An embedded X-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients

Abstract: We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$ erg/s exceeding those of superluminous supernovae (SNe), before declining as $\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at $E\ge 10$ keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with $v_{sh}$~0.1c with a dense environment ($\dot M\sim10^{-3}-10^{-4}\,M_{\odot}yr^{-1}$ for $v_w=1000$ km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released $\sim10^{50}-10^{51.5}$ erg over $\sim10^3-10^5$ s and resides within low-mass fast-moving material with equatorial-polar density asymmetry ($M_{ej,fast}\lesssim0.3\,\rm{M_{\odot}}$). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations.

• The paper reveals that AT2018cow’s luminous optical transient is powered by a central engine, likely a magnetar or accreting black hole.
• It employs extensive multi-wavelength observations over 100 days, from radio to gamma-rays, to trace rapid evolution and extreme energetics.
• The study identifies key shock and ejecta interactions with dense circumstellar material, reshaping our understanding of fast-evolving transients.

Overview of the Study on AT2018cow: An In-Depth Analysis

The paper, "An embedded x-ray source shines through the aspherical AT2018cow: revealing the inner workings of the most luminous fast-evolving optical transients" by Margutti et al., provides a comprehensive multi-wavelength examination of the fast-rising blue optical transient (FBOT) event, AT2018cow. Utilizing data spanning radio to $\gamma$-ray bands, the study presents an exceptional dataset over the transient's first 100 days, probing the intrinsic properties and defining a new class of astronomical occurrences distinct from traditional supernovae (SNe) and gamma-ray bursts (GRBs).

Key Observations

Optical and UV Observations

• AT2018cow was marked by a rapid increase in brightness, reaching a peak optical luminosity of $L_{\rm{pk}}\sim4\times 10^{44}\,\rm{erg\,s^{-1}$, surpassing that of many superluminous supernovae.
• Early spectra, up to 15 days, were featureless yet indicated high expansion velocities around $v\sim0.1\,c$ with temperatures as high as $3\times 10^{4}$ K.
• Distinct H and He lines with velocities $\sim4000$ km s$^{-1\ were apparent in later spectra, with persistent optical thickness and absence of significant cooling.</li> </ul> <h4 class='paper-heading' id='x-ray-and-radio-bands'>X-ray and Radio Bands</h4> <ul> <li>A rich X-ray timeline revealed a notably luminous and variable soft X-ray component, exceeding typical levels found in conventional transients.</li> <li><a href="https://www.emergentmind.com/topics/bigcodebench-hard-dataset" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">Hard</a> X-ray observations uncovered a transient component above 10 keV, unusually bright and variable, reminiscent of but not identical to <a href="https://www.emergentmind.com/topics/active-galactic-nucleus-agn-driven-shocks" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">AGN</a> activity.</li> <li>The radio emission was consistent with a blastwave at $v_{sh}\sim0.1\,c$$, interacting with a dense <a href="https://www.emergentmind.com/topics/circumstellar-medium-csm" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">circumstellar medium</a> (<a href="https://www.emergentmind.com/topics/concrete-score-matching-csm" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">CSM</a>), denoting$$\dot M$rates between$10^{-3}-10^{-4}\,\rm{M_{\sun}yr^{-1}$.</li> </ul> <h3 class='paper-heading' id='interpretation-and-proposed-models'>Interpretation and Proposed Models</h3> <p>The paper synthesizes these observations to propose mechanisms behind AT2018cow&#39;s unique attributes. The findings conflict with typical models driven by $^{56}$Ni decay due to the event&#39;s optical luminosity and rapid decline, pointing instead towards a central engine. The two major models explored are:</p> <ol> <li><strong>Central Compact Object Engine:</strong> <ul> <li>The core energy source might be a rapidly spinning magnetar or an accreting black hole, capable of releasing between $10^{50}-10^{51.5}$erg over timescales of$10^{3}-10^{5}$ s.
• The X-ray emission could originate from a magnetized nebula or an accretion disk, consistent with a persistent asymmetrical structure seen via H and He line profiles.

Shocked Ejecta/CSM Interaction:

• Alternatively, a deeply embedded radiative shock, forming from the interaction of SN ejecta with dense, aspherical circumstellar material, is entertained.
• This scenario allows for rapid evolutionary traits and the bright, extended radio emissions seen, harmonizing with asymmetric geometry and outgoing flow dynamics.

Theoretical and Practical Implications

The findings of AT2018cow have substantial implications for understanding the diversity of stellar endpoints and transient phenomena. Practically, they highlight the value of broad-spectrum coordinated campaigns, including hard X-ray and radio follow-ups, to identify and elucidate underrepresented classes of astrophysical events.

Future Directions

Future studies should aim to:

• Enhance detection strategies for similar FBOTs at larger redshifts using existing and upcoming wide-field observatories.
• Further model the geometrical asymmetries and interactions between high-energy photons and varying CSM distributions to simulate detailed multi-band light curves for similar transients.
• Explore potential progenitor scenarios, integrating these findings with population synthesis models to better predict the frequency and environments of such events globally. This includes leveraging insights on rapid mass-loss stages and fallback accretion scenarios for blue supergiants and other potential progenitor systems.

Through such efforts, the understanding of AT2018cow-like phenomena can be significantly refined, establishing connections or distinctions with traditional SNe and other explosive cosmic phenomena.