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

Brian D. Metzger|arXiv (Cornell University)|Mar 8, 2022

Gamma-ray bursts and supernovaePhysics and Astronomy168 references88 citations

TL;DR

This paper proposes that luminous fast blue optical transients (LFBOTs) and Type Ibn/Icn supernovae arise from tidal disruption and hyper-accretion of a Wolf-Rayet star by a black hole or neutron star in a binary system, with a long delay (≥100 yr) due to angular momentum loss from a relic circumbinary disk. The model explains the fast rise, high luminosity, low 56Ni yield, aspherical ejecta, and variable X-ray emission via disk-wind outflows and reprocessing of X-rays from a central engine, unified with SNe Ibn/Icn through merger delay timescales.

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 < days; peak luminosities >1e44 erg/s; low yields <0.1 Msun of 56Ni; aspherical ejecta with a wide velocity range (<3000 km/s to > 0.1-0.5 c with increasing polar latitude); presence of hydrogen-depleted-but-not-free dense circumstellar material (CSM) on radial scales from ~1e14 cm to ~3e16 cm; embedded variable source of non-thermal X-ray/gamma-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 (BH) or neutron star (NS) binary companion. In contrast with related previous models, the merger occurs with a long delay (>~ 100 years) 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 mass-loss from WR star (e.g., from the L2 point) during the earliest stages of the merger (~<1e14 cm) and the relic CE disk and its photoevaporation-driven wind (>~ 1e16 cm). Longer delayed mergers may instead give rise to supernovae Type Ibn/Icn (depending on the WR evolutionary state), potentially connecting these transient classes with LFBOTs.

Motivation & Objective

• To resolve the challenge of explaining the multi-wavelength properties of LFBOTs, including fast rise times, high luminosities, low 56Ni yields, and variable X-ray emission.
• To unify LFBOTs with Type Ibn/Icn SNe through a common progenitor mechanism involving delayed mergers of WR stars with compact objects.
• To explain the presence of dense, hydrogen-poor circumstellar material (CSM) on scales from 10^14 to 10^16 cm via relic circumbinary disk and photoevaporation-driven winds.
• To account for the central engine's X-ray variability and Compton hump features through reprocessing in fast, aspherical ejecta with optical depth τT ≈ few.
• To connect the observed radio synchrotron emission to shock interaction between fast polar ejecta and dense CSM, with electron energy distribution deviating from standard power-law.

Proposed method

• Models the tidal disruption of a Wolf-Rayet star by a black hole or neutron star in a binary system, with a long delay (≥100 yr) due to angular momentum loss from a relic circumbinary disk.
• Uses GRMHD-inspired accretion models with super-Eddington rates, yielding ˙M• ∝ t^−2 and X-ray luminosity consistent with AT2018cow.
• Simulates disk-wind outflows from the accretion flow, producing aspherical ejecta with fast polar components (v > 0.1c) and low mass (Mfast ≈ 0.1 M⊙), matching optical rise times.
• Calculates X-ray reprocessing in the fast polar ejecta, with optical light curves powered primarily by reprocessing and secondarily by CSM shock interaction.
• Estimates CSM mass and density from the relic common envelope disk and its photoevaporated wind, with radial extent ∼10^16 cm and n ≳10^5 cm⁻³, consistent with radio emission.
• Predicts 56Ni yields ≲10⁻² M⊙ from inner disk regions, consistent with low 56Ni in LFBOTs and Ibn/Icn SNe.

Experimental results

Research questions

• RQ1 Can a delayed merger of a Wolf-Rayet star with a black hole or neutron star explain the fast rise, high luminosity, and low 56Ni yield of LFBOTs?
• RQ2 How can the aspherical ejecta with velocities from 3000 km s⁻¹ to >0.1c be produced in a single event?
• RQ3 What physical mechanism generates the observed variable, X-ray-dominated emission with a Compton hump and quasi-periodic oscillations?
• RQ4 How is the dense, hydrogen-poor CSM on scales of 10^14–10^16 cm produced, and how does it contribute to radio and optical emission?
• RQ5 Can the observed properties of Type Ibn/Icn SNe be unified with LFBOTs through a common progenitor scenario involving delayed mergers?

Key findings

• The model reproduces the optical rise time of AT2018cow via the Arnett relation, requiring a fast polar ejecta mass of ≈0.1 M⊙ with velocities >0.1c.
• The X-ray light curve is powered by reprocessing of variable X-rays from a central engine, with a decay rate consistent with LX ∝ t^−4 after ~30 days.
• The Compton hump feature arises from reprocessing of a power-law spectrum through a fast polar shell with Thomson optical depth τT ≈ few, requiring Mfast ≈ 0.1 M⊙.
• The 56Ni yield in disk-wind outflows is predicted to be ≲10⁻² M⊙, consistent with the lack of a late-time secondary peak in optical light curves.
• The radio emission is explained by shock interaction between fast ejecta (v ≈ 0.1–0.2c) and CSM with n ≳10^5 cm⁻³ at r ≈ 10^16 cm, sourced by the relic circumbinary disk and its photoevaporated wind.
• Longer delay times (exceeding disk lifetime) lead to SNe Ibn/Icn with little or no H-rich CSM, unifying them with LFBOTs as merger-driven transients.