Luminous Fast Blue Optical Transients and Type Ibn/Icn SNe from Wolf-Rayet/Black Hole Mergers

Abstract

Progenitor models for the “luminous” subclass of Fast Blue Optical Transients (LFBOTs; prototype: AT2018cow) are challenged to simultaneously explain all of their observed properties: fast optical rise times of days or less; peak luminosities ≳1044 erg s−1; low yields ≲0.1M ⊙ of 56Ni; aspherical ejecta with a wide velocity range (≲3000 km s−1 to ≳0.1–0.5c with increasing polar latitude); presence of hydrogen-depleted-but-not-free dense circumstellar material (CSM) on radial scales from ∼1014 cm to ∼3 × 1016 cm; embedded variable source of non-thermal X-ray/γ-rays, suggestive of a compact object. We show that all of these properties are consistent with the tidal disruption and hyper-accretion of a Wolf-Rayet (WR) star by a black hole or neutron star binary companion. In contrast with related previous models, the merger occurs with a long delay (≳100 yr) following the common envelope (CE) event responsible for birthing the binary, as a result of gradual angular momentum loss to a relic circumbinary disk. Disk-wind outflows from the merger-generated accretion flow generate the 56Ni-poor aspherical ejecta with the requisite velocity range. The optical light curve is powered primarily by reprocessing X-rays from the inner accretion flow/jet, though CSM shock interaction also contributes. Primary CSM sources include WR mass loss from the earliest stages of the merger (≲1014 cm) and the relic CE disk and its photoevaporation-driven wind (≳1016 cm). Longer delayed mergers may instead give rise to supernovae Type Ibn/Icn (depending on the WR evolutionary state), connecting these transient classes with LFBOTs.

Hydrodynamics of Collisions and Close Encounters between Stellar Black Holes and Main-sequence Stars

“…For practically all values of s, the accretion rate onto the black hole is reduced and the majority of the disk mass is lost in the form of a radiatively driven wind as shown by (magneto) hydrodynamic simulations of radiatively inefficient accretion flows (e.g., Stone et al 1999;Igumenshchev & Abramowicz 2000;Hawley et al 2001;Narayan 2012;Yuan & Narayan 2014). In this case, the energy generated by accretion onto the black hole (which is reduced due to the reduced accretion rate) at the inner edge of the disk will be reprocessed by this wind and released at the photon trapping radius (e.g., Loeb & Ulmer 1997;Strubbe & Quataert 2009;Metzger & Stone 2016;Dai et al 2018;Kremer et al 2019a;Piro & Lu 2020;Metzger 2022). The emerging luminosity (which will be smaller than the accretion luminosity by a factor of roughly 100 due to adiabatic losses) is expected to be of order 10 40 −10 44 erg s −1 and peak in the optical/UV band for temperatures expected here (for more detail, see Kremer et al 2019a).…”

The X-Ray and Radio Loud Fast Blue Optical Transient AT2020mrf: Implications for an Emerging Class of Engine-driven Massive Star Explosions

“…Zapartas et al (2017) performed a population synthesis study of CCSNe, finding that a prolonged delay time can be achieved by binary interactions through common envelope evolution, mass transfer episodes, and/or merging. Explosions driven by the merging of a compact object with a massive star inside a common envelope have indeed been proposed as promising channels for producing AT2018cow-like events (Soker et al 2019;Schrøder et al 2020;Soker 2022;Metzger 2022).…”

Neutrino Emission from Luminous Fast Blue Optical Transients

“…This leads to the formation of an asymmetric CSM, dense in the equatorial region, and less dense in the polar one. The scenarios proposed by Gottlieb et al (2022b) and Metzger (2022) successfully fit the ultraviolet, optical and infrared spectra of AT2018cow; Metzger (2022) also provides a fit to the X-ray data of AT2018cow. However, it is yet to be quantitatively proven that the off-axis jet scenario of Gottlieb et al (2022b) is consistent with radio observations; no fit to the radio data is provided in Metzger (2022).…”

“…The jet interacts with the stellar envelope, inflating the cocoon surrounding the jet; the cocoon expands, breaks out of the star and cools, emitting in the ultraviolet, optical, and infrared. Metzger (2022) considers a delayed Wolf-Rayet starblack hole merger following a failed common envelope phase. This leads to the formation of an asymmetric CSM, dense in the equatorial region, and less dense in the polar one.…”