Optical Flares in the Luminous Fast Blue Optical Transient AT2022tsd ("Tasmanian Devil")

Technical Summary: Optical Flares in the Luminous Fast Blue Optical Transient AT2022tsd ("Tasmanian Devil")

Problem Statement
Luminous Fast Blue Optical Transients (LFBOTs), such as AT2018cow, AT2022tsd, and AT2020xnd, present a challenge to current astrophysical models. They are characterized by rapid rise and decay times (days), high peak luminosities ($\gtrsim 10^{43}$ erg s$^{-1}$), and hot, featureless blue continua, distinct from standard core-collapse supernovae (CCSNe) and superluminous supernovae (SLSNe). While various models have been proposed—including magnetar spin-down, black hole accretion, shock interaction, and tidal disruption events—none have successfully and self-consistently explained the rapid evolution, multi-wavelength properties (optical, X-ray, radio), and the specific phenomenon of rapid optical flares observed in events like AT2022tsd.

Methodology
The authors extend the Quark-Nova (QN) model to explain LFBOTs through a specific "partial conversion" channel. In this framework, a massive neutron star (NS) with a mass $M_{\text{NS}} \gtrsim 1.8 M_\odot$ undergoes a phase transition where its innermost core deconfines into quark matter, forming a highly magnetized hybrid star (HS), or "QCD magnetar," with a surface field $B_{\text{HS}} \sim 10^{15}$ G.

The model posits two critical conditions for an observable LFBOT:

1. The NS must be born rapidly rotating (period $P_{\text{NS}} \sim$ few ms) or spun up via accretion.
2. The QN transition must occur before the NS has significantly spun down ($t_{\text{QN}} < t_{\text{NS,SpD}}$), allowing the resulting HS to inherit rapid rotation.

The HS spin-down powers the ejection of $\sim 10^{-2} M_\odot$ of the NS's outer layers (the QN ejecta) at $\sim 0.1c$. The authors model the optical, X-ray, and radio emission by solving the spin-down luminosity equations and the radiative transfer through the ejecta. A key methodological innovation is the treatment of the QN ejecta's fragmentation. Unlike NS merger ejecta (stripped outside-in), QN ejecta are launched inside-out and remain cold and mechanically coherent. The authors propose that pre-existing density inhomogeneities in the NS crust seed fragmentation as radiation pressure support drops. They employ stochastic sampling using Weibull probability distributions (WPD) for fragment radii and Weibull failure time distributions (WFTD) for formation times to simulate the release of stored radiation energy when fragments become optically thin.

Key Contributions and Results
The paper presents a unified physical engine that reproduces the multi-wavelength light curves of AT2022tsd, AT2020xnd, AT2020mrf, and AT2018cow:

• Optical Flares: The model attributes the rapid optical flares (duration $\sim$ tens of minutes, luminosities comparable to the LFBOT peak) to the fragmentation of the QN ejecta. As the ejecta expands, distinct clumps become optically thin ($\tau_f < 1$) on light-crossing timescales. These clumps release radiation energy stored throughout their volume, which was maintained by the HS spin-down injection. The authors demonstrate that by tuning the fragmentation timing relative to the transparency timescale ($t_{\text{QN,tr}}$), the model can reproduce the presence of flares (as in AT2022tsd) or their absence (as in AT2018cow, where fragmentation occurs earlier and is dwarfed by the main light curve).
• X-ray Emission: X-rays are generated naturally by the relativistic HS spin-down wind. Synchrotron and inverse Compton radiation from the wind escape through the low-optical-depth gaps created by the fragmented ejecta, producing the observed intermittent X-ray emission without requiring additional assumptions.
• Radio Emission: Radio emission arises from the interaction of the non-relativistic QN ejecta with the ambient medium, consistent with standard synchrotron shock models.
• Thermal Evolution: The model reproduces the hot, blue, featureless continuum of LFBOTs. The effective blackbody temperature evolves as $T_{\text{BB}} \propto t^{-1/4}$ (for $t < t_{\text{QN,d}}$) and $t^{-1/2}$ (for $t > t_{\text{QN,d}}$), consistent with observed spectra. The heavy, neutron-rich composition of the ejecta leads to high opacity and line blending, explaining the lack of early spectral features.

Significance and Claims
The authors claim that this framework provides a self-consistent explanation for the diverse properties of LFBOTs, linking optical flares, X-ray variability, and radio emission to a single physical engine: the delayed conversion of a massive NS into a QCD magnetar.

Key implications highlighted in the paper include:

• Distinct Origin from Mergers: The model predicts that LFBOTs can occur in isolation (via binary accretion) or as a delayed event after a CCSN, distinct from neutron star mergers.
• Kilonova-like Emission: Because the QN ejecta is neutron-rich and favorable for r-process nucleosynthesis, the model predicts that LFBOTs should be accompanied by kilonova-like emission in environments where neutron star mergers are not expected. This is presented as a "smoking-gun" signature of the QN channel.
• Population Statistics: The estimated rate of LFBOTs derived from the NS birth magnetic field and spin period distributions ($\sim 0.01$ of the CCSN rate) is consistent with observational estimates ($\lesssim 1\%$).

The paper concludes that the fragmentation of QN ejecta offers a physically motivated mechanism for the rapid optical flares observed in AT2022tsd, distinguishing this channel from the tidal stripping mechanisms of NS mergers. The authors note that while the statistical agreement with observed flare distributions (via Q-Q tests) is tentative, the model successfully reproduces the key light curve features of multiple LFBOTs with a single set of physical parameters.