Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?

Abstract: The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint gamma-ray emission continuing for at least 8 days, and featureless spectra in the ultraviolet bands -- both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a 0.1 - 0.4 Msun Helium White Dwarf (WD) by a 100,000 to one million solar mass Black Hole (BH) located in the outskirts of galaxy Z~137-068 could provide an explanation for most of the characteristics shown in the multi-wavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10-150 keV band.

• The paper proposes that AT2018cow arises from a tidal disruption of a helium-rich white dwarf by an intermediate-mass black hole.
• Multi-wavelength observations and spectral modeling capture the event’s rapid evolution, highlighted by a near-blackbody UV-optical spectrum and burst-like X-ray variability.
• The study constrains key parameters, estimating WD masses of 0.1–0.4 M☉ and BH masses of 10⁵–10⁶ M☉, thereby advancing our understanding of stellar disruption phenomena.

Analyzing AT2018cow: Evidence for a White Dwarf Tidal Disruption Event

The paper entitled "Swift spectra of AT2018cow: A White Dwarf Tidal Disruption Event?" explores the origins of the unusual transient event AT2018cow, proposing it as a potential tidal disruption event (TDE) involving a white dwarf (WD) and a black hole (BH). This analysis leverages multi-wavelength observations, including data from the Swift satellite, to provide insight into the physical processes involved.

Key Observations and Modeling Approach

AT2018cow was initially notable for its high brightness, rapid rise and decay, and nearly featureless UV-optical spectrum. These features are atypical in comparison to other known transient events such as supernovae (SNe) or gamma-ray bursts (GRBs). The research presented in this paper suggests that the multi-wavelength characteristics of AT2018cow could be explained by the tidal disruption of a helium-rich WD by an intermediate-mass BH.

Spectral Characteristics and Multi-Wavelength Analysis

The spectral analysis reveals faint gamma-ray emission lasting for at least eight days and X-ray variability with a burst-like character. Notably, the UV-optical spectrum lacks typical emission lines, resembling a blackbody spectrum—a key observation that aligns with the proposed disruption scenario. The researchers find that, compared to other luminous transients, the observed characteristics of AT2018cow can be modeled as the aftermath of a TDE involving a WD, leading to the formation of a dense photosphere from disrupted material.

Kinematic and Energetic Considerations

A central challenge in understanding AT2018cow is explaining the rapid evolution and energy output of the event. The analysis suggests a photosphere radius on the order of 5 x 10^14 cm, requiring significant expansion velocities (~0.1c) and inferring a non-trivial kinetic energy budget on the order of 10^50 erg, which is consistent with a low-mass WD undergoing tidal disruption.

The presence of a jet is proposed to account for the observed X-ray signatures and possible gamma-ray emission. The X-ray light curve shows characteristics compatible with an off-axis jet, paralleling dynamics observed in some known TDEs such as Swift J1644+57.

Theoretical Implications and Constraints

In TDE modeling, the mass of both the white dwarf and the black hole are crucial parameters. The limits provided by the disk dynamics and spectral observations suggest a helium WD with a mass range of 0.1-0.4 solar masses and a black hole mass between $10^5$ and $10^6 M_\odot$. This conclusion is contingent upon the disruption radius being outside the Schwarzschild radius, emphasizing the role of dynamical instabilities and accretion processes in shaping the transient’s evolution.

Open Questions and Future Directions

While the proposed white dwarf tidal disruption scenario provides a coherent explanation for many aspects of AT2018cow, several open questions and ambiguities remain. The origin of the apparent non-thermal emissions and the exact mechanism leading to the formation of broad-line features in the early stages are areas ripe for further theoretical and observational exploration. Continued monitoring of similar events and advancements in hydrodynamical modeling will be critical in validating the TDE hypothesis and refining its parameters.

Furthermore, understanding the environment surrounding AT2018cow, which exhibits low density, poses additional challenges for constraining the circumstellar medium's role in the observed emissions. This study prompts further investigation into the frequency and conditions under which white dwarf TDEs occur, offering a potential new class of transient phenomena in astrophysics.

In conclusion, while uncertainties remain, this research elucidates a compelling scenario of a WD-BH interaction, expanding our understanding of transient astrophysical phenomena and the diverse outcomes of stellar disruptions by massive black holes. As such, it marks a significant step in the ongoing effort to categorize and model transient events within the broader context of stellar evolution and accretion physics.