AT2018cow: a luminous millimeter transient

Abstract: We present detailed submillimeter- through centimeter-wave observations of the extraordinary extragalactic transient AT2018cow. The apparent characteristics -- the high radio luminosity, the long-lived emission plateau at millimeter bands, and the sub-relativistic velocity -- have no precedent. A basic interpretation of the data suggests $E_k \gtrsim 10^{48}$ erg coupled to a fast but sub-relativistic ($v \approx 0.13c$) shock in a dense ($n_e \approx 3 \times 10^{5}\,$cm$^{-3}$) medium. We find that the X-ray emission is not naturally explained by an extension of the radio-submm synchrotron spectrum, nor by inverse Compton scattering of the dominant blackbody UVOIR photons by energetic electrons within the forward shock. By $\Delta t \approx20\,$days, the X-ray emission shows spectral softening and erratic inter-day variability. Taken together, we are led to invoke an additional source of X-ray emission: the central engine of the event. Regardless of the nature of this central engine, this source heralds a new class of energetic transients shocking a dense medium, which at early times are most readily observed at millimeter wavelengths.

• The paper demonstrates that AT2018cow exhibits a rapid rise and sustained millimeter plateau, suggesting shock interactions with a dense surroundings.
• It employs multi-wavelength observations with SMA, ATCA, and ALMA to capture a detailed transient light curve across various bands.
• Findings indicate an energy scale of ~10^49 erg and sub-relativistic speeds, hinting at a central engine contributing to its unique emissions.

Overview of "AT2018cow: A Luminous Millimeter Transient"

This paper explores the phenomenon of AT2018cow, an atypical extragalactic transient identified in June 2018. The object exhibits unprecedented characteristics marked by high radio luminosity, prolonged emission at millimeter wavelengths, and a velocity that is sub-relativistic but unusually fast. Through comprehensive submillimeter to centimeter-wave observations, the research aims to shed light on these unfamiliar attributes and explores AT2018cow's potential classification as a new category of energetic transients interacting with dense media.

Observational Analysis

The authors conducted extensive observations using various facilities, including the Submillimeter Array (SMA), Australia Telescope Compact Array (ATCA), and the Atacama Large Millimeter/submillimeter Array (ALMA). This multi-faceted observational strategy allowed them to capture a detailed light curve of the AT2018cow event across different wavelengths. A noteworthy aspect is the early detection capability at submillimeter wavelengths, a range that has been underexplored due to technical challenges and the high cost of observations.

By examining the data, the researchers observed a fast rise and long-lived emission plateau at millimeter bands, suggestive of a shock interacting with a dense medium. The absence of a straightforward explanation for the X-ray emissions further emphasizes the novelty of AT2018cow, propelling interpretations that might involve a central engine at the core of the transient's activities.

Modeling and Interpretation

Utilizing the Chevalier model for non-relativistic shocks in dense media, the study estimates key physical parameters of the AT2018cow event. The findings suggest an energy of $\sim10^{49}$ erg with a sub-relativistic velocity ($v \approx 0.1c$) that is consistently propagated, endorsing a scenario where significant energy conversion occurs within a dense circumstellar medium. The deduced electron density, magnetic field strengths, and blast wave properties position AT2018cow between supernova remnants and gamma-ray burst (GRB) afterglows, revealing an intriguing energy landscape.

Contrary to initial expectations, the X-ray emissions did not align well with inverse Compton processes or synchrotron spectra extensions. Instead, the analysis points to an active central engine possibly contributing additional energetic phenomena, akin to those seen in supernovae with pulsar wind nebulae or in tidal disruption events where accretion onto a compact object might play a vital role.

Implications and Speculations

The release and prolonged nature of millimeter-wavelength emissions underscore an interaction with unusually dense media, differentiating AT2018cow from known transient classes. This newly posited category enriches the tapestry of explosive astronomical phenomena, inviting further examination of millimeter-wave transients in other dense astronomical contexts such as supernovae and tidal disruption events potentially harboring central engines.

Future Prospects

The study concludes by advocating for an increased focus on millimeter-wavelength observations in the context of diverse transient events. Such a pursuit, facilitated by contemporary and future advancements in observational technology, promises to deepen our comprehension of high-energy astrophysical processes and drive forward the classification and understanding of anomalous celestial phenomena like AT2018cow. Robust theoretical models and computational simulations, alongside extensive multi-wavelength observational campaigns, are essential to unraveling the complexities unveiled by this first-of-its-kind luminous millimeter transient.