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

Abstract

We propose that luminous fast blue optical transients (LFBOTs) signal the delayed conversion of a massive neutron star (NS; M_NS > ~1.8 Msun) into a highly magnetized hybrid star (HS) with B_HS ~10^15 G surface field; a QCD magnetar. This is the partial conversion channel in the Quark-Nona (QN) model where the core of the NS enters a quark phase with spontaneous generation of extreme (i.e., up to > 10^18 G) magnetic field independent of the NS spin. The process ejects ~0.01 Msun of the NS outermost layers at ~0.1c (the QN ejecta) with a photon diffusion timescale of a few days. The powering of the QN ejecta by spin-down of a rapidly rotating HS (inherited from the parent NS) yields the LFBOT. The fragmentation of the QN ejecta allows optical flares to arise from clumps that become optically thin, releasing stored radiation energy (with luminosities comparable to the LFBOT peak) on light-crossing timescales of tens of minutes. X-rays from the relativistic HS spin-down wind escaping through optically thin gaps in the QN ejecta, and radio from QN ejecta-medium interaction arise self-consistently from a single physical engine. This framework reproduces key features of AT2022tsd, AT2020xnd, AT2020mrf, and AT2018cow. The neutron-rich, r-process-producing QN ejecta predicts kilonova-like emission associated with LFBOTs in environments that do not host neutron star mergers.