ∼200 days); possibly followed by (iii) a late-time flattening phase with L X ∼ 4 × 10 38 erg s −1 at δt  200 days with a soft X-ray spectrum, which, however, might be contaminated, or even dominated, by the host emission. The presence of luminous, lingering optical/UV emission during phase (iii) (Sun et al 2022;Chen et al 2023aChen et al , 2023bSun et al 2023), which is reminiscent of TDEs in the lowest mass bin (van Velzen et al 2019;Inkenhaag et al 2023;Mummery et al 2024), indirectly supports the physical association of the persistent, soft X-ray emission with AT 2018cow. Indeed, a further deep X-ray observation, able to probe the flux level measured in Obs4, could help to better discriminate the AT 2018cow contribution from the host galaxy one.…”

“…While the bright radio to millimeter emission of AT 2018cow stems from a nonrelativistic shock interaction with a dense medium (Ho et al 2019;Margutti et al 2019;Nayana & Chandra 2021), the persistent optical/UV blue colors requiring T eff ≈ 15,000 K weeks after discovery (e.g., Margutti et al 2019;Perley et al 2019;Xiang et al 2021), and the broadband X-ray properties above demand a different powering source, which can be in the form of a long-lived central engine. Slowly decaying, luminous (L > 10 7 L ☉ ) UV emission years after discovery has been recently reported by Sun et al (2022Sun et al ( , 2023, Chen et al (2023aChen et al ( , 2023b, Inkenhaag et al (2023), andMummery et al (2024), and similarities have been noted with radiation powered by accretion processes on compact objects. Along the same lines, a high-frequency (224 Hz) quasi-periodic oscillation (QPO) feature in the X-ray timing properties of AT 2018cow supports the presence of a compact object, either an NS or a BH with mass M < 850 M e (Pasham et al 2021; but see also Zhang et al 2022).…”

“…While for the modeling we formally associate the X-ray flux of Obs4 with AT 2018cow, this assumption is relaxed in the discussion on the nature of the accretor (see the text). The optical/UV fluxes (colored squares) are the HST data from Chen et al (2023aChen et al ( , 2023b. Triangles are NuSTAR data.…”