AT2018cow Powered by a Shock in Aspherical Circumstellar Media

TL;DR

The paper presents a quantitative, interaction-powered model for AT2018cow in which a fast, radiative shock plows through an aspherical, dense equatorial CSM embedded in a dilute CSM. X-rays originate from the hot immediate downstream while optical/UV photons are produced by reprocessing of these X-rays in the cooled downstream, with radio emission arising from the shock in the dilute CSM; the model relies on four hydrodynamic parameters and naturally explains the observed X-ray–optical coordination, the early 10 keV hump, and multi-wavelength light curves. A global radiative-shock instability accounts for observed X-ray fluctuations, and NIR excess is explained by non-thermally equilibrated reprocessed X-rays. The inferred energetics (E_j ∼ 1–5×10^{50} erg, M_ej ∼ 0.01–0.05 M_⊙ at v ∼ 0.1 c, and M_CSM ∼ 0.3 M_⊙ with ρ ∝ r^{-s}, s ≈ 2.4–3.1) point to a compact, asymmetric progenitor scenario (e.g., AIC or ultra-stripped SN) and show the framework can be generalized to other LFBOTs.

Abstract

We present a quantitative model for the luminous fast blue optical transient AT2018cow in which a shock propagating through an aspherical circumstellar medium (CSM) produces the X-ray and UV/optical/NIR emission. X-rays are emitted from hot post-shock electrons, and soft X-ray photons are reprocessed into optical/UV emission in the cool downstream. This naturally explains two previously puzzling features: (i) the coordinated evolution of the optical and soft X-ray after day 20, (ii) the hard X-ray hump above 10 keV that disappears around day 15 as the Thomson optical depth transitions from $τ_T \gg1$ to $τ_T \sim 1$. Our model is over-constrained, and it quantitatively reproduces the bolometric luminosity evolution, soft X-ray spectrum, and time-dependent soft/hard X-ray and soft X-ray/optical luminosity ratios. It also explains additional puzzles: X-ray fluctuations with $\sim4-10$ day timescales arise from a global radiative shock instability, while the NIR excess and the apparent receding blackbody radius result from reprocessed X-rays in matter far from thermodynamic equilibrium. The radio is naturally explained as originating from a shock driven by the same ejecta in the more dilute CSM. The light curve steepening after $\sim 40$ days likely indicates the shock reaches the edge of the dense CSM at $\sim {\rm few} \times 10^{15}$ cm. We infer explosion energy $\sim 1-5 \times 10^{50}$ erg, carried by an ejecta at $\sim 0.1c$ and a mass of $0.01-0.05 M_\odot$, in a dense asymmetric CSM with $\sim 0.3 M_\odot$, embedded in a more dilute CSM.

Figures (5)

• Figure 1: A schematic sketch of our model, showing on the top, the asymmetrical dense CSM embedded in a more dilute CSM. A single explosion launches a shock in both components; the X-ray and optical originate from a shock in the dense CSM, which decelerates as it collects mass, while the radio is emitted by the constant velocity shock in the dilute CSM.
• Figure 2: Phase space of the various regimes in the $(\tau_T,v_{\rm s})$ plane, for $s=2.5$. The red line shows our canonical model for AT2018cow ($t_{bo}=0.7\rm~d$, $v_{bo}=0.13~c$, $s=2.5$, $k=0.6$), and the top axis shows the time corresponding to this model. While the phase space itself only weakly depends on $s$ and is independent of all other parameters, the time depends on all model parameters, and is specific for our canonical model.
• Figure 3: A comparison of our model with $t_{bo}= 0.7 {\rm ~d}$, $v_{bo}=0.13c$, $s=2.5$, and $k=0.6$, to the optical and X-ray observations. The top panel shows light curves, and the bottom panel shows the ratios between the different bands.
• Figure 4: The predicted optical-IR spectrum from a CLOUDY simulation of the X-ray spectrum at day 30 reflected off a cold dense shell. The reflected spectrum is smoothed on a frequency scale corresponding to the expected Doppler broadening, though it is likely that additional broadening factors are also present. We plot the observations and model of Perley2019 for reference. Note that this simulation is a crude approximation that does not include all the physics; specifically, it does not include the regions at UV temperatures or their emission.
• Figure 5: Free–free spectrum from a cooling plasma compared to its low- and high-frequency approximations. As described in the text, $\nu^{-1/2}\exp(-h\nu/kT_{max})$ fits the predicted spectrum above for $h\nu \gtrsim 0.02 k_BT_{max}$. $\log(kT_{\max}/h\nu)$ is a good approximation at lower energies, and as expected, does not capture the high-energy cutoff.