Dust Echoes from Luminous Fast Blue Optical Transients

TL;DR

This paper proposes that dust formation in the circumstellar material of LFBOTs causes observable infrared echoes and affects their early light curves, providing new insights into their progenitors and environments.

Contribution

It introduces a model where dust in the CSM explains the NIR excess and color evolution of LFBOTs, linking dust formation to observable signatures.

Findings

Dust grains up to ~1 micron can form in the outflow before explosion.

Dust re-radiation creates a detectable NIR echo lasting weeks.

The NIR excess in AT2018cow can be explained by this dust echo.

Abstract

Luminous fast blue optical transients (LFBOTs) such as AT2018cow form a rare class of engine-powered explosions of uncertain origin. A hallmark feature of these events is radio/millimeter synchrotron emission powered by the interaction of fast (v > 0.1 c) ejecta and dense circumstellar material (CSM) extending to large radii > 1e16 cm surrounding the progenitor. Assuming this CSM to be an outflow from the progenitor, we show that dust grains up to ~1 micron in size can form in the outflow in the years before the explosion. This dusty CSM would attenuate the transient's ultraviolet (UV) emission prior to peak light, before being destroyed by the rising luminosity, reddening the pre-maximum colors (consistent with the pre-maximum red-to-blue color evolution of the LFBOT candidate MUSSES2020J). Re-radiation by the dust before being destroyed generates an near-infrared (NIR) "echo" of luminosity ~1e41-1e42 erg/s lasting weeks, which is detectable over the transient's rapidly fading blue continuum. We show that this dust echo is compatible with the previously unexplained NIR excess observed in AT2018cow. The gradual decay of the early NIR light curve can result from CSM which is concentrated in wide-angle outflow or torus, consistent with the highly aspherical geometry of AT2018cow's ejecta. Pre-maximum optical/UV and NIR follow-up of LFBOTs provide an new probe of their CSM environments and place additional constraints on their progenitors.