Late-time Hubble Space Telescope Observations of AT 2018cow. II. Evolution of a UV-bright Underlying Source 2–4 Yr Post-discovery

Abstract

In this second of a two-paper series, we present a detailed analysis of three Hubble Space Telescope observations taken ∼2–4 yr post-discovery, examining the evolution of a UV-bright underlying source at the precise position of AT 2018cow. While observations at ∼2–3 yr post-discovery revealed an exceptionally blue (L ν ∝ ν 1.99) underlying source with relatively stable optical brightness, fading in the near-UV was observed at year 4, indicating flattening in the spectrum (to L ν ∝ ν 1.64). The resulting spectral energy distributions can be described by an extremely hot but small blackbody, and the fading may be intrinsic (cooling) or extrinsic (increased absorption). Considering possible scenarios and explanations, we disfavor significant contributions from stellar sources and dust formation, based on the observed color and brightness. By comparing the expected power and the observed luminosity, we rule out interaction with known radio-producing circumstellar material (CSM) as well as magnetar spin down with B ∼ 1015 G as possible power sources, though we cannot rule out the possible existence of a denser CSM component (e.g., a previously ejected hydrogen envelope) or a magnetar with B ≲ 1014 G. Finally, we find that a highly inclined precessing accretion disk can reasonably explain the color, brightness, and evolution of the underlying source. However, a major uncertainty in this scenario is the mass of the central black hole (BH), as both stellar-mass and intermediate-mass BHs face notable challenges that cannot be explained by our simple disk model, and further observations and theoretical works are needed to fully constrain the nature of this underlying source.

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.…”

“…23 The UV spectrum is consistent with the F ν ∝ ν 2 expected for the Rayleigh-Jeans tail of an optically thick thermal spectrum. Chen et al (2023a) find indications of a chromatic flux decay, with a flattening of the near-UV spectrum in the last HST epoch, possibly pointing at cooling and expansion of the emitting region. The (slow) decay of the UV emission is not consistent with a stellar origin and suggests instead a physical association with AT 2018cow.…”

“…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.…”

Magnetar as the Central Engine of AT2018cow: Optical, Soft X-Ray, and Hard X-Ray Emission

“…The result is that the magnetization in the nebula may be very low, which is necessary to explain the the observed late-time steepening of superluminous SN optical light curves (Vurm & Metzger 2021). It should also be emphasized that AT2018cow has many other peculiar observational properties that are not discussed in the present work, such as high-amplitude quasiperiodic oscillation of AT2018cowʼs soft X-rays with a frequency of 224 Hz (Pasham et al 2021); possible 250 s X-ray quasiperiodicity (Zhang et al 2022); luminous UV emission detected 2-4 yr after the explosion (Sun et al 2022;Chen et al 2023); and persistent X-ray emission at the location of AT2018cow ∼3.7 yr after discovery (Migliori et al 2023). It is a great challenge to explain all these peculiar observational properties of AT2018cow based on the millisecond magnetar as the central engine and there is still a lot of work to be done.…”